[R-390] Suggestions for SSB adapter for R-392?

[2002tii]

Walt wrote:

>I have often wondered why somebody hasn't designed a very good solid
>state/transistor SSB/CW board(s) for the 455kc and 500kc ifs.
>Could sell the board with or without components.

Any number of them have been designed.  But built in volume, either 
assembled or as kits or bare boards, is another thing entirely.  That 
lack is most likely explained by the impossibility of making enough 
money on them to be worth the designer's while.  Hams and SWLs are 
notoriously cheap, and expect something that can be delivered to 
their door either assembled or with a full parts kit (including a 
nice case with holes already punched and graphics already applied) 
for $20, plus endless e-mail and/or phone support.

>Maybe a MC1496 or equivalent product detector.

Distinctly mediocre-to-poor performance.  But to do better costs money.

The most vexing design problem to solve is what to do about AGC.  You 
can rely on the AGC in the radio, but (i) if you use a different 
filter bandwidth in the detector box than in the radio, you get 
strange artifacts in the recovered audio; and (ii) many AGC systems 
in the radios folks would want to use with the box suck if there is 
no carrier (and for that matter, even if there is -- the 390A is a 
perfect example).  You can use the radio's AGC plus additional AGC in 
the box, but this also creates unwelcome artifacts.  You can turn the 
radio's AGC off and do it all in the box, but then you need to ride 
the radio's manual gain all the time.  Or, you can provide an AGC 
detector in the box, and feed its output back to the radio.  This is 
the best solution (in fact, the only good solution), but (i) you need 
to hack into most radios to implement it, and (ii) you need different 
detector characteristics, and thus different product versions, for 
each target radio.

Also, once you go to the trouble of designing the basic unit, it 
starts to make sense to add synchronous AM, PBT, several notch 
filters, and other essential features that the target radios lack -- 
thus raising the cost further.  I have several generations of 
prototypes of such a box that I have built over the last 25 years, 
and I am currently experimenting with a continuously-variable IF 
filter design.  But I hack into the radios' AGC circuits, so some 
other solution would need to be adopted to make boxes or kits for 
sale (and, as noted above, all of the other solutions I am aware of 
are too much of a performance compromise to bother with).  Plus, I 
now generally use modern radios that have the features I need built 
in, so there is much less incentive to spend time further developing 
the outboard box design.

>Designer could sell the boards. lots of radios could use them.

But very few owners would pay the price necessary to have one.

Best regards,

Don


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