15
VCO divider ratio
As mentioned above, the VCO frequency divider U42 normally divides by 10, but sometimes another ratio
may be more desirable. This is because the DDFS by its very nature produces spurs at some frequencies.
Since the DDFS can produce millions of frequencies, predicting ones which will produce these spurs is not
possible. However, if the VCO divider ratio were changed, the DDFS frequency would need changing to
keep the transceiver on the same frequency. Thus in the event that a channel frequency is chosen which
produces spurs on the DDFS output, changing the DDFS frequency will solve this problem.
Selcall (if enabled)
The selective system utilises Frequency Shift Keying (FSK) as the modulation technique. The tones are
generated and decoded by the DSP. The microprocessor however processes the data for encoding and
the decoded data. The design of the encoder/decoder allows individual channels to be set for operation on
either the International and OEM Selcall format (Codan compatible, based on CCIR 493-4) or the true
CCIR493-4 format and its associated tones. Tones used are as follows:-
International and OEM Selcall format
:-
High tone:- 1870 Hz
Low tone:- 1700 Hz
Centre tone:- 1785 Hz
CCIR 493-4 International Standard format:-
High tone:- 1785 Hz
Low tone:- 1615 Hz
Centre tone:- 1700 Hz
Audio mute
The DSP is used to provide three types of receiver muting. These are:-
SELCALL mute - mutes the receiver until it receives a SELCALL addressed to it.
SSL mute - mutes the receiver when signals are weaker than a set signal strength.
Syllabic mute - mutes the receiver unless speech is received.
Audio Filtering
The DSP provides additional audio filtering (beyond that given by the IF filter) as required. This extra
filtering is accessed from the front panel menu. The microprocessor sets the default amount of audio
filtering depending on the mode of operation.
Noise reduction
The DSP also implements a noise reduction mode (three selections) which is selected from the front panel.
Microprocessor / RF PCB – RF section
The RF section provides the complete exciter and receiver for the transceiver. It includes:-
First
mixer
Roofing filter at 45 MHz
45 MHz IF amplifier
45 MHz Noise blanker receiver
Second
mixer
Two noise blanker gates
SSB filter and optional other types
Envelope detector for AM and AGC
Product detector for SSB,FSK and CW signals
T-R
switching