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Controlled by SB select lines from connector 18, U15y and U15z connect either one or
both audio channels to the monaural audio driver U18a and to audio connectors 7, 10 and
34, rear panel J8, CPU/ DSP board and front panel mono level control and front-panel
phones jack J6.
3-7 FIRST LO SYNTHESIZER (81772): Refer to Figures (10-3) and (10-29). The
1st-LO synthesizer has three-loop architecture. For the fine loop (PLL3), U12 and charge
pump Q43-46, Q52 steer VCO Q47-D26-D27 over a range of 80 to 99.6 MHz in 400-
kHz steps. This VCO output is buffered by common-base amplifiers Q48 and Q49, then
divided by 400 in the N-divider of Mixing Loop U11. This configuration yields a fine-
tuning loop output of 200 to 249 kHz in 1-kHz steps.
For the coarse loop (PLL1), U10 and charge pump Q28, Q10-Q13 steer VCO Q14-D22-
D23 over a range of 45.7 to 75.7 MHz in 50-kHz steps. This VCO output is buffered by
common-base amplifiers Q15-Q16 and routed to phase-shift networks L13-L14-C58 and
L17-C69-C70 to form quadrature inputs for loop mixers U6 and U7 respectively.
For the mixing loop (PLL2), U11 and charge pump Q42, Q31-Q33 steer VCO Q35-D24-
D25 over the 1st-LO frequency range of 45.455 to 75.455 MHz in 1-kHz steps. This
loop is programmed with a fixed N-divider of 400 and a fixed R-divider of 1 so its VCO
runs at the VCO frequency of the coarse loop (PLL1), offset by the divided-down VCO
frequency of the fine loop (PLL3). VCO output is split and buffered by common base
amplifiers Q36-Q37, amplified by Q38, Q39, bandpass filtered at L26-L28, and routed to
connector 61. The output of Q36 is used to drive the signal-input port of mixers U6-U7
for translation to quadrature intermediate frequencies of 200 to 249 kHz.
Mixing-loop acquisition is aided by phase detector U8, comparator U9b, and charge-sink
circuit Q26-Q27. During certain transient conditions—such as power-up or for large
negative frequency steps when the mixing-loop VCO frequency is temporarily above the
coarse-loop VCO frequency—the output of U8 drops below the comparator threshold and
the charge-sink ramps the mixing-loop control voltage lower to steer the mixing-loop
VCO toward the correct offset.
A pre-steer circuit (U9a, Q18-Q19) forces the mixing loop control voltage to within ± 1V
of the coarse-loop control voltage. This ensures the maximum mixing-loop offset is
always less than 5 MHz, and the U6-U7 mixer outputs remain below the cutoff frequency
of low pass filters L15-16 and at L18-19.
3-8 SECOND LOCAL OSCILLATOR (81772): The 45-MHz injection frequency
required by the 2nd Mixer is developed by first dividing the 10-MHz reference by 2 in
U4 to produce a 5-MHz square wave, then selecting the 9th harmonic with 45-MHz
monolithic filter FL1. The resulting 45 MHz sine wave is amplified by Q8-Q9, bandpass
filtered at L2-L4, and applied to connector 55.
3-8
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