optical transceiver module
transmit-receive operation of baseband AM or 15 kHz
carrier frequency USB or NBFM, with self-contained
There's nothing very exciting about this unit. It's best
feature is that it was well made mechanically (and uses
plug-in assembles to make access simple). It even has
tie-wrapped cable assemblies... Components that were to
hand dominated the design process, hence the use of rather
large 1.4 MHz crystal filters for audio processing and SSB
Having spent a couple of years on the shelf (the unit, not
me), the inner workings had been totally forgotten, so it
seemed sensible to record the details here as the various
circuits and drawings were painstakingly located again on
one of the old PCs.
All but one part of this wish-list block diagram (and unit
front panel) came to fruition - at the moment, only a
single fixed (15 kHz) carrier frequency is provided.
The original idea was to have a locked VFO that would
increment in 1 kHz steps up to 25 kHz.
In operation, the 'bypass' mode allows the microphone and
loud-speaker amplifier to be connected directly to the
optical head unit, with initially, this being one the
Finningley transceiver units. On these transceivers, the
rx section has two outputs - 'baseband rx af' and
'unprocessed rx AF, the latter (with its flat frequency
response) being required for sub-carrier operation.
On transmit, the Finningley units were designed to be flat
up to 25 kHz anyway.
PCB circuit diagrams
Not shown on the block diagram is a SMPSU that converts
the 12v battery voltage down to 6v, which is used to power
the loud-speaker chip (the very useful, low consumption
Mic switching and agc
Loud speaker amplifier and NBFM detector (with
frequency conversion stage)
generation and clipping
External - on lid
(ie, part of battery pack assembly)
Main unit to