Digital Electronics: Principles, Devices and Applications

684 Digital ElectronicsI/PSamplerLPFIFAmplifierReciprocalCounterStepRecoveryDiodeLocalOscillatorMicroprocessorControlDisplayFigure 16.24Microwave counter architecture.shows the architecture of a microwave counter. Pulse microwave counters use similar architecture withadditional gating circuitry to position the gate comfortably within the pulse.16.15 Frequency Synthesizers and Synthesized Function/SignalGeneratorsFrequency synthesizers generate sinusoidal signals of extremely high frequency stability andexceptional output level accuracy. Frequency synthesizers and similar instruments such as synthesizedfunction/signal generators are used to provide test signals for characterization of devices, subsystemsand systems. Synthesized function generators, in addition to providing spectrally pure and accurate CWsinusoidal signals, also provide other waveforms such as ramp, triangle, square and pulse. Synthesizedsignal generators, in addition to providing spectrally pure and accurate CW signals, also havemodulation capability and can be used to generate AM, FM, PM and pulse-modulated signals. There isanother class of synthesized function generators called synthesized arbitrary waveform generators. Themajority of synthesized function generators have a limited arbitrary waveform generation capabilitybuilt into them. However, these are available as individual instruments also. All the above-mentionedinstruments have one thing in common, that is, the synthesis of a signal that lends ultrahigh frequencystability and amplitude accuracy to the generated waveform. They therefore have more or less similararchitecture for a given technique used for frequency synthesis.16.15.1 Direct Frequency SynthesisThe frequency synthesizer in its basic form uses a reference oscillator, which is an ultrastable crystaloscillator, and other signal-processing circuits to multiply the oscillator frequency by a fraction M/N(where M and N are integers) in order to generate the desired output frequency. One such arrangementis shown in Fig. 16.25. It comprises an assortment of frequency multipliers and dividers, mixersand band-pass filters (BPFs). The diagram shows the use of this architecture to generate 17 MHz.In this arrangement, if the BPF has a pass band centred around 3 MHz, the output will be 3 MHzas the mixer produces both sum and difference components. This method of frequency synthesis hasseveral disadvantages, not least that the technique is highly hardware intensive and therefore expensive.Another disadvantage is loss of phase continuity while switching frequencies, with the result that thistechnique has not found favour with designers.