Posted by
Geoffrey Mackenzie-Kennedy-2 on
May 28, 2005; 7:19pm
URL: http://elecraft.85.s1.nabble.com/K2-XTALS-tp378841p378844.html
Ron,
Thank you for your comments. Forgive me if I split up my reply.
On May 28, 2005 at 3:26 PM you wrote:
The issues with opposite sideband suppression that Geoff raised do not come
up because there is no need or desire to move the lower edge of the audio
bandpass closer to the carrier frequency for transmit.
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Perhaps. If the only consideration is the transmitted audio
spectrum, yes I would agree. As you correctly point out, passing more of the
low frequencies is usually a waste of power. Of course the frequency
response of the microphone and Tx audio circuitry needs to be included in
this consideration. But the group delay characteristics of the filter
should also be thought about. A typical Cohn ladder filter, for example, has
wicked gyrations at the corner frequencies which requires that the carrier
frequency be moved up the filter's skirt to reduce distortion at those audio
frequencies that contribute to intelligibility. This of course increases the
level of unwanted sideband unless at the same time, the Tx audio rolls off
very sharply at the lower audio frequencies. To avoid this, Gaussian,
Gaussian to 6db or 12db or the equiripple families of ladder filter which
approximate constant delay can be used, and are the most useful from the
distortion viewpoint. Although not relevant to this discussion, a narrowband
Cohn ladder as might be used in a CW receiver will ring.
Without seeing the delay and frequency response plots of the
modified filters, any further comment would be guessing on my part.
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My voice has been, on occasion, described as sounding a bit like a metal
trash can being dragged along a gravel road. That's my natural, sometimes
rough, low frequency drone.
Pass!!
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That's where actual listening tests are important. When listening to our
own voice in headphones, much of that low frequency sound comes right
through our bones. Unless that bone conduction is taken into consideration,
it's easy to arrive at a filter setting that sounds great when listening to
oneself in the headphones but which sounds unnatural and tinny to others on
the air. I suggest recording your signal then playing it back to hear what
others actually hear.
Totally agree.
73,
Geoff.
GM4ESD
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