Topband: Receivers

[Tom Rauch]

> > The GUF1 won't help the R4C a bit for CW. It is too
wide. It
> > barely makes a difference on SSB. IMO the GUF1 is not
worth
> > bothering with.
>
> The GUF-1 was an 8 Pole - 6 KHz filter and provided a
useful
> improvement on SSB without resorting to separate USB/LSB
filters
> in the R4C first IF.


"Useful improvement" is very subjective.

The problem is the second mixer has about the poorest
dynamic range of any mixer in history. It has this problem
because Drake injected a 50 kHz LO into a mixer operating on
~5.6 MHz. It's a terrible practice to inject a LO signal
below the operating frequency into a mixer, because good
mixer performance requires the mixer be driven hard into and
out of cutoff. Proper mixer operation would result in
harmonics of the LO being generated in the mixer tube. If LO
injection level is set high enough (NORMAL or proper
injection) to dominate the transfer characteristics of the
mixer, the receiver fills up with birdies and carriers as
you adjust the passband tuning.


In order to "hide" the very poor 2nd mixer, the roofing
filter must suppress anything outside of the desired
passband by about the difference between the 2nd mixer
dynamic range and the front end dynamic range...plus a tiny
bit of head room. It turns out to be about 80 - 55 = 25dB.
We need about 25 dB of suppression outside the signal BW,
then the front end sets the dynamic range. The stock front
end in the Drake is OK, on par with the better modern
receivers. You can look at the list and see which ones, but
remember...it is ONLY good at the BW of the roofing filter!!

With the stock 8 kHz filter, moderately strong stations
having emissions within  -4 to -5 and  +4 to + 5 kHz of your
dial setting are problems on all modes, even CW. Effectively
the "good performance" BW of the R4C is 8-10kHz, with the
dial frequency in the middle of that window. If you are on
3800 LSB and a moderately strong signal is on 3806 LSB with
his actual emissions extending down to 3803, you have the
performance of a very poor receiver getting rid of that guy.
You might as well be using a bottom line receiver like the
FT 101 instead of the R4C.

With the GUF filter, "good performance" BW of the R4C is 6-8
kHz.

This means you have narrowed your window by 2-3 kHz at best,
which is a 1 - 1.5 kHz narrowing up and down.

Everyone can view this differently, but in my opinion a
change like that is not worth very much. Can you imagine
operating where there are no strong emissions at all up 3-4
kHz when on LSB or down 3-4 kHz on USB? This also applies to
CW.

73 Tom