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5
CIRCUIT DESCRIPTION
The VCO output is amplified and buffered by monolithic amplifiers MA2 and MA3
before being fed to the PLL IC U6.
Amplifiers MA1, MA4 and MA5 increase the VCO output to approximately 4 mW
to drive the power amplifier. MA1 is not switched on until the PLL has locked and
had time to settle. This prevents any momentary off channel transmission when the
transmitter is keyed.
5.2 PLL Section
Temperature compensated crystal oscillator XO1 is the frequency reference source
for the PLL Synthesizer. The frequency stability of XO1 is better than 1 ppm and it
can be synchronized to an external reference for improved stability.
reference option board 11/9119 is required when using an external reference.
XO1 is frequency modulated by the processed transmit audio signal from U7b. This
extends the modulation capability down to a few Hz for sub-audible tones and digital
squelch codes. A two point modulation scheme is used with the audio also being fed
to the VCO to modulate the higher audio frequencies.
The 12.8~MHz output of XO1 is amplified by Q22 to drive the reference input of the
PLL synthesizer IC U6. This IC is a single chip synthesizer which includes a 1.1
GHz pre-scaler, programmable divider, reference divider and phase/frequency
detector. The frequency data for U6 is supplied through serial data link by the
microprocessor.
The phase detector output signals of U6 are used to control two switched current
sources. The output of the positive and negative sources (Q3 and Q6) produces the
tuning voltage which is smoothed by the loop filter components to bias the VCO
varactor diode D3.
5.3
Power Amplifier
The 4 mW output from the main board connects to the power amplifier board
through a short miniature 50 Ω coaxial cable.
Q2 on the power amplifier board increases the signal to approximately 200 mW. The
bias current of Q2 is controlled by Q1 and the power levelling circuitry to adjust the
drive to the output module U2.
U2 increases the power to 10--30 watts (depending upon options) before it is fed to
the directional coupler, low pass filter and output connector. The directional coupler
detects the forward and reverse power components and provides proportional dc
voltages which are amplified by U1a and U1b. The forward and reverse voltages
from U1a and U1b are compared to the DC reference voltage from RV1. The
difference is amplified by U1c, Q3 and Q4.
Page 10
External
RF Technology T220
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