[GreenKeys] Fredericks LEM-1

[David I. Emery]

On Sun, Dec 01, 2013 at 09:21:17PM -0600, Jim Haynes wrote:

> There seems to have been a common specification behind a whole bunch
> of demodulators: ST-8000, Frederick 1280, Dovetron, Electrocom 440,
> maybe others.  Something calling for the mark and space frequencies to
> be independently tunable.

	My educated guess would be SIGINT kinds of apps involving  VFT
multiplex complexes with tones for particular information streams at
various frequencies inside the multiplex.   And in the era these things
were built, a lot were used for SIGINT.

	For the most part it wasn't very possible to tune the "receiver"
for the right mark or space tone - depending on application these might
be looking at a fixed voice grade channel or tape recorded audio from a
remote receiver somewhere... so being able to select both the mark and
space tone was necessary since this could not be done by tuning the main
receiver dial.   Certainly and obviously necessary if the target signals
were part of a "tone pack" multiplex in which other streams  were also
being demodulated from the same intercept receiver audio at the same
time.

	The 1280A (for which I have or used to have a manual somewhere)
is a two channel superhetrodyne - upconverting the 0-6 KHz or so audio
to a IF around 15 KHz (approximately) with two separate PLL stabilized
LOs independently tunable for the mark and space channels.   Channel
selectivity is by matching switched capacitor filters at the IF...

	This design allows any shift or tones anywhere, compared to
others for similar purposes which upconvert both mark and space to to a
common set of bandpass filters and discriminators that set the shift.

	One note for this antique technology - the 1280s have IF outputs
for mark and space which can be used with an XY display for the old
fashioned green scope tube cross type tuning indicator. The fixed higher
frequency IF makes this work fairly well..


-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."