[Premium-Rx] WJ FT-203 IF to Tape Converter

[David I. Emery]

On Sun, Apr 23, 2017 at 05:00:52PM -0400, Peter Gottlieb wrote:
> My understanding is they were used with large, wideband FM tape recorders 
> to accurately record a transmission for later decoding.

	A lot of SIGINT in those days (and still) is figuring out what
format the information in various signals is and how to recover it (and
generally understand the modulation and coding and multiplexing and
other signal features).

	Analyzing signals in the analog era often involved recording 
them in some format that would allow playback over and over as various
measurements  and demodulators were tried.   And before sampling and
digitizing and digital capture and storage of signals became  practical
(and economically feasible at finite costs)  the standard technology was
recording and replay from fast mag tape.   

	And more often than not the signals were captured in places and
from platforms that didn't support real time analysis systems and
personnel so dragging back samples to the lab was routine.

	And often at the time of capture not much was known about the
signal format or where the useful information was in the signal, so
capturing it whole was a good idea as some critical signal component
might not be captured otherwise.

	This WJ box provided a means for capturing the IF of an
intercept or telemetry receiver for recording.   This involved down
conversion to a frequency range suitable for the tape, and in some cases
modulation of the signal onto an FM (or FM/VSB) carrier suitable for
recording on the tape (and demodulation and upconversion of that on
playback).   The WJ box probably takes a 21.4 MHz input and/or a 160 MHz
input (and maybe some common others as well like 30 or 70 or 140 MHz or
260 MHz).

	In particular, in telemetry and some other captures it was
common to record the analog IF and then later play it back through the
telemetry/signal processing system as an IF input to the receivers to
reproduce all the information again, perhaps with different settings
that might improve reception of some critical information during a fade
or interference...

	Today this stuff can be trivially done with SDRs and SSD arrays
with up to several tens of MHz of bandwidth captured in I and Q format 
(or more bw for more money).   There are even nice little boxes that will
record and playback signals this way available as (expensive) test
equipment.   And this is not just useful for spook work, it is also
often useful for analysis of subtle and transient system misbehavior due
to all kinds of obscure propagation, chip, software and firmware bugs,
interference etc... and also provides a means for capturing data to
verify channel and propagation models.

> I am not sure what you could do with them except maybe feed a SDR?

	Indeed it might provide a means to down convert receiver IF to
something that some HF oriented SDR might like... 

-- 
  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."