[MRCG] Clatternet 12/28/14

[Jim Haynes]

Dovetron was started by the late Hank Scharfe, W6SKC and to my knowledge
their first product was a rather high-end ham demodulator, a gray box
with a CRT tuning indicator.  What most of us have are the black box
later models with tunable mark and space frequencies and some of them
designed for TEMPEST, which mans low RF radiation from the device that
coule be picked up by an enemy.  So, yes, lots of military users, we
suspect, more of the NSA and intercept services rather than tactical
military that needed ruggedized equipment for the field.

There seems to have been a specification, which I've never seen, for
a demodulator of that general type.  HAL with the ST-8000, Dovetron with
the MPC-1000, Electrocom with the 400 series, and Frederick Electronics
with their 1280 all have some common properties.  Mainly mark and space
frequencies tunable, and hence arbitrary shift.  Someone suggested an
application was to monitor multichannel frequency-division carrier 
systems.  This seems reasonable considering that some of them are pretty
hard to tune to an ordinary signal but easy to set to specific mark and
space frequencies.  The Dovetron is perhaps the easiest of the bunch
to tune by hand.  Another claimed feature, most prominent in the Dovetron
and the HAL ST-8000 is the ability to cope with signals distorted by
multipath propagation.

There is quite a zoo of Dovetron models, or I will say that I've never
seen two of them exactly alike.  The original design was patented by
Scharfe (pat. 4,013,965, 1977) This seems to have been built on the
main circuit board.  Features were identical mark and space active 
filters, with the incoming tones heterodyned into them, and then various
ways of combining the detector outputs.  Apparently the original version
in the patent didn't achieve what was wanted in multipath correction,
because there is another model with all the detector combining circuits
stripped off the main board and a daughter board containing a "binary
bit processor" that plugs into the main board in place of some of the ICs.
Perhaps this was also disappointing, as there is a Mark II binary bit
processor that seems to have nothing special for multipath correction
but combines the mark and space detector outputs and then uses a threshold
corrector to deal with selective fading.  This is the model I have; and
I found a manufacturing defect in the board that made the threshold
corrector inoperative.  Fixing that improved performance somewhat.
There are at least two versions of the Mark II BBP board, differing
in a few resistor values.

Then mine also has a regenerator board using a UART to receive the
signal and put it back out, at a different speed if desired.  The 
regenerator is an important feature, because the main cause of errors
in simple RTTY is that a mechanical printer selector cannot cope with
errors in the START and STOP bits and is apt to produce several errored
characters when that happens.  Presence of the regenerator makes the
unit sensitive to the number of bits in the code being received, which
is set by internal straps.

Mine has a 3-position toggle switch marked OFF-SDS-CLEAR and I have
not learned what this does (but I haven't tried very hard).

Some of them seem to have a code converter in place of the simple
regenerator, allowing Baudot to be translated to ASCII.

The TEMPEST units have BNC connectors on all the signals in and out, and
no interal provision for a high voltage 60ma loop.  Someone said that it
is possible to add parts to the main board for a 60ma loop, as the 
original design provided for one.

Scharfe also patented the crossed-LED-bars tuning indicator, pat. 
4,229,698 in 1980.

And that pretty well exhausts my knowledge of Dovetron

Jim W6JVE

jhhaynes at earthlink dot net