[Elecraft] Data mode and filters

[Joe Subich, W4TV]

On 2015-05-12 12:40 AM, Jim Brown wrote:
> Not true, Joe. Both CW and FSK involve 100% modulation of a carrier
> by square waves. That's why rise time and shaping of the keying
> signal matters.

While that is true, using a class C amplifier with properly shaped
CW, FSK or JT65/JT9 will not significantly change the bandwidth of
the signal.  Yes, there will be some increased "click" at the top
of the rising edge but the impact will be nowhere near as significant
as an ALC system with severe overshoot driving a PA with only -30 dB
IMD on SSB.

 > I don't buy your complaints about my PSK31 measurements. It has never
 > been my practice to drive amplifiers into saturation.

If you are measuring 100W average power on PSK31 (anything more than
25 to 40 W depending on data) you are driving the KPA3 into saturation.
If you are not measuring 100W average power in your tests, you are not
fully testing the IMD of the KPA3 (at least as generated by the "all
knobs to the right" class of operators).

The vast majority of amateurs do not have calibrated scopes on the
output of their transmitter/amplifier to keep the instantaneous peak
output below the onset of compression in the final stage of power
amplification.

73,

   ... Joe, W4TV


On 2015-05-12 12:40 AM, Jim Brown wrote:
> On Mon,5/11/2015 8:39 PM, Joe Subich, W4TV wrote:
>> IMD is not an issue with CW or FSK (or JT65/JT9 for that matter) - they
>> are modes designed to work with saturated (class C) amplifiers. IMD is
>> a real issue with SSB when you separate out all the other junk caused
>> by bad ALC, excess IF gain and excess *audio* processing.
>
> Not true, Joe. Both CW and FSK involve 100% modulation of a carrier by
> square waves. That's why rise time and shaping of the keying signal
> matters. That's what Elecraft changed when they cleaned up their FSK a
> year or two ago. That's why some rigs click a lot wider than others.
> Compare the keying spectrum with the unkeyed steady carrier.
>
> Further, EVERY component of audio excites IMD, depending on the relative
> strength of all of the thousands of components. It is NOT to over
> processing audio -- the distortion of excessive processing and
> overdriven audio happens at baseband (audio) and in the audio chain.
>
> I don't buy your complaints about my PSK31 measurements. It has never
> been my practice to drive amplifiers into saturation.
>
> 73, Jim K9YC
>
>
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