The correct way to adjust the drive to an amplifier is to use a two-tone
test. Two audio tones are fed to the microphone input and the envelope
of the RF output of the radio (or radio/amplifier combination) is
observed on an oscilloscope. With two tones going into the radio there
should be only two tones at the output, but any nonlinearity in the
transmitter or amplifier (including especially overdriving) results in
other distortion products showing up in the transmitter's output. As
long as these unwanted tones are kept to a low level, their effects will
not be heard nor seen on the screen of the scope.
It's when the transmitter/amplifier is overdriven that the distortion
products rapidly rise to objectionable levels, widening the signal and
interfering with the frequencies above and below the QSO.
The ARRL Handbook contains an excellent chapter showing the correct
waveforms on an oscilloscope; in other words, they show what you should
be shooting for if you were adjusting your transmitter and amplifier
with a two-tone test.
If more hams learned how to use this technique, think how much cleaner
our bands would be, and how much better everybody would sound. No more
endless, "Hey, Vern, how do I sound now?" as the operator tweaks his mic
gain and drive.
So, why not have a button that injects two tones into the audio chain to
permit adjustment of the transmitter? Using DSP, two almost perfectly
clean tones (which is a requirement) could be generated and injected
easily, and it wouldn't cost that much to implement.
"But," you ask, "where do I get the oscilloscope to look at the output
signal?" It's true that most hams don't have access to this piece of
test equipment. However, a number of hams have something very close in
the form of a sweeping spectrum display on their radios. The Icom 756
series, the Orion and others have a "band scope" on their LCD displays
which are nothing more than spectrum analyzers; a visual representation
of the frequency you are tuned to. This spectrum display can be turned
into an oscilloscope with the addition of one component.
The spectrum display on your radio is a "frequency domain"
representation, because frequency is on the horizontal axis. An
oscilloscope, however, is a "time domain" instrument where time is on
the horizontal axis. The Inverse Fourier Transform is a mathematical
function that is able to turn a frequency domain graph into a time
domain graph which would serve us quite nicely to give an oscilloscope
display suitable for the two-tone test adjustment. With so much DSP
power available in our modern rigs, it might not be that hard to turn
that cool frequency scan display into a scope that could be used for any
number of purposes in our shacks.
As far as I know, this idea has never appeared on an amateur radio
transmitter. Should manufacturers think about how they could implement
this feature?
Al W6LX
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