The design requirements were:-
(a) frequency coverage 1.8 - 2.0 Mhz
(b) voltage controlled by a frequency synthesiser with an output level sufficient to drive the input of a Phase Locked Loop (PLL)
(c) a further buffered output for a digital frequency readout.
Synthesizers used in commercial radio receivers are largelybased on or PLLs. Manytypes of frequency synthesiser are available as , reducing cost and size. High end receivers andelectronic test equipment use more sophisticated techniques, oftenin combination.
The key to the ability of a frequency synthesizer to generatemultiple frequencies is the divider placed between the output andthe feedback input. This is usually in the form of a , with the output signalacting as a . The counter is preset to some initial count value, andcounts down at each cycle of the clock signal. When it reacheszero, the counter output changes state and the count value isreloaded. This circuit is straightforward to implement using , and because it is in nature, is veryeasy to interface to other digital components or a .This allows the frequency output by the synthesizer to be easilycontrolled by a digital system.
These small device is equipped with a FM tuner, digital PLL synthesis and the ability to save up to 50 radio stations, that option allows you to switch in between them quickly.
Here, we will take a brief look at solid-state RF signalgenerators. Some designs are based on a tuneable oscillator; more elaboratemodels feature a PLL which locks the oscillator to an internal or externalfrequency reference. More recent instruments, based on a PLL or DDSfrequency synthesiser, will be considered in Part 3.
Further practical aspects concern the amount of time the systemcan switch from channel to channel, time to lock when firstswitched on, and how much there is in the output. All of these are a function of the loopfilter of the system, which is a low-pass filter placedbetween the output of the frequency comparator and the input of theVCO. Usually the output of a frequency comparator is in the form ofshort error pulses, but the input of the VCO must be a smoothnoise-free DC voltage. (Any noise on this signal naturally causes of the VCO.).Heavy filtering will make the VCO slow to respond to changes,causing drift and slow response time, but light filtering willproduce noise and other problems with . Thus the design of the filter iscritical to the performance of the system and in fact the main areathat a designer will concentrate on when building a synthesisersystem.