The Vcos And The Rf Output; Voltage Controlled Oscillators (Sheet 7) - RF Technology Eclipse Series Operation And Maintenance Manual

5 CIRCUIT DESCRIPTION
The user programmable digital output of the DAC (MOD_VCO_EN) is used to enable
the modulation VCO (when High)

5.6.5 The VCOs and the RF Output

These are more closely described in 5.7, but it is worth noting that there are three
primary outputs of the VCOs. There is each VCO output itself, but also the signal
VCO_OUT. This is the difference frequency between them.
Generally the modulation VCO is set to oscillate at 320MHz. To get an output of
40MHz, the Channel VCO is set to 280MHz.
But if you wanted an output frequency of, for example, 40.00125MHz, then the
modulation VCO would change to 319.78125 MHz (i.e. it drops by 218.75kHz), and the
Channel VCO would become 279.78MHz (i.e. dropping by 220kHz).
By such small changes in the modulation VCO (maximum delta is +/- 240kHz), each
multiple of 1250Hz can be accommodated, but without any need to ever change the two
phase detector frequencies.
5.7

Voltage Controlled Oscillators (Sheet 7)

JFETS Q704, and Q705 are the heart of two Colpitts oscillators.
The capacitor feedback divider for Q704 (modulation VCO) is defined by C732 and
C733, and this shapes the negative impedance looking into the drain of Q704.
In the Channel VCO, C740 is effectively in series with Cgs of Q705. These, then define
the negative impedance looking into the drain of Q705.
L716, in parallel with L726 forms the tank coil for the modulation oscillator, and the
resonant capacitance is defined by the series combination of C750 and the capacitance
across D701. L712 has +ve reactance and it acts to reduce the minimum effective
capacitance seen back through C750, thereby increasing the tuning range slightly.
Similarly, L719 is the tank coil for the channel oscillator, and the resonant capacitance
is defined by the series combination of C751 and the capacitance across D704. L717
has +ve reactance and it acts to reduce the minimum effective capacitance seen back
through C751, thereby increasing the tuning range slightly.
The VCO frequencies are controlled by the bias applied to D701 and D704 respectively,
which is set by signals MOD_PLL_IN and CHAN_PLL_IN. These signals are the
phase detector outputs from the Modulation PLL and the Channel VCO respectively
(see Sheet 6).
Diodes D700 and D703 are used to provide some AGC for the JFETs. These Schottky
diodes will increase the –ve bias on the gate of the JFETs (thereby decreasing the drain
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