The invention provides a digital PLL (Phase-Locked Loop) based 
phase noise measuring device. The digital PLL based 
phase noise measuring device comprises a frequency power measuring unit, a 
phase discrimination unit, a filtering unit, a DC (
Direct Current) bias compensation unit, a low-
noise amplification unit, an acquisition, 
frequency discrimination and 
phase discrimination unit, a digital loop filtering unit, a 
phase discrimination constant detection unit, a self-adaptive digital 
gain control unit, a reference source unit, and a 
signal processing and displaying unit. According to the digital PLL based 
phase noise measuring device, a double-
balanced mixer is adopted as a phase 
discriminator, the phase 
noise of a measured source is extracted through a digital PLL, the specific 
noise extraction mode is divided into 
frequency mixer phase discrimination and 
digital delay line frequency discrimination, and DC caused by a loop circuit is counteracted through a 
DC bias compensation circuit, so that too big DC after amplification, induced ADC saturation and induced loop lock-losing are prevented. The implementation scheme of the digital PLL based phase noise measuring device is simple, totally excellent phase noise measuring sensitivity can be realized, the realized analysis 
frequency spectrum range is also very wide, and phase noise testing demands of most 
signal sources can be satisfied.