[Amps] OT: Anyone know of any RF Speech Processor Kits?

[wc6w at juno.com]

>-- Ken G3WCS <lists at kentek.co.uk> wrote:
>Sorry to be off topic but it does deal with getting a few extra db :-)
>
>Does anyone make kits for RF Speech Processors these days? I have an >old Datong unit but would like another one for another location.
>
>I'm a keen constructor so would enjoy building one up if kits are 
>available these days.
>
>Anyone know?
>
>Thanks and 73,
>
>Ken.

Hi Ken,
  I don't know of any current kits but, I've done quite a bit of work with RF clipper circuitry over the years and offer the following observations. 

  A good filter method design requires excellent filtering both preceding and following the clipping stage in order to avoid re-mixing out of band artifacts into the output.  In an analog design this translates to a pair of mechanical filters which are costly.  

  A phasing method design also requires an excellent post-clipping filter.

  The commercially produced outboard solutions, using either of the above schemes, that I've seen, fall short in this aspect. 
  
  Collins designed a baseband Hilbert Transform Clipper in the early '80's which they claim, in patent 4410764, to be essentially equivalent to RF Clipping.  Close but, there are some differences.

  I've never seen an example of this realized but, a Weaver method SSB generator/detector,  with a sine wave carrier & analog multipliers for mixers, should allow an implementation without requiring passband filtering at the "RF" frequency.  Though, this scheme would require 4 very good low-pass filters at baseband -- 4 pairs of LTC1062's come to mind.

  In this millennium, a DSP based implementation of the above would likely be preferred as the parts are becoming ever more cost effective.

  All of the above methods should incorporate a 5dB/Octave audio pre-emphasis circuit to maintain tonal balance with speech signals.

  Since up to 20dB of RF clipping may be usefully employed, the audio circuitry before the clipper must support this extra headroom and have a somewhat better than typical communication grade noise floor.

  Finally, I've found that a properly set-up RF clipper is the best choice for limiting in a transmitter.  If the ALC never blinks, you've got it right!

73 & Good morning,
  Marv WC6W





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