Ultrasonic transmitter with low distortion and simultaneous reception capability

Abstract

The ultrasonic transceiver overcomes many disadvantages of conventional ultrasonic transceivers by providing the ability to transmit high-power, high-frequency arbitrary waveforms with low distortion and the ability to monitor the transmitted signal through the receiver during the transmission period, and the transceiver circuit has a transducer element that emits an ultrasonic signal and receives a reflected ultrasonic signal, the transformer circuit has a secondary winding and a primary winding coupled to the transducer element, the H-bridge transmission waveform circuit is coupled to the primary winding of the transformer to generate a transmission waveform signal for the transducer element through the transformer, the FET clamp is coupled to the secondary winding of the transformer, and the receiver circuit has an input coupled to the FET clamp.

Description

Technology Field

[0001] The present disclosure relates to ultrasonic imaging, and more specifically, to a transceiver used with an ultrasonic transducer capable of transmitting a high-power, high-frequency arbitrary waveform with low distortion and monitoring the transmitted signal through a receiver during the transmission period. Background Technology

[0002] Ultrasonic systems typically utilize multi-element transducers to transmit ultrasonic pulses into a medium and receive echo signals returning from the medium and objects within it. Each transducer element may have its own independent transmitter and receiver, or a transceiver, enabling the electronic focusing and steering of the ultrasonic beam formed by the combination of transducer elements. To generate transmission pulses from piezoelectric transducer elements with sufficient power, transmitters typically operate at peak-to-peak voltage levels of several hundred volts. In contrast, the received echo signal level is typically less than a few millivolts, with a signal intensity significantly lower than that of the transmission pulse; therefore, a high-gain receiver is required for proper detection. Due to this large discrepancy between the transmitted and received signals, the receiver must be protected from prolonged recovery saturation, or at least from damage caused by high transmission currents.

[0003] Most ultrasonic systems employ what is known as a transmit / receive (T / R) switch to protect the input of the receiver during the transmission period. A typical ultrasonic transceiver circuit (10) is illustrated in FIG. 1 and includes a T / R switch (12) controlled by a control signal circuit (14). The T / R switch (12) is coupled to a transducer element (16) in response to the control signal circuit (14) switching the T / R switch (12) to an on state during the transmission period. The on state has the effect of creating a low-impedance path between the transmitter circuit (26) and the transducer element (16) and creating a high-impedance path from the transmitter circuit (26) to the receiver circuit (20), and the receiver circuit (20) includes a variable gain amplifier (21) having an input (34). When the T / R switch (12) is in the ON state, protection for the high-voltage transmission waveform signal (22) transmitted from the transmission waveform signal generator (24) of the transmitter circuit (26) is provided to the receiver circuit (20).

[0004] After the transmitter circuit (26) has completed the transmission of the waveform signal (22), the T / R switch (12) is switched from the transmission state to the reception state (28) (shown as a dotted line on the T / R switch (12)) by the control signal (14), which provides a low-impedance path from the transducer element (16) to the receiver circuit (20) and further prevents the transmitter circuit (26) from attenuating or degrading the returning echo signal. The receiver circuit (20) may have other limiting circuits at its input, such as a resistive connection to ground (36) to protect against large reception signals, and may also have a back-to-back diode passive receiver protection circuit (30) and a capacitance (38) shown in FIG. 1.

[0005] The transceiver circuit of FIG. 1 has several problems that limit its effectiveness. Most ultrasonic systems utilize a single T / R control signal (32) for all transceivers. In the case of multi-element transducers, the transmission waveform (22) is emitted at various delay times to steer and focus the ultrasonic beam in the medium (not shown). This generally requires a transmission period during which all transceivers maintain the transmission mode until the last transducer completes transmission. Consequently, there is a small area in the medium in front of the transducer where echoes cannot be received during the transmission period. Likewise, it is impossible to transmit from an element or group of elements during the reception process without interrupting the entire set of received signals. Additionally, the T / R switch (12), typically implemented with diodes and transistors, generally imparts some non-linear characteristics to the transmission waveform (22), causing some minor distortion of the waveform shape. For example, a sine wave transmission waveform (22) can be distorted in a way that generates higher harmonics, which can have an undesirable effect on the medium. Finally, the T / R switch (12) generally limits the amount of power that can be applied to the transducer (16) in transmission mode (on state) due to current limiting and heating effects in the electronic switch.

[0006] The present disclosure relates to an ultrasonic system and method including an ultrasonic transceiver that overcomes most of the disadvantages of conventional ultrasonic transceivers by providing the ability to transmit high-power and high-frequency arbitrary waveforms with low distortion and the ability to monitor the transmitted signal through a receiver during the transmission period.

[0007] According to one aspect of the present disclosure, a circuit is provided comprising: a transducer element configured to emit an ultrasonic signal and receive a reflected ultrasonic signal; a transformer circuit coupled to the transducer element—the transformer circuit comprises a transformer having a primary winding and a secondary winding, the secondary winding being coupled to the transducer element—a transmission waveform circuit coupled to the primary winding of the transformer and configured to generate a transmission waveform signal to the transducer element through the transformer circuit; and a receiver circuit configured to be coupled to the transducer element having an input coupled to the secondary winding of the transformer.

[0008] According to another aspect of the present disclosure, the transmission waveform circuit is an H-bridge circuit.

[0009] According to a further aspect of the present disclosure, the circuit comprises a clamp circuit coupled between the secondary winding of a transformer and the input of a receiver circuit. Preferably, the clamp circuit comprises an active clamp circuit having a pair of FET switches coupled in parallel to the input of the receiver circuit.

[0010] According to another additional aspect of the present disclosure, each of the pair of FET switches has a control terminal coupled to a transmission waveform generating circuit and receiving an on-signal connected to the duration of the transmission waveform signal.

[0011] According to another aspect of the present disclosure, an ultrasonic device is provided, and the ultrasonic device is,

[0012] A transducer circuit configured to transmit an ultrasonic signal, receive a corresponding echo signal, and generate a returning echo signal;

[0013] A variable gain receiver having an input coupled to a transducer circuit;

[0014] It includes a transceiver capable of generating a peak-to-peak waveform of more than 100 volts across a transducer and receiving a peak-to-peak echo signal of less than 1 volt returning from the transducer, and the transceiver,

[0015] A transformer having a primary winding and a secondary winding - the primary winding is coupled to the transducer -;

[0016] A transmitter circuit coupled to a transducer via a transformer—the transmitter circuit includes a transmission waveform generator configured to generate a transmission waveform, the transmission waveform generator is coupled to the primary winding of the transformer so that the primary winding is driven by the transmission waveform generator, the secondary winding is connected at one end to the transducer circuit and at the other end to the input of a variable gain receiver, and the maximum gain is at least 30 dB—; and

[0017] A protection circuit coupled between the secondary winding of a transformer and the input of a variable gain receiver—the protection circuit is configured to provide an impedance in the range of 0.1 ohms to 1.0 ohms from the input of the variable gain receiver to ground during the transmission period of the transmitter and receiver when activated, so that the input of the variable gain receiver provides effective grounding to the transformer secondary winding during the transmission period—including; where

[0018] A variable gain receiver can be activated during the transmission period and can monitor the amplitude and duration of the transmitted waveform by amplifying a small voltage across the protection circuit. Brief explanation of the drawing

[0019] The foregoing and other features and advantages of the present disclosure will be more easily understood, as will be better understood by looking at the following detailed description together with the accompanying drawings. Figure 1 is a schematic diagram of a conventional ultrasonic transceiver equipped with a transmitting and receiving circuit. FIG. 2 is a schematic diagram of an ultrasonic transceiver having a transmitting and receiving (T / R) circuit in which a T / R switch according to the present disclosure is replaced with a transformer. FIG. 3 is a schematic diagram of the ultrasonic transceiver of FIG. 2 having a T / R circuit utilizing an H-bridge circuit according to the present disclosure. FIG. 4 is a schematic diagram of the ultrasonic transceiver circuit of FIG. 3 having a T / R circuit utilizing a clamp circuit for protecting the receiver circuit according to the present disclosure. FIG. 5 is an operation waveform of a control signal for a clamp circuit of FIG. 4, along with a response waveform signal for a component of an ultrasonic transceiver circuit of FIG. 4 formed according to the present disclosure. FIG. 6 is a schematic diagram of a control circuit for a transmitter circuit having a waveform plot formed according to a representative embodiment of the present disclosure. Specific details for implementing the invention

[0020] In the description below, specific details are provided to provide a complete understanding of the various embodiments disclosed. However, those skilled in the art will recognize that the implementation may be carried out without one or more of these specific details, or using other methods, components, materials, etc. In other cases, well-known structures associated with switches, transducers, amplifiers, control signal generators, programmable logic devices, memories, and transformers have not been illustrated or described in detail to avoid making the description of the implementation unnecessarily obscured.

[0021] Unless otherwise required by the context, throughout the specification and claims, words such as “include” and “include” and variations thereof shall be interpreted in an open and inclusive sense, namely, “includes, but is not limited thereto.”

[0022] Throughout this specification, the references to “one embodiment” or “an embodiment” mean that a specific feature, structure, or characteristic described in relation to that embodiment is included in at least one embodiment. Accordingly, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification does not necessarily refer to the same embodiment. Furthermore, a specific feature, structure, or characteristic may be combined in any suitable manner in one or more embodiments. For the sake of simplicity and clarity of description, reference numbers may be repeated between drawings to indicate corresponding or similar elements or steps where deemed appropriate.

[0023] As used in this specification and the appended claims, the singular forms "a," "an," and "the" include the plural forms unless explicitly indicated otherwise. Additionally, it should be noted that the term "or" is generally used in its broadest sense, namely, "and / or," unless explicitly indicated otherwise.

[0024] General Overview

[0025] Ultrasonic systems generally utilize a transducer or a transceiver connected to a transducer element to transmit ultrasonic energy into a medium and receive echo signals returning to the transducer. The transceiver must be able to generate a high-voltage signal to drive the transducer and receive a very small voltage signal generated by the returning echo. In some ultrasonic applications, it may also be necessary to generate a specific transmission waveform with very little distortion to obtain appropriate results. An ultrasonic transceiver is described that overcomes most of the disadvantages of conventional ultrasonic transceivers by having the ability to transmit high-power and high-frequency arbitrary waveforms with low distortion and the ability to monitor the transmission signal through the receiver during the transmission period.

[0026] Improved transceiver design

[0027] Removal of T / R switch - One improvement that can be made to the existing transceiver design is to completely eliminate the T / R switch. This can be achieved by a transformer (40) that couples the transmission waveform to the receiving path, as illustrated in the transceiver circuit (42) of FIG. 2. During transmission, the high-voltage transmission waveform (42) (peak-to-peak voltage exceeding 100 volts) conducts the back-to-back diode (30), which protects the receiver circuit (20) at the input (34) by effectively grounding the input signal. Because there is a voltage drop of about + / - 0.8 across the back-to-back diode (30), the input signal is not completely grounded, but this is generally small enough not to damage the receiver circuit (20) and has minimal effect on the transmission waveform. When the transducer element (16) receives an echo signal from the medium, the transmission drive signal from the transmitter circuit (26) becomes zero, and the transformer (40) becomes a short circuit. The echo signal level received from the transducer element (16) is generally lower than the conduction threshold of the back-to-back diode (30), so that the diode becomes high-impedance and the received signal can flow to the input (34) of the receiver circuit (20). A typical variable gain receiver circuit (20) will have a maximum gain of more than 30 dB.

[0028] The removal of the T / R switch (12) offers several advantages. First, a transmission period is not required, as transmission from the transducer element (16) can occur at any time, even when the receiver circuit (26) is active or when another transceiver is transmitting or receiving. This enables applications where multiple transmissions can be employed during the echo signal acquisition period from the same or different sets of transducer elements (16). Second, since the echo signal reception period can begin before any transmission occurs from the transducer element (16), some transducer elements can receive the signal before transmitting, thus effectively eliminating the blind spot in front of the transducer element (16). Third, since the receiver circuit receives a highly attenuated version of the transmission waveform, it can monitor the transmission waveform to detect a faulty transmitter circuit (26) or a non-operational transducer element (16). Additionally, the transformer (40) effectively insulates the high-voltage circuit of the transmitter circuit (26) from the transducer element (16) that often comes into close contact with the body. Thus, the risk of shock to the scanning target due to an error in the transmitter circuit (26) is prevented.

[0029] H-bridge Transmission Generator - A more improved transceiver circuit (50) is illustrated in FIG. 3. A typical transmission waveform generator or transmitter circuit (26) for generating any acoustic waveform in FIG. 1 consists of a digital waveform signal generator (24), a digital-to-analog converter (not shown), and a power amplifier (25). It is difficult to generate high power levels with this approach because the power amplifier (25) consumes a significant amount of power, which is dissipated as heat. An ultrasonic system with multiple transmission channels, each with its own transmitter circuit (26), will have its transmission power limited by the ability to cool the circuit.

[0030] As illustrated in FIG. 3, the transformer isolation of the "no T / R switch" design allows for the generation of a transmission waveform by employing high-voltage field-effect transistor (FET) switches (52, 54, 56, 58) instead of expensive high-power linear amplifiers. The switches (52, 54, 56, 58) are arranged in an H-bridge circuit (60), and since each switch (52, 54, 56, 58) is controlled independently, the voltage supplied to the transducer element (16) can be one of three levels: +HV, 0, or -HV. These levels can be derived from a single HV supply of 100 volts or more, which facilitates waveform symmetry between positive and negative outputs, which is an important characteristic required to minimize uniform harmonic distortion. The H-bridge circuit (60) also enables the zero state to be actively driven, which alleviates ringing of the transducer element (16) and improves channel-to-channel uniformity. The actively driven zero state is also important for generating an accurate random acoustic waveform from the three-state transmitter (H-bridge circuit) (60).

[0031] FETs (52, 54, 56, 58) are driven by digital signals (A, B, C, D) (see, for example, FIG. 3 and FIG. 6) that control the output state of the transmitter circuit. The digital control signals can be easily generated by a programmable logic device or by a digital value read from a storage device. Since such devices are readily available commercially, they will not be described or illustrated in detail herein. The digital signals can generally program transitions to new output states that are timed by a high-frequency clock in the range of 250 MHz to 500 MHz.

[0032] For example, when the control signals (B, C) of the FETs (54, 56) are off and the FETs (52, 58) are turned on with the control signals (A, D), a high voltage HV is applied in the positive direction across the primary winding (62) of the transformer (40). When the control signals (A, D) of the FETs (52, 58) are turned off and the control signals (B, C) for the FETs (54, 56) are turned on, a high voltage is applied in the reverse direction across the primary winding (62) of the transformer (40), producing a negative output. Likewise, when the control signals (A, B) are turned off and the control signals (C, D) are turned on, both sides of the primary winding (62) are grounded and a 0-volt state is produced. Since the signals (A, C) and (B, D) are always complementary, they can be generated by a single driving signal as shown in FIG. 6. By using control signals (A, B, C, D) to control the configuration and timing of four H-bridge FETs (52, 54, 56, 58), a three-state output waveform with arbitrarily timed state transitions can be generated, which, when filtered by the impulse response of the transducer element (16), generates an arbitrary acoustic waveform in a medium acoustically coupled to the transducer element (16).

[0033] Therefore, this transceiver circuit (50) can generate an acoustic waveform that matches the acoustic waveform from a high-power linear amplifier. (See reference ["Method and System for Arbitrary Waveform Generation Using a Tri-State Transmit Pulser," by Flynn, J. et. al., PCT / US2014 / 047080]) In addition to low cost, the advantage of the three-state approach is that it can generate an ultra-high-power arbitrary acoustic waveform with low power consumption in the transmitter circuit itself, thereby reducing power and cooling requirements for the transmission circuit.

[0034] Reduction of nonlinear effects and distortion -Many ultrasonic applications require high linearity of the transmitted waveform and low distortion at the receiver to achieve their objectives. For example, there are ultrasonic applications that encode the transmitted waveform to uniquely identify a signal mixed with acoustic signals and noise. In receiving signal processing, it is important to have good linearity in both the transmitted waveform and the signal path leading to the receiver input to decode the transmitter's signal.

[0035] In the previous "no T / R switch" design of the transceiver circuit (42, 50), a back-to-back diode (30) was employed to protect the receiver circuit input (34) from high voltage and to serve as a path to ground the secondary winding (64) of the transmitter transformer (40) when transmission is enabled. While effective in protecting the receiver circuit (20) during transmission, this back-to-back diode (30) introduces some distortion to the transmission waveform and introduces non-linear characteristics to the signal path for receiving the echo signal from the transducer element (16).

[0036] To improve linearity and reduce distortion, the back-to-back diode (30) can be replaced with two parallel-coupled FET switches (72, 74) controlled by control signals (E, F), as shown in the transceiver circuit (70) of FIG. 4, to form an active clamp circuit (76). When the FETs (72, 74) are turned on, they exhibit very low resistance (about 0.1 ohms). This resistance is also relatively constant over the entire output current range of the transmitter circuit (60). This active clamp circuit (76) not only eliminates the cross-distortion of the passive clamp diode (30) but also increases the dynamic range of the receiver circuit (20), as a typical receiver can receive signals greater than + / - 0.8 volts. The attenuated transmission waveform signal at the input (34) of the receiver circuit (20) becomes an accurate representation of the transmission waveform due to the active clamp circuit (76) which provides consistent and linear resistance, and can be used to monitor transmission power and duration, and to detect a faulty transmitter circuit or transducer element. Such monitoring can be achieved by capturing the unsaturated output signal of the receiver during the transmission period without additional circuitry and by estimating the transmission power from the attenuated signal from the receiver. This requires knowledge of the receiver gain (during transmission) and the attenuation factor of the clamp circuit, which can be easily determined by anyone skilled in the art and will not be described in detail herein.

[0037] The operation waveform diagram for the control signal of the clamp circuit of FIG. 4 is shown in FIG. 5 along with the response waveform signal for the component of the ultrasonic transceiver circuit of FIG. 4. The control of the active clamp circuit (30) is indicated by signals (E, F), which are typically switched on together. The clamp control signal is generated to correspond to the transmission duration shown in the second waveform and minimizes the signal level at the variable gain amplifier point of the receiver circuit (20). Although this adds additional complexity to the design, it can be coupled to the transmission waveform duration and can be automatically generated by the digital transmission signal generator circuit (60) for most waveforms. During the reception period, the ability to transmit at any time is maintained. The diagram in FIG. 5 also shows the output of the receiver circuit (20) in the third waveform and the output of the transducer element (16) in the fourth waveform. Note that during transmission, a small signal of the receiver output is present, which can be used to estimate the transmission power. If desired, this can be amplified somewhat.

[0038] If the H-bridge circuit (60) is not properly designed and programmed, it may cause distortion in the transmission waveform. The FETs (52, 54, 56, 58) do not switch immediately, and transient voltage effects that damage the waveform may occur while one FET is turned on and the other is turned off. These effects can be minimized by fine-tuning the transition timing. A small delay can be introduced into the control signal that can be adjusted for propagation delay and component changes. This small delay can be programmed at system startup using a correction procedure that minimizes distortion components.

[0039] For example, FIG. 6 illustrates a representative control circuit (66) for the transmitter circuit (60) illustrated in FIG. 4. The control circuit (66) of this embodiment includes a programmable logic device (68) having outputs (P, N) coupled to a P-gate driver (70) and an N-gate driver (72), respectively. The P-gate driver (70) has an output that forms a C digital signal directly coupled to the gate of the N-FET (56). The output of the P-gate driver (70) is also an A digital signal input to the gate of the P-FET (52) through a capacitor (74). A positive +HV bias rail is connected to the gate of the P-FET (52) through a parallel diode resistor circuit (76). Similarly, the output of the N-gate driver (72) is a D digital signal coupled to the gate of the N-FET (58) and connected to the gate of the P-FET (54) as a B digital signal through the capacitor (78). The +HV bias rail is also coupled to the gate of the P-FET (54) through a parallel diode resistor circuit (80).

[0040] At the bottom of FIG. 6, a time-dependent waveform plot is shown for the P and N outputs, the A, B, C, and D gate signals, and the output signal (Output) received from the transducer element (16).

[0041] In general, the terms used in the following claims should not be interpreted as limiting the claims to the specific embodiments disclosed in the specification and claims, but should be interpreted to include all possible embodiments, together with the full range of equivalents covered by such claims. Accordingly, the claims are not limited by the present disclosure.

[0042] This application claims priority to U.S. Patent Application No. 17 / 747,398 filed May 18, 2022, the entirety of which is incorporated herein by reference.

Claims

Claim 1 As a circuit, a transducer element configured to emit an ultrasonic signal and receive a reflected ultrasonic signal; a transformer circuit coupled to the transducer element, wherein the transformer circuit includes a transformer having a primary winding and a secondary winding, and the secondary winding is coupled to the transducer element; a transmission waveform circuit coupled to the primary winding of the transformer and configured to generate a transmission waveform signal to the transducer element through the transformer circuit; and a receiver circuit having an input coupled to the secondary winding of the transformer and configured to be coupled to the transducer element via the secondary winding. A circuit comprising a clamp circuit coupled between the secondary winding of the transformer and the input of the receiver circuit, wherein the clamp circuit comprises an active clamp circuit having a pair of field-effect transistor (FET) switches coupled in parallel to the input of the receiver circuit, and each FET switch of the pair of FET switches has a control terminal coupled to the transmission waveform circuit to receive an ON signal connected to the duration of the transmission waveform signal. Claim 2 In claim 1, the transmission waveform circuit comprises an H-bridge circuit. Claim 3 In paragraph 2, the H-bridge circuit comprises four FETs that generate a tri-state transmission waveform signal, and each of the four FETs is configured to be independently switched on / off. Claim 4 In paragraph 3, the timing of the on / off switching of each of the FETs of the H-bridge circuit is adjustable with a small delay to minimize distortion in the output of the transmission waveform circuit. Claim 5 As an ultrasonic device, a transducer circuit comprising a transducer configured to transmit an ultrasonic signal, receive a corresponding echo signal, and generate a returning echo signal; a variable gain receiver having an input coupled to the transducer circuit; and a transceiver capable of generating a peak-to-peak waveform of more than 100 volts across the transducer and receiving a peak-to-peak echo signal of less than 1 volt returning from the transducer, wherein the transceiver comprises a transformer having a primary winding and a secondary winding—the secondary winding being coupled to the transducer—; A transmitter circuit coupled to the transducer through the transformer—the transmitter circuit includes a transmission waveform generator configured to generate a transmission waveform, the transmission waveform generator is coupled to the primary winding of the transformer so that the primary winding is driven by the transmission waveform generator, the secondary winding is connected to the transducer circuit on one side and to the input of the variable gain receiver on the other side, and the maximum gain is at least 30 dB—; and a protection circuit coupled between the secondary winding of the transformer and the input of the variable gain receiver—the protection circuit, when activated, has a resistance of 0.1 ohms to 1 ohm from the input of the variable gain receiver to ground during the transmission period of the transmitter / receiver.An ultrasonic device configured to provide an impedance in the 0 ohm range, wherein the input of the variable gain receiver provides effective grounding to the transformer secondary winding during the transmission period; wherein the variable gain receiver can be activated during the transmission period and can amplify a small voltage across the protection circuit to monitor the amplitude and duration of the transmission waveform, and the protection circuit includes an active clamp circuit having a pair of field-effect transistor (FET) switches coupled in parallel to the input of the variable gain receiver, and each FET switch of the pair of FET switches has a control terminal coupled to the transceiver to receive an ON signal connected to the duration of the transmission waveform signal. Claim 6 In paragraph 5, the impedance of the protection circuit is resistive when activated and constant over the current range of the transmitter circuit output, in an ultrasonic device. Claim 7 An ultrasonic device according to claim 5, wherein the activation of the protection circuit for the input of the variable gain receiver is performed automatically based on the duration of the transmission waveform. Claim 8 An ultrasonic device according to claim 5, wherein the transmitter circuit can be activated multiple times during the reception period, and the variable gain receiver is minimally affected during each activation by simultaneous activation control of the protection circuit. Claim 9 An ultrasonic device according to claim 5, wherein the transmission waveform generator is configured to generate a tri-state output waveform composed of states of -HV, 0, and +HV, and HV can be set in a range from less than 2 volts to greater than 100 volts. Claim 10 An ultrasonic device according to claim 9, wherein the three states of the three-state output waveform are generated from four field-effect transistors (FETs) configured in an H-bridge circuit, and each of the four FETs is configured to be independently switched on / off. Claim 11 In claim 10, the duration of each state of the above 3-state output waveform is programmed to generate a 3-stage output waveform that generates any desired acoustic output waveform when filtered by the impulse response of the transducer, an ultrasonic device. Claim 12 An ultrasonic device according to claim 10, wherein the timing of the on / off switching of each of the FETs of the H-bridge circuit is adjustable with a small delay to minimize distortion in the output of the transmission waveform circuit. Claim 13 delete

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