[GreenKeys] Collins 709D-1 FSK keyer & 706A-1 TU

[Jim Haynes]

On Thu, 4 Dec 2014, Duncan Brown wrote:
> On 02-Dec-14 04:34, Geo. Hutchison wrote:
>
>>  The 706A-1 used  an input  bandpass filter centered on 2550 Cycles, two
>>  6AK5's as limiters, a discriminator circuit featuring two matched audio
>>  transformers with  tuned filters (2125 and 2975 cycles) each to one of a
>>  pair of of 6AL5's as detectors, and after a bit of DC amplification drove
>>  a pair of 5686's as keyer tubes.

This sounds a lot like a typical amateur design of the late 1950s.  Tuning
the filters to 2125 and 2975 is convenient, especially for use with a 
tuning indicator scope, but turns out to be a bad idea.  This was shown
by the late Don Wiggins W4EHU in an article in RTTY for November 1960.

When the signal goes through a limiter, a weaker interfering signal causes
the frequency coming out of the limiter to have some spiky variations.
If the discriminator is linear well beyond the bandwidth of the signal
these variations will average out to zero, so putting the discriminator
output through a low-pass filter will eliminate them.  But when the
discriminator peaks are at the mark and space frequencies the interference
spikes do not average out to zero and thus distort the signal.  Wiggins
advocated putting the narrow bandwidth ahead of the limiter, in the form
of a two-peaked filter.  But hardly any amateur designs followed this
principle.  Possibly the Alltronics-Howard Model L terminal unit did -
I believe Wiggins was involved in designing that one.