8525B/8528 Technical Service Manual Technical Description 4-9
Publication No: 15-02036 Issue 6
4.4 Local Oscillators (04-02450)
Two synthesized oscillators are used to drive the first and second mixers.
The first oscillator operates between 45.250MHz and 75MHz in 2kHz steps
to convert the 250kHz to 30MHz received signal to 45MHz for the first IF.
The second oscillator operates between 43.352 and 43.350MHz in 10Hz
steps to convert the 45MHz first IF to the 1650kHz second IF.
Each oscillator consists of a voltage controlled oscillator (VCO), a divider, a
phase/frequency detector and loop amplifier/filter together with a reference
crystal oscillator and divider.
The division ratios required are provided in serial data form from the
microprocessor from data stored in the memory. The reference frequency
crystal (6.6MHz) is held at a constant temperature by a power PTC
thermistor and is divided by 4 to provide the 1650kHz drive to the
modulator/demodulator in USB operation. For LSB operation, a separate
1647kHz crystal oscillator is required.
4.4.1 Voltage Controlled Oscillators (VCO)
As VCO1 and VCO2 are virtually identical, only VCO1 will be described in
detail:
• FET V2 operates as a Hartley oscillator tuned by varicaps D17 to D20.
The output is levelled by schottky diode D21 which generates a
negative bias voltage for V2 gate.
• Unity gain feedback amplifier V3/V4 provides a buffer between the
oscillator and the cascade connected amplifier V5/V6. The amplifier
provides the +7dBm drive to the ring diode mixer and also drives the
divide by 64/65 prescaler IC6.
• VCO2 differs from VCO1 only in having a single varicap because of
the reduced frequency range required.
4.4.2 Phase/Frequency Detectors and Dividers
IC5 generates a DC voltage which controls the varicaps in VCO1 in order to
lock the oscillator to its nominated frequency. A 6.6MHz oscillator in IC5
controlled by the crystal Z2, is divided within the IC to give a 2kHz
reference for a phase/frequency detector also in the IC. VCO1 output is
divided in a prescaler, IC6, and applied to further dividers in IC5, allowing
for the VCO frequency to be changed in 2kHz steps. The divided output is
compared with the 2kHz reference frequency.
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