[Elecraft] Re: Speech Processors

Thu Jan 30 01:21:00 2003

Interesting indeed, Jerry - thanks for the details.

Let me just note that the tests that I usually make with SSB
operation, processed or not, usually involves exciting the transmitter
with a suitable two-tone signal. With that as an input, I have a fixed
ratio of peak to average power. If the waveform is not distorted, a
first-order confidence check is to compare average and peak power and
verify the 50% ratio. I also observe the waveform visually on a scope
and over the years have developed a fair eye for spot incipient
distortion. I monitor all outgoing r-f from the transmitter on a scope
displaying the r-f output envelope, thus I can monitor SSB speech for
any peak flattening or actual clipping.

The bottom line is that I have found that I can maintain a clean SSB
output signal with relatively basic equipment and careful attention to
control settings.  I agree that a storage scope would be far superior
for estimating average power relative to peak power, which is simply
the keydown power output of the transmitter. But, with practice, I
have found that closely observing the envelope waveform  gives a great
deal of information - at least enough to avoid airing an undesirable
signal.  <:}

Thanks again for the suggestions about the VCA chip . . .

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 Hancock" <[email protected]>
To: <[email protected]>; "VR2BrettGraham" <[email protected]>
Sent: Wednesday, January 29, 2003 11:45 PM
Subject: RE: [Elecraft] Re: Speech Processors

> If you are creative, you can recalculate the compression
characteristics by
> downloading the datasheet for the chip and changing the resistors.
You could
> even change the resistors to variable.  I think the chip (as
designed and
> implemented In the k2) is set at the lower end of the compression
curve.
> You could also acquire one of the chips and mount it in a mic with
variable
> gain and compression.  You would then bypass the chip in the k2.
>
> If you take a look at the chip datasheet I think you will find that
it is
> quite capable of compressing (and distorting) audio with resulting
increase
> in average power with the best of them.  I like the chip and the
> implementation in the k2.
>
> Unless you are using a storage scope or measuring the heat from a
dummy load
> it is very difficult to ascertain the "average" PEP of an SSB
signal.  Logic
> follows that it the input signal is being compressed then the output
> peak/average ratio is changing.
>
> Audio only - obviously.
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