e, 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 programmable divider. The card system overrides the other forms of sequential tuning and allows instantaneous pre-selection of stations by using the. desired card.
Digital Frequency Readout. The tuner uses the familiar cold cathode neon indicator tubes which were chosen for reliability, long life and reasonable cost. The display is actuated by the same binary code which the code generator supplies to the programmable divider to set its divide ratio. Aside from displaying the frequency in MHz which the tuner is receiving at any given moment, the display serves as a self-checking feature for the code generator as well as the card reader. The binary code from the code generator is decoded into decimal form and used to drive the display; if an incorrect or non-allowable code is presented to the divider such as one caused by a damaged card, the readout will immediately show the error.
Fig. 2 - The programmable divider allows tuner to synthesize multiple frequencies.
The digital readout in the tuner is not simply hooked up to. a frequency counter which counts the oscillator frequency minus 10.7 MHz. The frequency counter is a "passive" addition which will work with any existing tuner while the PLL design is an "active" system which requires an electronically tuned r-f section. In the passive system, manual fine tuning is still required. With a PLL system the oscillator is forced to lock at each channel center, which always falls exactly on the assigned frequencies of the broadcast stations.
The digital PLL system provides an r-f oscillator of crystal stability on all frequencies plus an absolutely accurate digital display of the frequency being received. The binary code generator setting the divide ratio allows the operator to scan the FM band manually at a preselected speed or to let the circuit search for a station or stereo station automatically. If one station of known frequency is desired, a pre-punched card can be used to tune to it immediately. This feature will appeal to the discriminating music listener who ha