| Millimeter-wave frequency synthesizer is one of the key components of millimeter-wave radar or communication system. The performance of the system will be greatly enhanced when coherent frequency synthesizer is used. So it is necessary and important for us to study the coherent frequency synthesizer design and its applications. The key techniques and the applications of the coherent frequency synthesizer at W-band are studied in this paper. The main researches are presented as the follows:1. The main coherent frequency synthesis techniques are expounded firstly. The W-band frequency synthesis techniques mainly belong to composite frequency synthesis. According to the millimeter-wave circuit is mainly based on feed-back circuit or not, there are two main millimeter-wave frequency synthesis techniques:millimeter-wave direct coherent frequency synthesis and millimeter-wave indirect coherent frequency synthesis. The performance of the millimeter-wave coherent frequency synthesizer is mainly depended on the characteristic of the RF components or the choice of the synthesizer circuits.2. For millimeter-wave short-range radar used for speed test, the sensitivity of the radar will be deteriorated by the signal (Tx-IF) which firstly leaks from the transmitter signal then mixes to intermediate frequency with local oscillator. The coherent analysis shows that it is an effective way to reduce the phase noise of the Tx-IF by enhancing the correlationship of the phase noise of the transmitter signal and that of the local oscillator. The output signal to noise radio (SNR) will be improved in the same way. The method is successfully applied in the design of the W-band coherent continuous wave frequency synthesizer. The frequency of coherent continuous wave the synthesizer is 95 GHz and the phase noise is-90 dBc/Hz@10kHz. After mixing with the coherent LO, the phase noise of the Tx-IF is-103 dBc/Hz@10kHz. With such W-band coherent frequency synthesizer, the W-band short-range radar can be used to test the speed of the target whose velocity varies from 10 m/s to 2000 m/s. When the cross section of the target is about 0.5 mm2, and the effective range is no larger than 10 m, the output signal to noise radio of the radar system is proved better than 28 dB. 3. For millimeter-wave stepped-frequency radar, many parameters will affect the high range resolution of the radar, including the stepped-frequency synthesizer which provides the signals to the radar. In this paper, the parameters of the stepped-frequency synthesizer which will affect the radar performance are analyzed, including the amplitude fluctuation, frequency error, spurious signal level, phase noise, frequency switching time and the phase difference of the transmit-receive signal. The analysis is advantageous to the demand of the frequency synthesizer and it is helpful to the design of the frequency synthesizer. Then the W-band stepped frequency synthesizer is designed out. In the synthesizer, a method is advanced to speed up the frequency switching time, which is based on the circuit of DDS driving PLL. Without any peripheral circuits, the frequency switching time is about 1μs, and performance of the phase noise or spurious signal suppression are not influenced. The phase noise of the W-band stepped-frequency synthesizer is-90 dBc/Hz@10kHz, the spurious signal suppression is-55 dBc and the frequency switching time of the LO is 1μs. It provides high quality signals to the system. With such frequency synthesizer, the range resolution of the radar system is proved better than 0.6 m, and high range resolution imaging is achieved.4. Besides the output frequency, the output power is also important to the frequency synthesizer, because effect range of the radar system depends on the output power level. In this paper, a power combining network is introduced, the W-band coherent power combiner. With 4 IMPATT diodes whose normal rated power is 75 mW, the W-band coherent power combiner can have an output power of 250 mW. The bandwidth of the combiner is 60 MHz, and the efficiency is about 80%. The research shows that the W-band coherent power combiner can make a breakthrough at the single W-band solid diode power limit, which is valuable in the practical radar systems.
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