8525B/8528 Technical Service Manual Technical Description 4-11
Publication No: 15-02036 Issue 6
4.4.5 1650/1647kHz Local Oscillators
When receiving (or transmitting) a USB signal, a 1650kHz local oscillator is
required. This is obtained by dividing the buffered oscillator output of IC7
(6.6MHz) by 4 in IC9.
When receiving (or transmitting) an LSB signal, a 1647kHz local oscillator
is required. This is obtained from IC10c and crystal Z3. A high on J3 pin 11
enables IC9 and disables IC10c giving 1650kHz (USB). A low on J3 pin 11
gives 1647kHz.
A tune output is provided from IC10a to the gate of source follower V16 and
V17 when the tune line J3 pin 12 is low. The DC component of the tune
output holds V26 conducting which switches V23 to open circuit,
disconnecting the crystal filter from V16 and V17.
In the 8528S with compatible AM fitted, resistor R76 is removed,
preventing V23 from switching off. This is necessary to permit the carrier to
be re-inserted for AM transmission. Because of excessive loading of SSB
filter Z1 at 1647kHz, the 1650kHz LO is automatically selected in the tune
mode when LSB is selected.
4.4.6 Clarifier
Operation of the clarifier control varies the frequency of VCO2 in 10Hz
steps. This is achieved by varying both the VCO and reference frequency
dividers according to a ‘look-up’ table in the transceiver’s operating system.
Thus the phase comparison frequency is not constant but varies from 1.1 to
2.0kHz.
The range of the clarifier differs between land and marine transceivers:
• Land—The limits of the clarifier are 0.001% (±10ppm) of carrier
frequency. Consequently the number of steps are also a function of the
carrier frequency. At low frequencies dummy steps are inserted to
increase the time the clarifier takes to go from end to end of its range.
• Marine—The limits of the clarifier are ±180Hz at all frequencies, i.e.
±18 steps.
An audible indication is given when the clarifier reaches its upper or lower
limit. The clarifier automatically resets to mid frequency when the channel
is changed.
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