[GreenKeys] HAL ST-8000 (New Toy)

[Jim Haynes]

On Sun, 6 May 2018, W2HX wrote:

> tune up the radio very rapidly. Just tune until the ellipses on the CRT
> matched the lines.
> 
And I will add that practically every amateur T.U. from the early days
was designed wrong, some of mine included.  The design was a limiter-
discriminator, and we always tuned the "filters" of the discriminator
so the peaks were 850 Hz apart (or, later, 170 Hz apart)  for exactly
that reason.  You could tune the transmitter to the peaks indicated on
the X-Y scope and its shift would be correct.

Now the trouble with that, which was originally stated in the Sprague
article in November, 1944 issue of Electronics, is that the discriminator
needs to be linear far beyond the mark and space frequencies, so you don't
want the peaks at those frequencies.  All this was explained by Prof.
Don Wiggins W4EHU around 1960.  The effect of putting a signal plus a
weaker interfering signals through a limiter is a signal where the average
frequency is that of the stronger signal, but the weaker signal causes
spikes in the instantaneous frequency.  If the discriminator is widely
linear the spikes average out to zero when the signal goes through the
post-discriminator low-pass filter.  But if the discriminator is not
linear widely enough those spikes do not average out and the limiter is
less effective in suppressing the weaker interfering signal than it could
be.