EPR spectrometer having microwave source comprising fixed frequency microwave oscillator and tunable HF generator

Abstract

The present disclosure relates to an EPR spectrometer having a microwave source including a fixed frequency microwave oscillator and a tunable HF generator. An electron paramagnetic resonance (= EPR) spectrometer (1) comprising-a sample resonator (2) comprising a sample region (3) for a measurement sample (4) to be held in the sample resonator (2); the invention relates to a microwave detector (1) comprising a sample resonator (2), a tunable microwave source (7) for generating a tunable microwave signal to be applied to the sample resonator (2), a magnet system (5) for generating a variable magnetic field at least to be applied to a sample region (3) in the sample resonator (2), and a microwave detector (9) connected to the sample resonator (2) for detecting microwave radiation, characterized in that the tunable microwave source (7) comprises-a fixed frequency microwave oscillator (20) for generating a fixed frequency microwave signal,-a tunable frequency generator (27) for generating a tunable high frequency (= HF) signal, where the tunable HF signal has a lower frequency than the fixed frequency microwave signal, and-a mixer (32) for mixing the tunable HF signal with the fixed frequency microwave signal, and a mixer for mixing the fixed frequency microwave signal and the tunable HF signal to generate a tunable microwave signal. The invention provides an EPR spectrometer with high spectral resolution, which is simple to produce and operate and low in cost.

Description

Technical Field

[0001] The present invention relates to an electron paramagnetic resonance (EPR) spectrometer, comprising

[0002] a sample resonator comprising a sample area for a measurement sample to be held in the sample resonator;

[0003] - a tunable microwave source for generating a tunable microwave signal to be applied to the sample resonator,

[0004] - a magnet system for generating a variable magnetic field to be applied at least to a sample region in the sample resonator, and

[0005] - and a microwave detector connected to the sample resonator for detecting the microwave radiation.

[0006] Such a process is known from US Pat. No. 5,309,118.

[0007] Electron paramagnetic resonance (EPR), also known as electron spin resonance, is a powerful tool in instrumental analysis for studying the chemical composition of samples with a permanent magnetic moment, generated by unpaired electrons in the sample. The sample is exposed to an external magnetic field and the resonant microwave absorption is measured.

[0008] In practice, the sample is placed in the sample area of ​​the sample resonator. A microwave source generates microwave radiation (also referred to as an applied microwave signal) that is applied to the sample resonator. The applied microwave signal has a frequency that corresponds to the resonant frequency of the microwave resonator containing the sample. It should be noted that the presence of the sample affects the resonant frequency of the microwave resonator, so different samples with different chemical compositions may result in different resonant frequencies of the microwave resonator. It is further noted that external factors such as temperature may affect the resonant frequency of the sample resonator. Therefore, the microwave source is tunable in order to adapt to specific measurement situations. Typical EPR spectrometers operate in the frequency range of 9-10 GHz (X-band) or 33-35 GHz (Q-band).

[0009] Furthermore, the sample is exposed to a magnetic field. This results in a splitting of the energy levels of unpaired electrons in the sample, which depends on the strength of the magnetic field and the chemical properties of the sample. If the energy of the applied microwave signal is equal to the splitting of the energy levels, the microwave energy is absorbed by the sample, which is recorded at the microwave detector. In continuous wave (CW) EPR, to obtain the EPR spectrum of the sample, the magnet system generates a slightly amplitude-modulated magnetic field, for example at 100 kHz, while the microwave frequency is kept constant.

[0010] In order to obtain high-resolution EPR spectra of a sample, the tunable microwave source should have high spectral purity, resulting in low phase noise.

[0011] US Pat. No. 5,309,118 describes a microwave source for EPR spectroscopy, in which a resonant cavity source incorporates a resonant active element directly into the cavity. Hard-to-obtain GUNN-effect diodes are used as active elements. Furthermore, the cavity geometry is varied by a movable closing plate to change the microwave frequency. Water cooling is applied during operation.

[0012] Although this type of microwave source can generate microwave radiation with excellent spectral purity, production and operation involve relatively high costs and effort.

[0013] Microwave radiation can be generated by a voltage-controlled oscillator (VCO). However, VCOs operating in the X-band or Q-band generate significant phase noise. Therefore, VCOs are only used in entry-level EPR spectrometers.

[0014] Wei et al., Photonics Research, Vol. 6, No. 1, January 2018, pp. 12-17, propose a frequency synthesizer for radar systems. It includes a mode-locked laser (MLL) stabilized to an all-fiber reference, from which a 10 GHz microwave signal is extracted. A direct digital synthesizer generates a signal up to 1 GHz. The DDS output is mixed with the stabilized 10 GHz signal to produce a tunable microwave signal between 9 and 11 GHz.

[0015] CN 209881735 describes a Ku-band low phase noise microwave source comprising a local oscillator circuit, a DDS circuit, a mixer, and a frequency multiplication filtering circuit using a harmonic filter. Summary of the Invention

[0016] Purpose of the Invention

[0017] The object of the present invention is to provide an EPR spectrometer with high spectral resolution which is simple and inexpensive to produce and operate.

[0018] Brief description of the invention

[0019] According to the invention, this object is achieved by an EPR spectrometer as described at the outset, characterized in that the tunable microwave source comprises

[0020] - a fixed frequency microwave oscillator for generating a fixed frequency microwave signal,

[0021] a tunable frequency generator for generating a tunable high frequency (=HF) signal, wherein the tunable HF signal has a lower frequency than the fixed frequency microwave signal, and

[0022] - A mixer for mixing the fixed frequency microwave signal and the tunable HF signal to generate a tunable microwave signal.

[0023] The present invention proposes an EPR spectrometer in which a microwave signal to be applied to a sample resonator is generated using a fixed-frequency microwave oscillator, a tunable frequency generator, and a mixer.

[0024] A fixed-frequency microwave oscillator generates a fixed-frequency microwave signal. This can be accomplished in a relatively simple manner with high spectral purity or low phase noise at a fixed microwave frequency that is chosen to be close to or within the microwave frequency band desired for EPR measurements (e.g., X-band or Q-band).

[0025] Furthermore, a tunable frequency generator generates a tunable high-frequency (HF) signal. Compared to a fixed-frequency microwave signal, an HF signal has a lower frequency, typically by a factor of 3 or more, often by a factor of 5 or more, or even by a factor of 10 or more. This can also be achieved in a relatively simple manner with high spectral purity or low phase noise. Typically, tunable HF signals are generated in the range of 3 GHz or less, and often in the range of 1 GHz or less.

[0026] The fixed-frequency microwave signal and the tunable HF signal are mixed by a mixer to generate a tunable microwave signal having a first mode and a second mode. The first mode has a microwave frequency lower than the fixed-frequency microwave signal, and the second mode has a microwave frequency higher than the fixed-frequency microwave signal. By adjusting the frequency of the tunable HF signal, the microwave frequencies of the two tunable microwave modes can also be adjusted. One of the modes can be selected for application to the sample resonator of an EPR spectrometer; the other mode is typically filtered out. Preferably, the first mode (the lower frequency mode) is selected for application to the EPR spectrometer.

[0027] By mixing a fixed-frequency microwave signal of high spectral purity (and low phase noise) with a tunable HF signal of high spectral purity (and low phase noise), a tunable microwave signal of high spectral purity (and low phase noise) can be obtained. This is relatively simple, especially compared to directly generating a tunable microwave signal of high spectral purity in an EPR spectrometer as known in the prior art.

[0028] Typically, the fixed-frequency microwave signal is chosen to have a frequency at one end of (or even exceeding) the desired bandwidth interval for EPR measurements. This avoids generating two frequencies within the desired bandwidth interval, which would complicate obtaining high spectral purity.

[0029] Preferred variations of the present invention

[0030] A preferred embodiment of the EPR spectrometer of the present invention provides that the fixed-frequency microwave oscillator is adapted to provide a fixed-frequency microwave signal having a phase noise of -135 dBc or less at an offset of 100 kHz, and the tunable frequency generator is adapted to provide a tunable HF signal having a phase noise of -135 dBc or less at an offset of 100 kHz. Using such a low-noise input signal for the mixer, a tunable microwave signal having a phase noise of -135 dBc or less at an offset of 100 kHz can also be provided in a simple manner by the mixer. Preferably, the EPR spectrometer is adapted to provide the fixed-frequency microwave signal and the tunable HF signal, and the resulting tunable microwave signal, each having a phase noise of -140 dBc or less at an offset of 100 kHz.

[0031] In a highly preferred variant of the EPR spectrometer according to the invention, the EPR spectrometer further comprises an automatic frequency control (=AFC) system, wherein the AFC system is adapted to - receive a measured microwave signal provided by the microwave detector;

[0032] - deriving an AFC adjustment signal from the measured microwave signal, wherein the AFC adjustment signal indicates a deviation of a current frequency of the measured microwave signal from a current resonant frequency of the sample resonator; and - feeding the AFC adjustment signal into a tunable frequency generator for continuously controlling the current frequency of the tunable HF signal, such that the current frequency of the tunable microwave signal is continuously adjusted to the current resonant frequency of the sample resonator. By means of the AFC system, the sample resonator can be reliably and continuously operated at its resonant frequency, thereby correctly performing EPR measurements (without distortion / artifacts due to detuning of the applied microwave frequency relative to the resonant frequency). In the example of the AFC system, the AFC system can also be adapted to modulate the tunable HF signal with a modulation frequency, resulting in modulation of the tunable microwave signal with the modulation frequency, and to derive the AFC adjustment signal from the measured microwave signal using said modulation; however, it is noted that other variations for obtaining the AFC adjustment signal can also be applied.

[0033] In a preferred embodiment, the fixed-frequency microwave oscillator comprises a resonant cavity, particularly a cylindrical resonant cavity. This is a simple and inexpensive way to provide a fixed-frequency microwave oscillator with high spectral purity or low phase noise. The resonant cavity should be made of a highly conductive material. In one example, the resonant cavity is made of aluminum. In addition, the cavity surface may be provided with a highly conductive (internal) coating. In one example, the cavity surface is provided with a silver coating. The resonant cavity has a fixed geometry. It is further noted that in alternative embodiments, the fixed-frequency microwave oscillator may comprise a dielectric resonator or an optical fiber.

[0034] Advantageously, the fixed-frequency microwave oscillator includes a selective return chain element, a low-noise amplifier, a phase shifter, and a coupler connected in a ring-shaped series arrangement. This allows for the generation of a highly stable and pure fixed-frequency microwave signal. For example, the selective return chain element may be a resonant cavity, a dielectric resonator, or an optical fiber. The phase shifter is typically a 180° phase shifter.

[0035] In a preferred embodiment, the tunable frequency generator comprises a direct digital synthesizer (=

[0036] DDS). This has proven in practice to be very suitable for the present invention and allows for very good spectral purity or low phase noise of tunable HF signals. A DDS may comprise a phase / frequency control register to which a control signal is applied, a numerically controlled oscillator (=NCO), a digital-to-analog (=D / A) converter, and a reconstruction filter; a reference signal (also called a reference clock) is applied to the phase / frequency control register, the NCO, and the D / A converter.

[0037] In a preferred further development of the above embodiment, the tunable frequency generator further comprises a coupler for obtaining a portion of the fixed frequency microwave signal, and

[0038] A frequency divider for generating a divided frequency signal from the portion of the fixed-frequency microwave signal and providing the divided frequency signal to the DDS as a reference signal. This allows the DDS to operate in a simple and precise manner. The frequency divider can particularly employ a divisor of 4. The divided frequency signal is used by the DDS as a reference signal (reference clock).

[0039] In a preferred embodiment, the tunable frequency generator comprises a voltage controlled oscillator (=VCO).In this way, a tunable HF signal can also be provided in a simple and inexpensive manner.

[0040] Advantageously, the mixer only retains the difference between the fixed frequency microwave signal and the tunable HF signal.In this way, the tunable microwave signal can be obtained efficiently with high spectral purity or low phase noise.

[0041] Advantageously, the mixer is a single sideband mixer. This is simple and cheap to implement.

[0042] Alternatively, the mixer is a single-balanced mixer or a double-balanced mixer or a triple-balanced mixer. These mixer types can suppress AM noise contained in the tunable HF signal and have improved linearity.

[0043] In a preferred embodiment, the EPR spectrometer further includes a low-noise main amplifier for amplifying the tunable microwave signal provided by the mixer before applying it to the sample resonator. This allows the amplitude of the tunable microwave signal to be tailored to the needs of the EPR measurement and improves the signal-to-noise ratio of the EPR measurement.

[0044] Preferably, the embodiment provides

[0045] a) A fixed frequency microwave oscillator (20) is adapted to generate a fixed frequency microwave signal with a frequency of Ffix, wherein

[0046] 8.5GHz≤Ffix≤10.5GHz,

[0047] and the tunable frequency generator is adapted to generate a tunable HF signal having a frequency up to Fmax, wherein

[0048] 0.5GHz≤Fmax≤1.5GHz,

[0049] Specifically, where Ffix = 10 GHz, and Fmax = 1 GHz,

[0050] or

[0051] b) The fixed frequency microwave oscillator is suitable for generating a fixed frequency microwave signal with a frequency of Ffix, wherein

[0052] 32.0GHz≤Ffix≤36.0GHz,

[0053] and the tunable frequency generator is adapted to generate a tunable HF signal having a frequency up to Fmax, wherein

[0054] 1.0GHz≤Fmax≤3.0GHz,

[0055] In particular, where Ffix = 35 GHz and Fmax = 2 GHz, this has been shown to allow EPR measurements of samples in the X-band or Q-band with excellent resolution in a simple and inexpensive manner.

[0056] The present invention also includes a method for measuring an EPR spectrum of a measurement sample,

[0057] The measurement sample is located in the sample area of ​​the sample resonator.

[0058] wherein a tunable microwave signal is applied to the sample resonator,

[0059] wherein a variable magnetic field is applied at least to the measurement sample in the sample region in the sample resonator,

[0060] and wherein a microwave detector connected to the sample resonator detects the microwave radiation, characterized in that

[0061] The invention relates to a method for generating a tunable microwave signal by mixing a fixed-frequency microwave signal obtained from a fixed-frequency microwave oscillator with a tunable high-frequency (HF) signal obtained from a tunable frequency generator, wherein the tunable HF signal has a lower frequency than the fixed-frequency microwave signal, in particular wherein the method is performed with an inventive EPR spectrometer according to the invention as described above. With the inventive method, a tunable microwave signal with high spectral purity or low phase noise can be obtained, and conversely, EPR measurements with high spectral resolution can be obtained in a simple and inexpensive manner.

[0062] A preferred variant of the above method provides that the fixed-frequency microwave signal has a phase noise of -135 dBc or less at an offset of 100 kHz, and the tunable HF signal has a phase noise of -135 dBc or less at an offset of 100 kHz. A tunable microwave signal having a phase noise of -135 dBc or less at an offset of 100 kHz can then be obtained in a simple manner.

[0063] In a particularly advantageous variant, the method applies automatic frequency control (=AFC), comprising

[0064] - receiving a measured microwave signal provided by a microwave detector;

[0065] - deriving an AFC adjustment signal from the measured microwave signal, wherein the AFC adjustment signal is indicative of a deviation of a current frequency of the measured microwave signal from a current resonant frequency of the sample resonator;

[0066] The AFC adjustment signal is fed into the tunable frequency generator, thereby continuously controlling the current frequency of the tunable HF signal, so that the current frequency of the tunable microwave signal is continuously adjusted to the current resonant frequency of the sample resonator. The AFC system allows the sample resonator to reliably and continuously operate at its resonant frequency, thus correctly performing EPR measurements (without distortion / artifacts).

[0067] In a preferred further development of this variant, the automatic frequency control (=AFC) also includes

[0068] - the modulation of the tunable HF signal by means of a modulation frequency results in the modulation of the tunable microwave signal by means of a modulation frequency,

[0069] - Using said modulation to derive an AFC adjustment signal from the measured microwave signal. This is a simple way of setting up AFC. However, note that other ways of obtaining the AFC adjustment signal may alternatively be applied.

[0070] Further advantages can be derived from the description and the accompanying drawings. According to the invention, the features mentioned above and below can be used individually or together in any combination. The embodiments mentioned should not be understood as an exhaustive enumeration, but rather as having exemplary characteristics for describing the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The invention is illustrated in the accompanying drawings.

[0072] Figure 1 Schematically illustrates an exemplary embodiment of an EPR spectrometer according to the present invention;

[0073] Figure 2 Schematically shows Figure 1 A tunable microwave source for an EPR spectrometer, comprising a DDS;

[0074] Figure 3 Schematically shows Figure 1 The main components of the AFC system of the EPR spectrometer;

[0075] Figure 4 Shown with Figure 3 The AFC system generates an AFC adjustment signal, wherein the frequency of the tunable microwave signal is equal to the resonant frequency of the sample resonator;

[0076] Figure 5 Shown Figure 3 generating an AFC adjustment signal in an AFC system, wherein a frequency of the tunable microwave signal is greater than a resonant frequency of the sample resonator;

[0077] Figure 6 Shown Figure 3 generating an AFC adjustment signal in an AFC system, wherein a frequency of the tunable microwave signal is less than a resonant frequency of the sample resonator;

[0078] Figure 7 Schematically shows a variant including a VCO according to the present invention. Figure 1 An alternative tunable microwave source for EPR spectrometers. DETAILED DESCRIPTION

[0079] Figure 1 An exemplary embodiment of an EPR spectrometer 1 of the invention is schematically shown.

[0080] The EPR spectrometer comprises a sample resonator 2 including a sample region 3 for a measurement sample 4. In the embodiment shown, the measurement sample 4 comprises a sample tube held in the sample region 3, and the sample tube contains some measurement substance having unpaired electrons.

[0081] A magnet system 5 generates a magnetic field that is applied to the sample region 3. The applied magnetic field is slightly modulated by the magnet system 5 and controlled by an electronic control unit 6, which typically includes a lock signal generator (not shown in detail). The modulated magnetic field has a primarily constant component and an additional varying component; here, the varying component varies sinusoidally at a frequency of, for example, 100 kHz, and the amplitude of the varying component is significantly smaller than that of the constant component. For example, the amplitude of the varying component can be 1% or less compared to the amplitude of the constant component.

[0082] The EPR spectrometer 1 also includes a tunable microwave source 7, details of which are given in Figure 2 The tunable microwave signal generated by the tunable microwave source 7 at its output 7a is fed to a circulator 8, which forwards the tunable microwave signal to the sample resonator 2 and also receives return microwave radiation from the sample resonator 2, which is forwarded to a microwave detector 9. Some of the tunable microwave signal is also directed through a reference arm unit 10, and the reference arm signal is also fed into the microwave detector 9.

[0083] The sample resonator 2 is critically coupled. As long as no energy transitions occur in the sample material of the measurement sample 4, the sample resonator 2 completely absorbs the incoming microwave signal. If the energy transitions into resonance, the absorption in the sample resonator 2 becomes incomplete, which can be detected as a measured microwave signal at the microwave detector 9.

[0084] A first part of the measurement microwave signal containing sample information is provided at a sample detection output 11b of the microwave detector 2. The sample detection output 11b is typically connected to a lock-in amplifier (not shown).

[0085] Furthermore, a second portion of the measured microwave signal is provided at the AFC processing output 11a of the microwave detector 9 and fed to an AFC system 12, details of which are given in Figure 3 The AFC system 12 generates an AFC adjustment signal at the AFC system output 12a, which is fed to the tunable microwave source 7. By means of the AFC adjustment signal, the current frequency of the tunable microwave signal generated by the tunable microwave source 7 can be controlled and maintained at the current resonant frequency of the sample resonator 2.

[0086] Figure 2 Shown in more detail Figure 1 Tunable microwave source 7. It should be noted that Figure 2 The designs of the tunable microwave source 7 described in are exemplary designs, and other designs may be chosen without departing from the scope of the present invention.

[0087] The tunable microwave source 7 includes a fixed-frequency microwave oscillator 20. The fixed-frequency microwave oscillator 20 includes a selective return chain element 21, here a resonant cavity 22. The resonant cavity 22 is a resonant cylindrical cavity 23 operating in the TM010 (transverse magnetic) resonant mode. Note that the resonant frequency of the cylindrical resonant cavity 23 depends on its diameter, not its length. In the illustrated embodiment, the resonant cavity 22 is machined from aluminum and has an internal silver coating (not shown in detail). Typically, the quality factor QF of the resonant cavity 22 should be ≥ 2000, preferably ≥ 4000. For coupling, two strip traces on a printed circuit board can be used in the resonant cavity 22. Furthermore, the fixed-frequency microwave oscillator 20 includes a low-noise amplifier 24, a phase shifter 25 (here operating with a 180° phase shift), and a first coupler 26. Components 21, 24, 25, and 26 are electrically connected in series to form a closed loop. The generated fixed-frequency microwave radiation circulates within this loop. This loop has a gain greater than 1, corresponding to the energy tapping of the first coupler 26 .

[0088] At the first coupler 26, a portion of the fixed-frequency microwave radiation is coupled out, and thus a fixed-frequency microwave signal is obtained at the output end 26a of the first coupler 26. The fixed-frequency microwave signal obtained at the output end 26a has a phase noise of -135 dBc or less at an offset of 100 kHz, preferably a phase noise of -140 dBc or less. In the example described, the fixed-frequency microwave radiation and the fixed-frequency microwave signal have a frequency of 10.06 GHz and are sinusoidal.

[0089] The tunable microwave source 7 further comprises a tunable frequency generator 27. The tunable frequency generator 27 here comprises a second coupler 28 for tapping off a portion of the fixed-frequency microwave signal from the output 26a and feeding said portion into a frequency divider 29. The frequency divider 29 generates a divided-frequency signal, here one-quarter the frequency of the fixed-frequency microwave signal. The divided-frequency signal is fed as a reference signal (also referred to as a "reference clock," not mixed with a clock phase signal, see below) into a direct digital synthesizer (DDS) 30. The DDS 30 generates a tunable HF signal at an output 30a. The tunable HF signal obtained at the output 30a has a phase noise of -135 dBc or less, preferably -140 dBc or less, at an offset of 100 kHz. In the example described, the tunable HF signal has a frequency that can be varied by the DDS 30 between 0 and 1 GHz and is of sinusoidal type.

[0090] To determine the frequency of the tunable HF signal, the DDS 30 uses a rough default programming (which takes into account the known characteristics of the sample resonator) and the AFC adjustment signal from the AFC system (see Figure 1 and below). The AFC regulation signal is output at the AFC system output 12a (see also Figure 1 and Figure 3 ) and is fed into the AFC adjustment signal input 31 of the DDS 30. In the variant shown, for the AFC system used, the DDS 30 also implements a slight frequency modulation of the tunable HF signal, here according to a clock phase signal with a modulation frequency of 78.125 kHz. The resulting modulation of the tunable microwave signal and its influence on the detected microwave signal at the microwave detector are analyzed to generate the AFC adjustment signal (see below Figure 3 and the following text Figure 4-6 ).

[0091] The remainder (also referred to as " carrier wave ") of the fixed frequency microwave signal obtained at the output 28a of the second coupler 28 and the tunable HF signal obtained at the output 30a of the DDS 30 are fed to a mixer 32. The mixer 32 mixes the fixed frequency microwave signal (or its remainder here) and the tunable HF signal by multiplication, thereby obtaining the tunable microwave signal at the output 32a of the mixer 32. In the illustrated variant, the mixer 32 is a single sideband mixer (not shown in detail). In addition, only the resulting lower frequency mode (of the frequency difference) is further used here, and the higher frequency mode (of the sum of the frequencies) is filtered out (not shown in detail).

[0092] The tunable microwave signal obtained at the mixer output 32a then undergoes amplification at the low-noise main amplifier 33. The tunable microwave signal obtained after this amplification at the output 7a of the tunable microwave source 7 has a phase noise of -135dBc or less at an offset of 100kHz, preferably a phase noise of -140dBc or less.

[0093] Figure 3 Shown Figure 1 The main components of the AFC system 12. It should be noted here that the described variants are examples of the present invention and that other types of AFC systems can also be applied according to the present invention.

[0094] An AFC system 12 is connected to the microwave detector 9 at its AFC processed output 11a, where the measured microwave signal (or a portion thereof) is present.

[0095] The signal is split and fed in parallel to a first amplifier 34 and a second amplifier 35. The first amplifier 34 maintains the signal's polarity, while the second amplifier 35 inverts the signal's polarity. Aside from the polarity, the amplification factors of amplifiers 34 and 35 are identical (in absolute value). Typical amplification factors (in absolute value) are 10 or greater, but can be smaller if desired. For the AFC's functionality, the polarity assignment is crucial, not the amplification factor. Therefore, the AFC system 12 can even operate without amplification, simply with polarity inversion for one of the split signal paths.

[0096] The signals of the amplifiers 34, 35 are then fed into a rectifier unit 36 ​​having a switching function. The rectifier unit 36 ​​also receives a clock phase signal at a clock phase signal input 37. This clock phase signal indicates the modulation that the tunable HF signal undergoes in the DDS (see above). Figure 2 and the following Figure 4-6 ) and is of a sinusoidal type. During the first half of each sine wave of the clock phase signal (phase 0 to π), the first amplifier 34 (instead of the second amplifier 35) is connected to the output terminal 36a of the rectifier unit 36. During the second half of each sine wave of the clock phase signal (phase π to 2π), the second amplifier 35 (instead of the first amplifier 34) is connected to the output terminal 36a of the rectifier unit 36.

[0097] The rectified signal obtained at the output 36a is fed into the integrator unit 38, and the integrated signal is obtained at the output 12a of the AFC system 12. The integrated signal is used as the AFD adjustment signal. The AFC system output 12a is connected to the AFC adjustment signal input 31 of the DDS (see also Figure 2 ).

[0098] Figures 4 to 6 Schematically shows the Figure 3 The setting of AFC12 is generated in the exemplary variant. Note that Figure 5 、 6 The plot axes in the graph correspond to Figure 4 The axes in the figure, so in Figure 4 They are explained only once in . Note also that the resonances of the measurement sample are not discussed here, as they are not relevant for automatic frequency control (AFC). Note further that AFC is usually performed simultaneously with the EPR measurement of the measurement sample.

[0099] refer to Figure 4 , graph 40 shows the intensity I(DET) of the microwave signal measured at the microwave detector (upward axis) as a function of the frequency f (rightward axis) of the tunable microwave signal applied to the sample resonator; graph 40 is also called the absorption curve of the sample resonator.

[0100] The sample resonator has a resonator frequency RF. Note that the resonator frequency RF depends primarily on the geometry of the sample resonator or its resonator cavity, but can vary slightly due to the type and contents of the measurement sample arranged in the sample resonator, as well as due to other variable conditions such as temperature. Therefore, the resonant frequency of the sample resonator is always the current resonant frequency. Since EPR measurements should be performed using microwave radiation with a frequency that is precisely equal to the current resonant frequency of the sample resonator, the microwave source used in the EPR measurement should be tunable and should be adjusted to the current resonant frequency.

[0101] When the current frequency of the tunable microwave signal is equal to the resonator frequency RF, the sample resonator absorbs virtually all microwave energy, and the microwave signal measured at the microwave detector is at a minimum (and effectively zero). However, if the current frequency of the tunable microwave signal is higher than RF or lower than RF, in both cases the intensity of the measured microwave signal becomes greater.

[0102] In order to determine whether the current frequency of the tunable microwave signal is above or below the resonant frequency RF, the frequency of the tunable HF signal is slightly modulated around the center frequency CF according to the clock phase signal (=CPS). In this way, the frequency of the tunable microwave signal is also slightly modulated. At the same time, the center frequency CF of the tunable microwave signal represents the current frequency of the tunable microwave signal.

[0103] The clock phase signal is shown in diagram 41. It shows time t on the right axis and the elongation E(CPS) of the clock phase signal on the upward axis. The clock phase signal is of sinusoidal type and has a frequency of 78.125 kHz, which is referred to as the modulation frequency MF. Applying this modulation, the field strength of the tunable microwave signal TMS can be described as

[0104] TMS=A0*sin{2π*[CF+M0*sin(2π*MF*t)]*t}

[0105] Where A0 is the amplitude of the tunable microwave signal, CF is the center frequency of the tunable microwave signal, M0 is the modulation amplitude of the clock phase signal, MF is the modulation frequency of the clock phase signal, and t is time. In other words, the frequency of the tunable microwave signal oscillates over time within a range of ±M0 around the center frequency CF, and this oscillation has a frequency MF. It should be noted that CF is typically on the order of approximately 10 GHz or 35 GHz, corresponding to the microwave frequency at which the EPR measurement is performed, while MF is much smaller here at approximately 78 kHz. Note that graph 40 gives an indication of the typical modulation amplitude M0 compared to the width of the resonance curve, as seen in the small CPS curve 41*, which shows a time extension in the upward direction and a frequency extension in the right / left direction.

[0106] for Figure 4 Graph 42 shows the detected intensity I(DET) of the microwave signal measured at the microwave detector over several oscillation periods T of the clock phase signal, when the center frequency CF is equal to the resonant frequency RF (note that T is equal to one oscillation period of the clock phase signal, T=1 / MF, and corresponds to a phase of 2π), see also Graph 40. When the clock phase signal is at 0, T / 2 (or π), T (or 2π), 3 / 2*T (or 3π), 2T (or 4π), and so on, its extension is zero, and the current frequency of the tunable microwave signal is exactly RF. The intensity I(DET) in graph 42 is then at its minimum value of approximately 0 at these points in time. Between these points in time, the intensity increases to a maximum and then decreases again in each case; between each two selected points in time, it does not matter whether the frequency of the tunable microwave signal is above or below RF, since the situation is symmetrical if CF=RF.

[0107] Then, the detected microwave signal is rectified and amplified (see Figure 3 34, 35, 36), wherein the polarity is maintained in each first half-wave of each sinusoidal oscillation of the clock phase signal, while the polarity is reversed in each second half-wave, see amplified polarity scheme APS 43. The resulting rectified signal RS is shown in graph 44, where time t is shown on the right-pointing axis and the rectified signal RS is shown on the upward-pointing axis. The rectified signal RS oscillates "symmetrically" about the axis of zero signal strength, with approximately equal areas under the signal curve on the positive and negative sides.

[0108] When the rectified signal is integrated, the resulting integrated signal IS of graph 45 is constant at zero. The integrated signal IS corresponds to the AFC adjustment signal. A value of zero indicates that the current center frequency CF is equal to the resonant frequency RF and that no center frequency correction is required.

[0109] In contrast, Figure 5The case where the center frequency CF is greater than the resonant frequency RF is shown, compare graph 50. The same clock phase signal as before is applied, compare graph 51, and the small CPS curve 51* in graph 50 is also shown.

[0110] At the extended zero crossings (or “zero crossings”) of the clock phase signal, i.e., at 0, T / 2, T, 3 / 2*T, 2T, etc., the intensity I(DET) of the measured microwave signal shown in graph 52 is non-zero and positive, increasing from t=0 to a maximum value around T / 4, then decreasing again until T / 2. The intensity then decreases further, reaching a minimum value of approximately 0 at 3 / 4*T, but then increases again until T.

[0111] Then, the same amplification polarity scheme APS 53 as before is applied. Comparing graph 54, the rectified signal RS is generally large and positive from 0 to T / 2, and generally small (in absolute value) and negative between T / 2 and T. Specifically, the area (in absolute value) under the RS curve from 0 to T / 2 is larger than the area from T / 2 to T. The resulting integrated signal IS, as shown in graph 55, is then positive. This positive value of the integrated signal IS, or AFC adjustment signal, indicates that the center frequency CF of the tunable microwave signal should be lowered to approach the resonant frequency RF.

[0112] Now, Figure 6 The case where the center frequency CF is less than the resonant frequency RF is shown, compare graph 60. The same clock phase signal as before is applied, compare graph 61, and in graph 60 a small CPS curve 61* is also shown.

[0113] At the extended zero crossings (or "zero crossings") of the clock phase signal, i.e., at 0, T / 2, T, 3 / 2*T, 2T, etc., the intensity I(DET) of the measured microwave signal shown in graph 62 is non-zero and positive, decreasing from t=0 to a minimum value of approximately zero at around T / 4, and then increasing again until T / 2. The intensity then increases further, reaching a maximum value at 3 / 4*T, but then decreases again until T.

[0114] Then, the same amplification polarity scheme APS 63 as before is applied. Comparing graph 64, the rectified signal RS is generally small and positive from 0 to T / 2, and generally large (in absolute value) and negative between T / 2 and T. Specifically, the area (in absolute value) under the RS curve from 0 to T / 2 is smaller than the area from T / 2 to T. The resulting integrated signal IS, as shown in graph 65, is then negative. This negative value of the integrated signal IS, or AFC adjustment signal, indicates that the center frequency CF of the tunable microwave signal should be raised to approach the resonant frequency RF.

[0115] The DDS analyzes the AFC adjustment signal. If the AFC adjustment signal is positive, the frequency of the tunable HF signal decreases slightly. If the AFC adjustment signal is negative, the frequency of the tunable HF signal increases slightly. As long as, or as soon as, the AFC adjustment signal reaches zero, the current frequency of the tunable HF signal remains constant. In this way, closed-loop control of the current frequency of the tunable microwave signal can be established, maintaining it very precisely at the current resonant frequency of the sample resonator.

[0116] Figure 7 An alternative design of a tunable microwave source 7 according to the invention is shown, which can be used in Figure 1 The EPR spectrometer shown. Figure 7 The tunable microwave source 7 is similar to Figure 2 The tunable microwave source shown is not shown, so only the main differences are explained below.

[0117] The tunable microwave source 7 comprises a fixed frequency microwave oscillator 20 as already described and at the first coupler 26 a portion of the fixed frequency microwave radiation is coupled out so that a fixed frequency microwave signal is correspondingly available at the output 26a of the first coupler 26 as previously described.

[0118] The tunable microwave source 7 further comprises a tunable frequency generator 27 , which here comprises a voltage-controlled oscillator (=VCO) 39 .

[0119] The VCO 39 generates a tunable HF signal at the VCO output 39a. The tunable HF signal is of sinusoidal type here. The frequency of the generated tunable HF signal depends on the control voltage present at the VCO 39. The control voltage is obtained here by adding a basic signal (which results from a rough default programming, taking into account the known characteristics of a sample resonator not shown further), an AFC adjustment signal from the AFC system or a signal derived therefrom (see above), and a clock phase signal or a signal derived therefrom (see above). The AFC adjustment signal is output at the AFC system output 12a (see also Figure 1 and Figure 3 ) and is fed into the AFC adjustment signal input 31 of the VCO 39. For the AFC system used here, the VCO 39 also slightly frequency modulates the tunable HF signal in accordance with the clock phase signal, here also using a modulation frequency of 78.125 kHz (not shown further).

[0120] The tunable HF signal obtained at the VCO output 39a has a phase noise of -135 dBc or less, preferably -140 dBc or less at an offset of 100 kHz. In the example described, the tunable HF signal has a frequency that can be varied by the VCO 39 between 0 and 1 GHz.

[0121] The fixed frequency microwave signal obtained at output 26a and the tunable HF signal obtained at VCO output 39a are fed into mixer 32. The tunable microwave signal obtained at mixer output 32a then undergoes amplification at low noise main amplifier 33, as previously described.

[0122] Reference Signs List

[0123] 1EPR spectrometer

[0124] 2 sample resonators

[0125] 3. Sample area

[0126] 4. Measuring samples

[0127] 5-magnet system

[0128] 6 Electronic Control Unit

[0129] 7. Tunable microwave source

[0130] 7A output (tunable microwave source)

[0131] 8 Circulator

[0132] 9 Microwave detector

[0133] 10 reference arm units

[0134] 11a AFC processing output (of microwave detector)

[0135] 11b Sample detector output (for microwave detector)

[0136] 12 AFC system

[0137] 12a AFC system output

[0138] 20 Fixed frequency microwave oscillator

[0139] 21 Selective return chain elements

[0140] 22 resonant cavity

[0141] 23 Resonant cylindrical cavity

[0142] 24 low noise amplifiers

[0143] 25 Phase Shifter

[0144] 26 (first) coupler (fixed frequency microwave oscillator)

[0145] 26a output (first coupler)

[0146] 27 Tunable frequency generator

[0147] 28 (Second) Coupler (Tunable Frequency Generator)

[0148] 28a output (second coupler)

[0149] 29-way divider

[0150] 30 Direct Digital Synthesizer (=DDS)

[0151] 30A DDS output

[0152] 31 AFC adjustment signal input

[0153] 32 mixers

[0154] 32a output (mixer)

[0155] 33 Low noise main amplifier

[0156] 34 First amplifier (maintain polarity)

[0157] 35 Second amplifier (change polarity)

[0158] 36 rectifier units

[0159] 36a output (rectifier unit)

[0160] 37 clock phase signal input

[0161] 38 integrated units

[0162] 39 Voltage Controlled Oscillator (VCO)

[0163] 39a VCO output

[0164] 40 Graph of measured microwave frequency / intensity as a function of the frequency of the tunable microwave signal (CF=RF)

[0165] 41 Graph / Extension of a clock phase signal as a function of time

[0166] 41*Small CPS curve

[0167] 42 Graph / intensity of measured microwave signal as a function of time

[0168] 43 Amplification Polarity List

[0169] 44 Graph of the rectified signal as a function of time / extension

[0170] 45 Graph / integrate signal as a function of time

[0171] 50 Graph of measured microwave frequency / intensity as a function of frequency of a tunable microwave signal (CF>RF)

[0172] 51 Graph / Extension of a clock phase signal as a function of time

[0173] 51*Minimum CPS curve

[0174] 52 Graph / intensity of the measured microwave signal as a function of time

[0175] 53 amplification polarity scheme

[0176] 54 Graph of the rectified signal as a function of time / extension

[0177] 55 Graph / integrated signal as a function of time

[0178] 60 Graph of measured microwave frequency / intensity as a function of frequency of a tunable microwave signal (CF < RF)

[0179] 61 Graph / Extension of a clock phase signal as a function of time

[0180] 61*Minimum CPS curve

[0181] 62 Graph / intensity of measured microwave signal as a function of time

[0182] 63 amplification polarity scheme

[0183] 64 Graph / Extension of the rectified signal as a function of time

[0184] 65 Graph / Integrated Signal as a Function of Time

[0185] APS amplification detection solution

[0186] CF is the center frequency / current frequency of the tunable microwave signal

[0187] CPS clock phase signal

[0188] Microwave signals measured / detected by DET

[0189] Extension of E(…)…

[0190] The intensity of I(…)…

[0191] IS integrated signal

[0192] f frequency

[0193] RF resonant frequency (sample resonator)

[0194] RS rectified signal

[0195] T time

[0196] T vibration period (clock phase signal)

Claims

1. An electron paramagnetic resonance (EPR) spectrometer (1), comprising - a sample resonator (2) comprising a sample area (3) for a measurement sample (4) to be held in the sample resonator (2); - a tunable microwave source (7) for generating a tunable microwave signal to be applied to the sample resonator (2), a magnet system (5) for generating a variable magnetic field to be applied at least to the sample region (3) in the sample resonator (2), and - and a microwave detector (9), connected to the sample resonator (2) for detecting microwave radiation, Characterized by The tunable microwave source (7) comprises - a fixed frequency microwave oscillator (20) for generating a fixed frequency microwave signal, - a tunable frequency generator (27) for generating a tunable high frequency (=HF) signal, wherein The tunable HF signal has a lower frequency than the fixed frequency microwave signal, and - a mixer (32) for mixing the fixed frequency microwave signal and the tunable HF signal to generate a tunable microwave signal.

2. The EPR spectrometer (1) according to claim 1, characterized in that The fixed frequency microwave oscillator (20) is adapted to provide a fixed frequency microwave signal having a phase noise of -135 dBc or less at an offset of 100 kHz, And the tunable frequency generator (27) is adapted to provide a tunable HF signal having a phase noise of -135dBc or less at an offset of 100kHz.

3. The EPR spectrometer (1) according to claim 1, characterized in that The EPR spectrometer (1) further comprises an automatic frequency control (=AFC) system (12), wherein the AFC system (12) is adapted to - receiving a measured microwave signal provided by a microwave detector (9); - deriving an AFC adjustment signal from the measured microwave signal, wherein the AFC adjustment signal is indicative of a deviation of a current frequency (CF) of the measured microwave signal from a current resonant frequency (RF) of the sample resonator (2); and - feeding the AFC adjustment signal into the tunable frequency generator (27) for continuously controlling the current frequency of the tunable HF signal so that the current frequency (CF) of the tunable microwave signal is continuously adjusted to the current resonance frequency (RF) of the sample resonator (2).

4. The EPR spectrometer (1) according to claim 1, characterized in that The fixed frequency microwave oscillator (20) comprises a resonant cavity (22), in particular a cylindrical resonant cavity (23).

5. EPR spectrometer (1) according to one of the preceding claims, characterized in that The fixed frequency microwave oscillator (20) comprises a selective return chain element (21), a low noise amplifier (24), a phase shifter (25) and a coupler (26) connected in series in a ring shape.

6. EPR spectrometer (1) according to one of claims 1 to 5, characterized in that The tunable frequency generator (27) comprises a direct digital synthesizer (=DDS) (30).

7. The EPR spectrometer (1) according to claim 6, characterized in that The tunable frequency generator (27) also includes - a coupler (28) for obtaining a portion of the fixed frequency microwave signal, and - a frequency divider (29) for generating a divided frequency signal from the portion of the fixed frequency microwave signal and providing the divided frequency signal as a reference signal to the DDS (30).

8. The EPR spectrometer (1) according to claim 1, characterized in that The tunable frequency generator (27) comprises a voltage controlled oscillator (=VCO) (39).

9. EPR spectrometer (1) according to one of the preceding claims, characterized in that The mixer (32) only holds the difference between the fixed frequency microwave signal and the tunable HF signal.

10. The EPR spectrometer (1) according to one of the preceding claims, characterized in that The EPR spectrometer (1) further comprises a low-noise main amplifier (33) for amplifying the tunable microwave signal provided by the mixer (32) before applying the tunable microwave signal to the sample resonator (2).

11. EPR spectrometer (1) according to one of the preceding claims, characterized in that a) A fixed frequency microwave oscillator (20) is adapted to generate a fixed frequency microwave signal with a frequency of Ffix, wherein 8.5GHz≤Ffix≤10.5GHz, and the tunable frequency generator (27) is adapted to generate a tunable HF signal with a frequency up to Fmax, wherein 0.5GHz≤Fmax≤1.5GHz, Specifically, where Ffix = 10 GHz, and Fmax = 1 GHz, or b) The fixed frequency microwave oscillator (20) is adapted to generate a fixed frequency microwave signal with a frequency of Ffix, wherein 32.0GHz≤Ffix≤36.0GHz, and the tunable frequency generator (27) is adapted to generate a tunable HF signal with a frequency up to Fmax, wherein 1.0GHz≤Fmax≤3.0GHz, Specifically, where Ffix = 35 GHz and Fmax = 2 GHz.

12. A method for measuring an EPR spectrum of a measurement sample (4), in, The measurement sample (4) is located in the sample region (3) of the sample resonator (2), wherein a tunable microwave signal is applied to a sample resonator (2), wherein a variable magnetic field is applied at least to a measurement sample (4) in a sample region (3) in a sample resonator (2), and wherein a microwave detector (9) connected to the sample resonator (2) detects the microwave radiation, It is characterized by generating a tunable microwave signal by mixing a fixed frequency microwave signal obtained from a fixed frequency microwave oscillator (20) with a tunable high frequency (=HF) signal obtained from a tunable frequency generator (27), Among them, the tunable HF signal has a lower frequency than the fixed frequency microwave signal, In particular, the method is performed with an EPR spectrometer (1) according to one of the preceding claims.

13. The method according to claim 12, characterized in that The fixed frequency microwave signal has a phase noise of -135dBc or less at an offset of 100kHz. And the tunable HF signal has a phase noise of -135dBc or less at an offset of 100kHz.

14. The method according to claim 12 or 13, characterized in that The method applies automatic frequency control (=FC), which includes - receiving a measured microwave signal provided by a microwave detector (9); - deriving an AFC adjustment signal from the measured microwave signal, wherein the AFC adjustment signal indicates a deviation of a current frequency (CF) of the measured microwave signal from a current resonant frequency (RF) of the sample resonator (2); - feeding the AFC adjustment signal into the tunable frequency generator (27) to continuously control the current frequency of the tunable HF signal so that the current frequency (CF) of the tunable microwave signal is continuously adjusted to the current resonance frequency (RF) of the sample resonator (2).

15. The method according to claim 14, characterized in that Automatic frequency control (=AFC) also includes - modulation of the tunable HF signal by means of a modulation frequency (MF), resulting in modulation of the tunable microwave signal by means of a modulation frequency (MF), - deriving an AFC adjustment signal from the measured microwave signal using said modulation.

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