Ramsey Electronics FZ-146 Specifications download pdf (Page 94)

FX 146 • 94

VCO CIRCUIT SUMMARY:

The control voltage for the D3 and D23 varactor diodes is supplied through

R47 and R25 by the output of U5:A in the PLL synthesizer circuit.

We know already that there must be a 21.4 MHz difference between the

receive and transmit frequencies of the VCO. This swing cannot be

accomplished by PLL programming alone. The VCO must be able to stay "in

range" with the synthesizer. D3 and D23 work in series during transmit,

which reduces their total capacitance to one-half (per the standard formula

for capacitors in series). For example, if a given control voltage runs both

diodes at 5 pf, the actual capacitance is 2.5 pf. In receive, the +8R through

D1 causes D3 to be shunted by C39, which causes D23 alone to control the

VCO L-C circuit, introducing twice as much capacitance and thereby

lowering the frequency.

Transistor Q5 is a common base buffer which affords good isolation, low

input impedance and broadband characteristics. The output from Q7 is fed

into U3, 12017, a Ã64/65 prescaler which divides down the VHF signal to a

lower frequency which the PLL chip can process. The output is further

buffered and amplified by Q16, the VCO buffer which feeds U1 through C35

for receive, and Q10 through C56 for transmit.

Another important role of the VCO is that it is the stage that is modulated by

microphone amplifier U4, which is discussed in Stage M. Notice that the

microphone amplifier output is coupled through C62 directly to the voltage

control line for the varactor diodes. Therefore, any voltage variation imposed

on that line will vary the VCO frequency in step with that variation. If the

frequency is varied in step with speech patterns or other audio signals such

as tone shifts, we are generating 'frequency modulation', hence 'FM'.

D5 and R31 perform an interesting function. Remember that the VCO control

voltage has a range of about 1.0 volts DC (low frequency) to 7.0 volts (high

frequency). Therefore, more modulation voltage is needed at higher VCO

frequencies. As the VCO control voltage increases, D5 turns on and places

R31 in parallel with R33, reducing the resistance in the line to half and

thereby increasing available modulation voltage.

Finally, it should make sense that the VCO should have a very pure source

of well-filtered DC, completely free of AC hum, alternator whine or other

disturbance. R19 and 47 uf C40 form a basic low pass filter. Transistor Q4

serves as an electronic capacitance multiplier. The actual effect of the filter

is that the beta of Q4 multiplies the 47 uf to the equivalent of installing a

huge 4000-5000 uf capacitor in the VCO area.

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Ramsey Electronics FZ-146 Specifications Page 94