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 has a small number of favorite stations and selects definite programs. The scan feature on the other hand might appeal to the less critical listener who usually scans the band until he hears something he likes.
Other Features. One of the extremely useful by-products of this system is automatic interstation muting. The tuner just won't tune between channels. Noise muting is also provided however for silencing empty channels; this type of muting is defeated by a front panel switch. During the automatic "Stereo Station" scan mode, all mono stations are muted. When tuning from one station to another regardless of which tuning mode is used, the sound disappears without the usual transient swish or thump and reappears out of complete silence again with the absence of annoying noise bursts and distortion.
All muting is done after the multiplex decoder by two FET series-gate switches which reduce the signal by at least 60 dB in the muted condition without introducing a dc transient.
The r-f section in the tuner employs selected high-gain, low-noise FET's for both r-f gain and mixer functions. A FET is also used for impedance matching and low noise in the first stage of the i-f amplifier. Two 6-pole elliptical filters shape the passband of the i-f and achieve a selectivity in excess of 70 dB which allows this tuner to select any one station from a crowded area on the band.
The "Station" light on the front panel indicates the presence of a carrier and is actuated by a zero-crossing detector coupled to the output of the ratio detector; the station light is also a double check on the PLL and reference standard because it is actuated only if the station is tuned to exact center. On noise which is present on empty channels, the station light is extinguished automatically. The "Stereo" indicator will light up in the presence of a 19-kHz subcarrier when the signal level is sufficient to give an acceptable signal-to-noise ratio. A "Card Program" indicator shows at a glance whether or not card tuning is being used. Aside from provid