[Amps] IMD vs CW BW.

[Jim Brown]

Here's a pdf file collecting a lot of measurements I've made on the rigs 
and amps I could easily get to measure.  
http://k9yc.com/P3_Spectrum_Measurements.pdf

Remember that what we call CW is really 100% amplitude modulation of a 
carrier by a rectangular wave. The rise and fall time have an infinite 
spectrum, and the components excite intermod.  Slide 8 is a steady 
carrier with no keying. The sidebands at -70dBC are 60 Hz hum. Slide 11 
is the same for an FTDX5000.

Slide 14 and many that follow are a series of dits at about 30 wpm. The 
stuff below about 55 dB is IMD.

Slide 63 shows an amp that is adding IMD to a CW signal. Yes, the 
sidebands are clicks. Slides 70-72 compare that amp to the Titan and the 
KPA500 at their rated power.

On all of these rigs and amps, the differences are more than -40dBC. 
That matters when the broad signal is very strong and the bands are 
full. Like in a contest. :)  I see lots of signals that are 20-30 dB 
over S9, so their -50 dBC sidebands can wipe out a lot of signals I want 
to work. And when one of these broad radios shows up next to my running 
frequency, either he has to more or I do, because I can't hear half the 
guys calling me.

The K3 keying is carefully shaped to provide clear keying at QRQ (up to 
100 wpm). If I remember correctly, they're using a raised cosine 
function to shape it. Isn't DSP wonderful? Last I've heard, the better 
(later) firmware for some SDR radios is doing the same thing or 
something similar.

Most other rigs give the user a menu setting that allows them to vary 
rise and fall time; clicks increase with faster rise and fall times. 
Below about 35 wpm, bandwidth is independent of keying speed and depends 
only on the keying waveform; above that speed, information theory takes 
over, and the higher speed causes bandwidth to increase. I don't work 
faster than about 35 wpm myself, but some of the QRQ guys on 40M are 
running WAY up there, mostly with keyboards, I think.

Not all of the broadness is clicks -- on the worst radios, some of the 
really broad stuff is phase noise. That's the "whooshing" noise that we 
hear corresponding to keying. BTW  -- that Mark V Field had been "fixed."

K6XX has a big station about 3 miles from me on top of the ridge that I 
have get over to get to EU. We both run K3s and tube amps. Because we 
keep our signals as clean as possible, we can work 500 Hz apart on CW, 
often not knowing the other is there.

73, Jim K9YC

On Fri,12/30/2016 8:34 PM, Jim Thomson wrote:
> Date: Thu, 29 Dec 2016 01:27:49 -0800
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: amps at contesting.com
> Subject: Re: [Amps] FCC Denies Expert Linears' Request for Waiver of
> 15 dBRule
>
> The IMD from this amp is significantly higher than my "good" amps, so
> I'll eventually replace it, because I can't use it for contesting
> (because the IMD makes it broad on CW). The Titan remains my contesting
> amp, clean as the K3 that drives it at legal limit. But the SPE gives me
> 3 dB more than my KPA500 on 6M (and instant on at HF), which is why it's
> on my operating desk. :)
>
> Glen says the 1.3K he bought is close to 14-15 dB gain, but that the
> vendor offered to tell him how to get the additional available gain. It
> would, indeed, be nice if SPE used the additional gain to reduce IMD.
> But the DXpeditioners love the 1.3K-FA -- it's light weight (thanks to
> an SMPS), very close to legal limit. And when you're DX in the middle of
> nowhere, you don't have neighbors who care if you're a bit broad. :)
>
> 73, Jim K9YC
>
> ##  Im trying to wrap my head around  poor  IMD  vs being broad on CW mode.
> Im assuming a clean CW signal  from the xcvr.... driving an amp with mediocre
> IMD.   Exactly what effect are you seeing.   Does it sound like key clicks.
> How much broader are you talking about, several hundred hz or what.
>
> Jim   VE7RF
>
>
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