[GreenKeys] Fredericks LEM-1

[David I. Emery]

On Mon, Dec 02, 2013 at 12:41:30PM -0500, Duncan Brown wrote:
> And now back to the LEM-1
> 
> Thanks, Jack - you seem to be the only one who has seen a LEM-1. 
> Probably the Fredericks 1280 M&S outputs that would drive the external 
> scope drove the LEM-1?  Any idea what the "DIV" button in the front 
> panel is for?  My first thought was "diversity" but that wouldn't make 
> much sense in a BER tester unless it could show you the improvement to 
> the BER in diversity mode.  But don't see any diversity output.

	I have never seen a LEM-1 except in the recent photos so my
input is strictly worth what was paid for it...

	Clearly the IF linear outputs of two 1280s could be combined in
some kind of diversity combiner... so that does indeed make sense.

	And clearly the 1280 has the ability to recover timing and
output cleaned up bits in various modes with correct timing which seems
to be the primary mode in which it was used.  So  I'd presume there
might be some kind of digital (232 or 188) output which would be the
recovered bits.   Apparently there IS a "DATA" output which I'd guess
could be recovered RS-232/Mil-188 level retimed bits.

	But thinking a bit further - maybe this box just OUTPUTs
RS-232/188 LSFR pseudo-random bits out the "DATA" lines to the FSK
modulator card in a 1280 and accepts the receive tones from the 1280,
perhaps from a far end loopback on a circuit and counts errors between
what it output and what it gets back.

	That would explain the error counter and the BER indication.
And suggests the unit is a "Link Error Monitor".

	Diversity would in that case be an optional mode where it
used the combination of two receive channels instead of displaying the
performance of each one separately.

	As I am sure some readers know, the 1280 has a FSK modulator
card so it can both send and receive tones...

	Maybe MBC is "Master Bit Clock" and "Q" is quality - which I
guess could be an analog voltage related to SNR and/or BER.

	Seems to me I remember an era many many years ago when certain
State Department related US government HF circuits had 600 baud FSK
signals on them  on a regular basis.   This would seem possibly related.

-- 
  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."