Operating instructions

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RF Technology R800 Page 10
5 CIRUCIT DESCRIPTION 5.3.0 VCO Section
The limiter/discriminator I.C. U3 further amplifies the signal and passes it through CF2. CF2
does not contribute to the adjacent channel rejection but is used to reduce the wide band noise
input to the limiter section U3.
The limiter section of U3 drives the quadrature detector discriminator. C31 and I.F. tuned
circuit L10 comprise the discriminator phase shift network.
U3 also has a received signal strength indicator output (RSSI). The RSSI voltage connects to
the test socket for alignment use. The RSSI voltage is also used by the microprocessor for
the adaptive noise squelch, carrier squelch and low signal alarm functions.
Dual op-amp U2 is used to amplify and buffer the discriminator audio and RSSI outputs.
5.3 V.C.O Section
The Voltage controlled Oscillator uses a bipolar junction transistor Q6 which oscillates at the
required mixer injection frequency. A fixed tuned ceramic coaxial resonator CR1 is used to
set the tuning range. Varactor diode D18 is used by the P.L.L. circuit to keep the oscillator
locked on the desired frequency. Transistor Q7 is used as a filter to reduce the noise on the
oscillator supply voltage.
5.4 P.L.L. Section
The synthesizer frequency reference is supplied by a temperature compensated crystal
oscillator (XO1). the frequency stability of the TCXO is better than 1ppm over the operating
temperature range.
The 12.8MHz output of XO1 is amplified by Q8 to drive the reference input of the P.L.L.
synthesizer I.C. U4. This I.C. is a single chip synthesizer which includes a
1.1GHz pre-scaler, programmable divider, reference divider and phase/frequency
detector. The frequency data is entered by a serial data link from the microprocessor.
The phase detector output signals from U4 are used to control two switched current sources.
The output of the positive and negative sources Q10 and Q15, produce the tuning voltage
which is smoothed by the loop filter components to bias the V.C.O. varactor diode D18.
5.5 Audio Signal Processing
A 4KHz low pass filter (U27b) is used to remove high frequency noise from the signal. A
300Hz high pass filter (Y26a,b) then removes the sub-audible tones. A 240Hz notch filter
(U26c,d) is used to improve the rejection of tones above 200Hz. The high pass and notch
filters can be bypassed by internal jumpers JP1 and JP3.
The audio frequency response can be set for either a 750uS de-emphasis or flat characteristic
by JP2. JP2 switches the feedback networks of amplifier U27c to achieve the desired
response.
After de-emphasis and filtering, the audio signal is applied to the inputs of two analog
switches (U17a,b). These switches are controlled by the microcontroller and squelch or mute