Professor Philip Davies

Emeritus Professor


Microwave and millimetre wave optical electronics.

Research interests

Microwave and millimetre wave optical electronics.


  • Chartered Engineer and Fellow of the Institution of Electrical Engineers 
  • Member of the Institute of Electrical and Electronics Engineers


  • Member of IEE Scholarship Committee 
  • Member of the IEEE Communications Chapter UK Committee
  • Member of Technical Committee: European Conference on Networks and Optical Communications
  • External examiner for the BSc by Coursework at the University of Surrey and the University of Northumbria and the MSc by Coursework at Brunel University and Manchester Metropolitan University


  • Member of IEE Accreditation Committee
  • Member of EPSRC Information Technology - Communications College


Showing 50 of 62 total publications in the Kent Academic Repository. View all publications.


  • Shen, P. et al. (2007). Analysis and demonstration of a fast tunable fiber-ring-based optical frequency comb generator. Journal of Lightwave Technology [Online] 25:3257-3264. Available at:
    Fiber-ring-based optical frequency comb generators are analyzed to understand their behavior and limitations. A numerical frequency-domain model is described for studying dispersion and other phase mismatch causing effects in the fiber ring cavity, as well as for predicting the spectral and temporal evolutions of the comb in time. The results from this analysis are verified with experimental measurements. A flat optical comb, with a terahertz span within a 6-dB power envelope and containing 100 comb lines, with a suppressed central comb line, is demonstrated. The comb shows an excellent coherence dependent on the phase noise from the radio frequency synthesizer that drives the comb generator. Improvement in the error correction loop also enables the comb spacing to be set at precise 12.5-MHz intervals without having to adjust the system. Fast frequency switching of the comb line spacing is demonstrated for the first time. The comb line spacing,can be switched to any operation frequency with a resolution of 12.5 MHz between 6 and 12.5 GHz, as limited only by the microwave circuit used. The switching time is less than 1 s, and the spectral profile of the comb is maintained.
  • Oven, R., Yin, M. and Davies, P. (2004). Characterization of planar optical waveguides formed by copper-sodium, electric field assisted, ion exchange in glass. Journal of Physics D: Applied Physics [Online] 37:2207-2215. Available at:
    Planar waveguides have been formed by the electric field assisted diffusion of copper ions from a metallic copper film into a borosilicate glass (Corning 7740). The waveguides have been analysed using conventional prism coupling techniques and have been compared with the electric field assisted diffusion of silver into the same glass type. The measured refractive index profiles for copper Guides are shown to have nearly Fermi function shapes, which are consistent with the theory of field assisted ion diffusion in glass. It is shown, however, that the guide depth formed by copper or silver ions is laraer than that predicted by calculations based on the nominal C composition, possibly indicating that only a fraction of the sodium ions within the glass are mobile. Annealing experiments have been performed on the guides in order to determine the copper self-diffusion coefficient in the Glass. The coefficients are extracted from the data by fitting a non-linear diffusion equation model of ion motion in glass to the experimental profiles.
  • Yin, M., Oven, R. and Davies, P. (2004). Low-loss Cu+-Na+ ion exchanged optical channel waveguides in glass. Electronics Letters 40:1265-1266.
    Electric field assisted Cu+-Na+ ion exchanged optical channel waveguides have been produced in Coming 7740 borosilicate glass using a Cu film source. Average losses of 1.0, 0.45 and 0.3 dB/cm, at wavelengths of 633, 850 and 1300 nm, respectively, have been measured.
  • Huggard, P. et al. (2002). Generation of Millimetre and Sub-Millimetre Waves by Photomixing in a 1.55 Micrometre Wavelength Photodiode. IEE Electronics Letters 38:327-328.
  • Huggard, P. et al. (2002). Efficient Generation of Guided Millimeter-Wave Power by Photomixing. IEEE Photonics Technology Letters 14:197-199.
    A 70-GHz bandwidth commercial photodiode has been coupled to W-band waveguide and used as a photomixing source from 75 to 170 GHz. Maximum power conversion efficiency of 1.8% was obtained at 75 GHz, where an optical input of +10 dBm yielded a nonsaturated millimeter-wave (mm-wave) power of -7.5 dBm. Optimizing the photomixer backshort turning at individual frequencies showed that the mm-wave power decreased with frequency to a level of -30 dBm at 170 GHz. Fixed tuning allowed the generation of power across the full waveguide band from 75 to 110 GHz, with a variation within 5 dB across the majority of the band.
  • Wang, X. et al. (2000). Indirect optically injection-locked oscillator for millimeter-wave communication system. IEEE Transactions on Microwave Theory and Techniques [Online] 48:2596-2603. Available at:
    A millimeter-wave optically injection-locked oscillator has been designed and fabricated. It represents the first millimeter-wave hybrid transistor oscillator locked to a long-wavelength optical signal. The oscillator could be used as a photoreceiver in remote base stations in future picocellular communication systems employing fiber-optic backbones; its high "gain" (output relative to input locking signal) and relative flatness in output power may make it attractive compared to simple millimeter-wave amplifier configurations. Measurement results show the output signal exhibits little variation (less than 0.7 dB) with input optical power changes of 6 dB. A locking bandwidth of 2.6 MHz has also been achieved.
  • Shaar, A., Woodcock, C. and Davies, P. (1999). Prime sequences for asynchronous pulse repetition interval agile radar. IEE Transactions on Aerospace and Electronic Systems [Online] 35:543-548. Available at:
    We propose the use of prime sequences in pulse repetition agile radar to increase the immunity of the radar to reconnaissance and deceptive jamming. The prime sequences are mapped into state agile mapped (SAM) sequences and are used to determine the pulse repetition interval (PRI) of the radar. We show that this produces a system with low mutual interference between radars and with a low probability of ambiguous range measurement resulting in good resistance to active deceptive jamming.
  • Razavi, K. and Davies, P. (1998). Semiconductor laser sources for the generation of millimetre-wave signals. IEE Proceedings: Optoelectronics [Online] 145:159-163. Available at:
    Various millimetre-wave generation techniques utilising semiconductor laser sources are considered. A comparative study highlights issues such as cost and experimental complexity, millimetre-wave signal quality (phase noise and stability), millimetre-wave signal power and signal tunability. Optical millimetre-wave generation techniques are categorised into systems using dual-semiconductor-laser sources, single-semiconductor-laser sources and dual-mode semiconductor laser sources. Within the dual-mode laser category a novel device structure for generating millimetre-wave signals in the 60 GHz region is proposed and simulation results are presented.
  • Foord, A., Davies, P. and Greenhalgh, P. (1996). Synthesis of microwave and millimetre-wave filters using optical spectrum-slicing. Electronics Letters [Online] 32:390-391. Available at:
    The experimental demonstration of a new type of microwave filter is reported. The filter uses samples of an optical spectrum, which are passed through a dispersive element to implement the delays of a transversal filter structure. This method allows for rapid tuning and for the synthesis of a wide range of filter characteristics.
  • Wake, D., Lima, C. and Davies, P. (1996). Transmission of 60-GHz signals over 100 km of optical fiber using a dual-mode semiconductor laser source. IEEE Photonics Technology Letters [Online] 8:578-580. Available at:
    High-purity 60-GHz signals, generated using a dual-mode DFB semiconductor laser, have been transmitted over 100 km of standard optical fiber with no observable degradation in purity. This device is shown to be a simple and compact optical source of high-purity 60-GHz signals for long-reach fiber-fed millimeter-wave radio systems.
  • Foord, A., Davies, P. and Greenhalgh, P. (1995). Optical Demultiplexing for Subcarrier Multiplexed Systems. IEEE Transactions on Microwave Theory and Techniques [Online] 43:2324-2329. Available at:
    Subcarrier multiplexing (SCM) is an attractively simple technique for establishing multiple independent channels over a single fiber. At the receiver, electrical mixing converts a selected channel to baseband. The photodiode, oscillator, mixer, and preamplifier must, however, operate up to the highest subcarrier frequency. The use of an optical filter for channel selection allows any subcarrier to be accessed with only baseband electronics. To obtain the best performance with this approach, sub-subcarrier modulation is needed. In this paper we propose a method suitable for externally modulated CATV systems and show that the sensitivity is comparable to the best conventional receivers.
  • Davies, P., Foord, A. and Razavi, K. (1995). Millimeter-Wave Signal Generation by Optical Filtering of Frequency-Modulated Laser Spectra. Electronics Letters [Online] 31:1754-1756. Available at:
    Optical mixing on fast photodiodes is a potentially convenient method for generating millimetre-wave signals in fibre radio applications. The authors describe multiplication of the modulation frequency of a DFB laser using a periodic optical filter to enable mixing between FM sidebands, and report experimental generation of a 19.2GHz signal.
  • Wake, D., Lima, C. and Davies, P. (1995). OPTICAL-GENERATION OF MILLIMETER-WAVE SIGNALS FOR FIBER-RADIO SYSTEMS USING A DUAL-MODE DFB SEMICONDUCTOR-LASER. IEEE Transactions on Microwave Theory and Techniques [Online] 43:2270-2276. Available at:
    This paper presents a new approach to the optical generation of millimeter-wave signals using a dual-mode multisection distributed feedback semiconductor laser. This simple device is capable of generating high power signals between 40 and 60 GHz with extremely high spectral purity and stability. The two optical modes produced by this laser are heterodyned on an ultrafast photodiode to give a beat signal at the mode difference frequency. The phase noise of the beat signal is greatly reduced by phase-locking the modes using an electrical drive signal applied to the laser at a subharmonic of the beat frequency. Millimeter-wave signals are obtained with a linewidth of less than 10 Hz, a phase noise of less than -85 dBc/Hz at 100 kHz offset, and a locking range of about 500 MHz. Millimeter-wave fiber-radio systems are seen as a major application area for these new compact optical sources.
  • Davies, P. (1995). The First Canterbury Workshop on Microwave and Millimetre-wave Optoelectronics. International Journal of Optoelectronics 31:1754 -1756.
  • Lima, C. and Davies, P. (1995). Degradation of microwave signals generated by a modelocked semiconductor laser due to up-conversion of low-frequency noise. Electronics Letters [Online] 31:641-642. Available at:
    The authors show that the frequency up-conversion of low-frequency noise present in drive oscillators can degrade the performance of microwave signals produced by a modelocked semiconductor laser. Experiments with low-frequency noise injection are used to demonstrate this effect. The results show also that the filtering effect of the external cavity resonances serves to reduce the sidebands of the up-converted low-frequency noise.
  • Lima, C. and Davies, P. (1995). Noise in microwave signals generated by pulsed semiconductor lasers. International Journal of Optoelectronics 10:443-446.
    Gain-switched and mode-locked semiconductor lasers have been considered as potential optical soul ces for the generation of microwave and millimetre-wave signals. High frequency harmonics can be obtained from these pulsed semiconductor lasers to transport information, overcoming the bandwidth limitation of direct and external modulation schemes. An understanding of the causes that degrade the noise performance of microwave harmonic signals generated by these sources is presented.
  • Lima, C., Wake, D. and Davies, P. (1995). Compact optical millimetre-wave source using a dual-mode semiconductor laser. Electronics Letters [Online] 31:364-366. Available at:
    The authors demonstrate the optical generation of extremely narrow linewidth millimetre-wave signals between 40 and 60GHz using a single-chip semiconductor laser. A dual-mode long-cavity multisection DFB semiconductor laser is driven at a subharmonic of the free-running-mode brat signal frequency to produce phase-locked millimetre waves with a 3dB linewidth of less than 10Hz and a 3dB locking range of similar to 500MHz.
  • Lima, C. and Davies, P. (1994). Effects of extra low-frequency noise injection on microwave signals generated by a gain-switched semiconductor laser. Applied Physics Letters [Online] 65:950-952. Available at:
    Low-frequency upconverted noise (1/f laser intensity noise and the low-frequency noise of the driver source) has been identified as the dominant noise generation mechanism in microwave signals generated by a Fabry-Perot gain-switched semiconductor laser. An experimental investigation is carried out using extra low-frequency noise injection added to the drive signal. Results show the dependence of the broadband intensity noise level, formed by the overlapping of the upconverted noise sidebands present at each harmonic, on gain-switching input parameters.
  • Urey, Z. et al. (1993). Noise performance of a GaAs MESFET as an optical detector and as an optoelectronic mixer in analogue optical links. Electronics Letters [Online] 29:147-149. Available at:
    Noise measurements with a GaAs MESFET employed as a photodetector or as an optoelectronic mixer are reported. Despite its higher noise it is shown that with proper biasing the GaAs MESFET provided a higher carrier-to-noise ratio than a pin photodiode, and as an optoelectronic mixer provided a comparable signal-to-noise ratio to that of a conventional pin and microwave mixer combination. The implications of using GaAs MESFETs in these configurations are discussed.


  • Razavi, K. and Davies, P. (1997). Millimetre wave generation by filtering the FM-IM spectra of a directly modulated DFB laser. [Online]. Koepf, G. A. et al. eds. Piscataway NJ: I E E E. Available at:
    The non-linear characteristics of semiconductor lasers have been exploited in order to generate low phase noise microwave and millimetre-wave frequencies for radio-over fibre applications. This is achieved using the modulation characteristics of a MQW-DFB semiconductor laser. A high FM index MQW-DFB laser is directly modulated at the resonant frequency of its FM characteristic response in order generate high order harmonics. By optimising the rf modulation frequency drive power, high order sideband components can be generated, selected and mixed on a photodetector to generate microwave and millimetre-wave frequencies. This paper presents results utilising the direct modulation technique, which demonstrate the generation of beat frequencies up to 25GHz. Optimization of the optical filtering process and of the phase-noise dependence of the generated beat frequency on the modulation drive source are also discussed.

Book section

  • Lima, C., Davies, P. and Wake, D. (1997). A new optical source for generation of 40-60 GHz signals using a dual mode multisection DFB semiconductor laser. in: Optoeletr, I. M. T. and Tech, S. eds. 1995 SBMO/IEEE MTT-S International Microwave and Optoelectronics Conference. Piscataway NJ: IEEE Press, pp. 647-652.
  • Lima, C., Razavi, K. and Davies, P. (1995). Generation of microwaves by optical filtering. in: European optical communications & networks : EFOC & N. Information Gatekeeper, pp. 125-127.
  • Davies, P. and Gomes, N. (1995). Subcarrier multiplexing in optical communication network. in: Wilson, B., Ghassemlooy, Z. and Darwazeh, I. eds. Analogue Optical Fibre Communications. IET, pp. 1-32. Available at:
    In this chapter, the principles behind subcarrier multiplexing (SCM), the factors affecting component and system performance and its application potential are discussed. SCM makes efficient use of available bandwidth and, even when techniques to reduce bandwidth in purely digital systems are considered, SCM remains less complex and expensive. The RF/microwave components required are often readily available from other systems (e.g. radio, radar, satellite) again reducing costs. The performance requirements for SCM systems are different to those for typical digital optical communications systems. Generally, these are more stringent regarding noise and nonlinearities, especially for multichannel applications. However, the correct choice of components and system design, perhaps with the use of compensation techniques, does allow these requirements to be met. The compatibility of SCM with many other types of RF/microwave systems and its flexibility makes it useful in a variety of applications, from antenna remoting and CATV to local-area networks. Probably the most significant feature of this flexibility, for the near-term application of SCM techniques, is its ability to allow an evolutionary development of existing networks into broadband-ISDN with low initial installed costs.
  • Lima, C., Davies, P. and Bird, C. (1994). Electrical locking bandwidth of a diffraction grating external cavity mode-locked semiconductor laser. in: Leos '94 - Conference Proceedings. Institute of Electrical and Electronics Engineers, pp. 169-170.

Conference or workshop item

  • Shen, P., Gomes, N. and Davies, P. (2009). RF Distortion Effects on a Fiber Ring Based Optical Frequency Comb Generator. in: 2009 Asia-Pacific Microwave Photonics Conference, APMP 2009.
  • Shen, P. et al. (2008). Multi-Wavelength Clock Signal Generation Implementated by an Optical Frequency Comb Generator. in: Optical Fiber Communications Conference (OFC 2008). Optical Society of America, pp. 406-408.
    Multi-wavelength clock signal generation by filtering an optical comb by a DWDM filter is demonstrated. 12 channel clock signals are generated simultaneously with jitter better than the ITU-T recommendations.
  • Shen, P. et al. (2007). Generation of 2 THz Span Optical Comb in a Tunable Fiber Ring Based Optical Frequency Comb Generator. in: IEEE International Topical MEeting on Microwave Photonics, MWP.
  • Shen, P., Gomes, N. and Davies, P. (2006). Polarisation Mode Noise in Remote Transfer of Ultra-Low Jitter Timing Signal. in: National URSI Symposium (abstract only), Book of Abstracts: Paper 31.
  • Shen, P. et al. (2005). Fiber Ring Based Optical Frequency Comb Generator with Comb Line Spacing Tunability. in: IEEE LEOS Summer Topical Meeting on Optical Frequency Measurement and Generation. pp. 69-70. Available at:
    A fiber ring based Optical Frequency Comb Generator with tunability in comb line spacing, fast tuning of 10 MHz per step with speed much better than 0.1 s, and a high coherence is demonstrated.
  • Shen, P. et al. (2005). Polarization Mode Noise in Ultra-Low Drift Phase Reference Distribution System over Fiber Network. in: IEEE Intl. Topical Meeting on Microwave Photonics, MWP2005. pp. 297-300.
    The impact of polarization mode noise on the temporal drift of a delivered analogue phase reference signal through an optical fiber network is studied. The relationship between the dispersion of the state of polarization, change of the state of polarization and the maximum temporal drift of the LO signal is established. The resulting temporal drift can be larger than the PMD value of the receiver under certain conditions. It is a significant contributor to the temporal drift in the ultra-low drift phase reference distribution system, especially when motion of the fiber has to be present in the system.
  • Shen, P. et al. (2005). Fiber Ring Based Optical Frequency Comb Generator and its Application. in: 9TH and 10th European NEFERTITI Workshops on Millimeter Wave Photonic Devices and Technologies for Wireless Communications and Imaging Applications (MPWI).
  • Shen, P. et al. (2004). Photonic Generation and Delivery of Millimetre-Wave Reference Signals. in: Microwave and Terahertz Photonics, Photonics Europe. p. .
  • Huggard, P. et al. (2004). Integrated 1.5mm Photomixer Local Oscillators for Heterodyne Receivers at Millimetre-Wavelengths. in: Proceedings SPIE Vol. 5466, presented at Microwave and Terahertz Photonics, Photonics Europe, Strasbourg.
  • Huggard, P. et al. (2004). Application of 1.55m Photomixers as Local Oscillators and Noise Sources at Millimetre Wavelengths. in: Joint 29th International Conference on Infrared and Millimeter Waves and 12th International Conference on Terahertz Electronics.
  • Shen, P. et al. (2003). High-purity millimetre-wave photonic local oscillator generation and delivery. in: Microwave Photonics, 2003. MWP 2003. International Topical Meeting. pp. 189-192. Available at:
    Tunable millimetre wave generation by spectrally slicing a phase modulated lightwave is demonstrated. The low phase noise signal (-95 dBc/Hz at 100 kHz offset) can be delivered through a 9 km fibre link, with no noticeable power penalty.
  • Huggard, P. et al. (2003). Generation of narrow and broad band millimetre and sub millimetre waves by photomixing in 1.55?m-wavelength photodiodes. in: 11th IEEE International Conference on Terahertz Electronics.
  • Shen, P., Gomes, N. and Davies, P. (2002). High Frequency Photomixer and Comb Generator for the ALMA Project. in: Semiconductor and Integrated Optoelectronics (SIOE '02). pp. 25-27.
  • Huggard, P. et al. (2002). Generation of Millimetre and Sub-Millimetre Waves by Photomixing in a 1.55 Micrometre Wavelength Photodiode. in: 13th Intnl. Space THz Symposium. pp. 221-228.
  • Gomez-Rojas, L. et al. (2000). High Performance Optical Receiver Using a PIN Photodiode and Amplifier for Operation in the Millimeter-wave Region. in: 30th European Microwave Conference, 2000. pp. 1-3. Available at:
    A highly efficient photodiode plus amplifier hybrid module in the mm-wave region is demonstrated. The design uses a broadband edge-coupled PIN photodiode and a MMIC amplifier interconnected through a microstrip matching circuit. An improvement of 9 dB in output power is demonstrated at 27 GHz compared to the same PIN diode connected to a 5O¿ amplifier without matching.
  • Gomez-Rojas, L., Gomes, N. and Davies, P. (2000). Simple technique for fine tuning of impedance matching circuits for the mm-wave region. in: IEE Seminar on Packaging and Interconnects at Microwave and mm-Wave Frequencies (Ref. No. 2000/083). pp. 711-712. Available at:
    This paper describes a simple technique to achieve fine tuning in bandpass impedance matching circuits. The technique takes advantage of the parasitic inductance of the required bondwire interconnection between the active device and the microstrip circuit using the shape of the bondwire as the means of tuning. An experimental tuning range of 160 MHz at 20 GHz was obtained for a single bondwire of 0.5 mm in length. The technique can easily be extended to other applications, which require bondwire interconnects.
  • Gomez-Rojas, L. et al. (2000). Efficient photoreceivers for millimetre-wave cellular system base stations. in: International Topical Meeting on Microwave Photonics, 2000. MWP 2000. pp. 194-197. Available at:
    Hybrid integrated photoreceiver modules exhibiting high performance at millimetre-wave frequencies are demonstrated. Novel PIN photodiode plus amplifier resonant modules and injection-locked oscillators are presented and their potential performance in cellular system base stations discussed
  • Wang, X. et al. (2000). 30 GHz microstrip HEMT oscillator using indirect optical injection locking. in: Perkins, T. ed. IEEE MTT-S International Microwave Symposium (IMS2000). Ieee, pp. 1753-1756.
    A 30 GHz oscillator using indirect optical injection locking and operating with long-wavelength (1550 nm) optical signals has been designed and fabricated for the first time. A maximum millimeter-wave power of 5.3 dBm is reported. Initial results show a 650kHz locking range.
  • George, D. et al. (1999). Effects of direct current modulation on a 60 GHz mm-wave carrier generated by master/slave optical sideband injection locking. in: International Topical Meeting on Microwave Photonics, 1999. MWP '99. pp. 65-68. Available at:
    Direct modulation of one of the lasers in a master/slave optical sideband injection locking mm-wave generation scheme is investigated experimentally and theoretically. It is shown that this modulation produces a predominantly phase modulated mm-wave signal.
  • George, D. et al. (1997). Further Observations On The Optical Generation Of Millimetre-wave Signals By Master/slave Laser Sideband Injection Locking. in: International Topical Meeting on Microwave Photonics, 1997. MWP '97. pp. 5-7. Available at:
  • Davies, P. et al. (1996). Technology requirements for fibre radio systems. in: International Conference on Telecommunications, ICT’96.
  • Gomez-Rojas, L., Greenhalgh, P. and Davies, P. (1995). Multi-cavity optical filters using in-fibre Bragg gratings. in: 1995 Annual/8th Annual Meeting of the IEEE Lasers-and-Electro-Optics-Society (LEOS 95). IEEE, pp. B378-B379. Available at:
  • Razzooqi, K., Gomes, N. and Davies, P. (1993). Physically based circuit model of GaAs MESFET as an optical port for microwave systems. in: IEEE MTT-S International Microwave Symposium Digest, 1993. pp. 1553-1556. Available at:
    A GaAs MESFET large-signal equivalent circuit model with optical illumination effects has been developed. The model includes accurate representation of the drain current dependence on the operating voltages under different operational conditions. The model for the GaAs MESFET was implemented in PSPICE. Simulated results obtained are shown to compare well with the measured results, with a good fit to measured GaAs MESFET I-V characteristics over a wide bias voltage range and under both dark and illumination conditions. The RF response of the MESFET is also modeled and simulated. Such a circuit model is important in the design of optically controlled microwave circuits involving a GaAs MESFET as an optical interface.


  • Shen, P., Gomes, N. and Davies, P. (2006). Optical Delay Device.
  • Shen, P., Gomes, N. and Davies, P. (2006). Optical Frequency Comb Generator.
  • Davies, P., Gomes, N. and Sheng, P. (2005). Electromagnetic transmission/reception system. [Online]. Available at:
    An electromagnetic transmission and reception system comprises a transmitter section and a receiver section. The transmitter section has a first signal source, a second signal source at a lower frequency than the first signal source, and means for generating from the first and second signal sources a plurality of signals with fixed frequency spacing derived from the second signal source frequency. One or more pairs of the plurality of signals are selected, and for the or each pair, the signals of the pair are combined to derive an output signal having a frequency derived from the difference between the frequencies of the signals of the pair. The receiver section combines a received signal, which comprises a received version of the output signal, with a local oscillator signal for frequency down-conversion of the received signal. This local oscillator signal is generated by the transmitter section. This provides a system in which the generation of signals of the desired frequency is achieved by mixing signals of specific frequencies. The reception uses frequency down-conversion using the same signal sources as used for the transmission. This enables a low noise system to be implemented.
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