plest solution to eliminating fine tuning of the oscillator is to use a crystal-controlled oscillator. Since the oscillator frequency is always 10.7 MHz above the receiving frequency, 100 crystals covering from 98.8 MHz, to 118.6 MHz in 200-kHz steps would be required for all channels. A 100-position rotary selector switch would serve as a tuning knob and provide a mechanical or electrical readout determined by its shaft position. This system effectively eliminates the need for fine tuning but is far too expensive since the quartz crystals alone amount to about $200 in parts cost. Using frequency mixing techniques, it is possible to bring the number of crystals down to 20 for dual conversion and 15 for triple conversion but the additional switchable filter requirements also make this approach expensive. Furthermore, multi-conversion designs may be troubled by spurious response and poor image rejection performance due to the nonlinearity of the mixer circuits.
The Crystal Oscillator. The oscillator of the Scott 433 digital FM tuner is crystal controlled on every frequency but uses only a single quartz crystal as a reference standard. This is accomplished by making the oscillator part of a digital phase locked loop (PLL) circuit. In order to understand this principle, let us look at the simplest PLL circuit which locks an oscillator to a reference frequency.
When the system shown in Fig. 1 is first turned on, the voltage-controlled oscillator frequency will not be exactly the same as the reference frequency. The output of the frequency-phase comparator is an error voltage which tunes the VCO in a direction to minimize the error until phase-lock condition is established and fosc = fref. Since the control voltage for the VCO is ideally a dc voltage, the low-pass filter is used to remove any high frequency components which might be present at the output of the comparator.
Scott 433 digital FM tuner circuitry is described in text by its designer.
In order to generate a large number of frequencies from a single reference, a programmable divider is inserted into the PLL as shown in Fig. 2. The loop behaves as before except that a submultiple of the VCO is now presented to the comparator and the frequency relationship becomes
fosc / N= fref.
fosc = Nfref.
In North America, stations are assigned to fall on 100 channels from 88.1 to 107.9 inclusively with a spacing of 200 kHz. Since the channel spacing requirement is 200 kHz, the reference frequency of the crystal oscillator must be 200 kHz if its multiples are to fall on each FM channel plus 10.7 MHz. Let us calculate what the divide ratio must be when the tuner is receiving 88.1 MHz or the bottom channel on the band.
The oscillator frequency will be
88.1 + 10.7 = 98.8 MHz
Substituting into the earlier equation to find the divide ratio N:
N = fosc = fref. = 98.8/0.2 = 494
This means that the voltage-controlled oscillator will be at a frequency of 200 kHz multiplied by 494 or 98.8 MHz. The next channel higher at 88.3 MHz will require a divide ratio of 495 and so on until 107.9 MHz, the top of the band, is reached at a divide ratio of 593.
For the tuner to scan the entire FM band, the programmable divider must therefore be able to divide from 495 to 593 inclusively.
Consequently every time a new station is desired, the divide ratio must be altered. Since several IC counters are available with a variable modulo or programmable count sequence, one of these is used here. The divide ratio is altered by inserting a binary code which affects the length of the count sequence and hence the divide ratio. Each channel or frequency requires a unique code which is presented to the divider.
The code itself is derived electronically and can be generated in a sequence which will make the tuner appear to scan across the FM band. It actually "steps" across the band rather than scans continuously since it only pauses (phase locks) on assigned channels. The tuner can also be tuned or programmed by cards which present a binary code to the code generator which in turn decodes it and passes it on to the 