Tait T830 Series, Руководство по обслуживанию

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31/01/96 Copyright TEL
2.3 VCO
The VCO transistor (Q1) operates in a common source configuration, with an LC tank
circuit coupled between its gate and drain to provide the feedback necessary for oscilla-
tion. The VCO control voltage from the loop filter (IC6) is applied to the varicaps (D1-
D4) to facilitate tuning within an 8MHz band of frequencies. A trimcap (VC1) is used
for coarse tuning of the VCO. The output from the oscillator circuit drives a cascode
amplifier stage (Q2, Q3) which su10dBm (typically) to a further stage of amplifi-
cation, Q5. This is the final amplifier on the VCO PCB, and de22dBm (typically)
to the exciter drive amplifier.
A low level "sniff" is taken from the input to Q5 and used to drive the divider buffer for
the VHF prescaler (IC3). The prescaler divide ratio is 40/41.
The VCO operates at the actual output frequency of the exciter, i.e. there are no multi-
plier stages. It is modulated by superimposing the audio signal onto the control voltage
and by phase modulating the reference signal.
2.3.1 Two-Point Modulation
Both the VCO and reference oscillator are modulated so that the phase detectors of IC5
see no frequency error under modulation. Thus, the synthesiser loop will not attempt to
correct for modulation and the response of the transmitter remains unaffected.
2.3.2 VCO Supply
The VCO is supplied with two switched supplies, Tx reg. and +9V rail under the control
of the Tx reg. supply.
The VCO and buffer amplifier are supplied from the +9V switched supply by Q6 via the
C multiplier (Q5, C34).
The output amplifier is supplied from Tx reg.