[Elecraft] Speech Processors

Sun Jan 26 16:03:00 2003

The usual warning and precaution somebody makes when RF processors are
suggested is to make sure that the radio can handle the increased duty
radio.

I have used RF processing since designing and building my own into a
Heathkit SB-401 in 1971. My favorite is the Comdel which I still have
but haven't used in quite some time - also have a like new spare for
it - and the VOMAX split-band audio processor.

By way of explanation, there are two styles of "r-f processors." The
most obvious operates in the IF chain of the transmitter where the
signal is clipped and then filtered to remove harmonic distortion
products. That is the approach I used with the SB-401. The other
approach is to take the mic signal and use it to modulate a miniature
SSB transmitter operating say at 500 KHz like the Comdel does. An SSB
signal is generated at 500 KHz suppressed carrier frequency and then
is clipped. The clipped signal is then filtered and downconverted in a
product detector back to baseband audio which is then applied to the
transmitter mic input. The same LO is used for the up- and
down-conversion so that no frequency shift occurs.

Ten Tec made a unit like that as did Drake as Jerry describes.

The split-band Vomax design is implemented in DSP in the Icom PRO
series and is very effective. The analog approach requires great care
in component selection, etc. to maintain phase continuity and avoid
distortion.

In my experience with both units, the Comdel approach is capable of
more effective processing (more average power increase and enhanced
intelligibility in noise), but the Vomax sounds a little smoother to
some. So far, I have tried neither with the K2 since its audio
compression seems to do a good job of maintaining a fairly constant
modulation level.

I found that setting the rf clipping level for a 6 dB increase in
average power output on a two-tone test signal was a good compromise
among effectiveness, a clean signal and minimal overheating. With the
Comdel, for example, a two-tone input signal clipped for 6 dB average
power increase would produce an output signal that could not be
distinguished from the input - no visible distortion. Very few
stations would even know that I was using rf processing.

So, just a little history and the reminder that the final amp stage of
the K2 will be exercised a little more with pre-processed speech
coming in the front door. I have no idea what duty cycle the K2 finals
can handle continuously - probably missed reading it in the specs.

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489  IC-765 #2349 IC-756 PRO  #2121 IC-756 PRO2 #3235

----- Original Message -----
From: "Jerry T Dowell" <[email protected]>
To: <[email protected]>
Sent: Sunday, January 26, 2003 1:53 PM
Subject: [Elecraft] Speech Processors

> For those who desire a good speech processor for the K2, the Drake
SP75
> works just fine! It is a good RF processor and has continuously
variable
> amount of clipping. It is much more effective than the simple
compressor in
> the K2.
>
> Were it not for the fact that I spend 99% of my time on CW, I would
be
> tempted to build an RF processor into an EC1 cabinet, based on the
Drake
> design. A nice project for those long winter evenings.
>
> 73,
> Jerry  AI6L