[Elecraft] Data mode and filters

[Joe Subich, W4TV]

Jim,

> But Joe, you are fixating on a single specification, which is only
> one part of what establishes the cleanliness of a radio. On CW and
> FSK, there is the keying waveform, and on all modes there is phase
> noise.

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.

The K3 can be 4 to 6 dB *cleaner* than it already is if the proper
approach were taken in the KPA3.  Since I don't use an amplifier, I
want the cleanest "exciter power level" possible and it is annoying
that given all the other attention to detail, the K3 has a dirty PA
(IMD comparable to the IC-706mkIIg on my test bench) simply to make
it compatible with a battery pack.

> Six months ago, when I began doing my own measurements with a
> P3/SVGA, you insisted that the IMD in a K3 would make a mess of
> PSK31, so I measured PSK31 signals. They are in the set of P3
> measurements I have posted here several times, and they clearly show
> that you were wrong! :)

Your PSK31 "measurements" are clearly not believable.  Look at the
IMD values in most PSK software when the K3 is run at more than 30
to 40 watts *average* power output.  You will see the PSK IMD rise
significantly because the KPA3 is going into compression.  If you
measure at 100W *instantaneous* peaks (using an oscilloscope and
looking for 70.7 Volts peak RF across 50 Ohms), the average power
in a PSK31 signal (e.g. measured on the K3 Wattmeter or a Bird 43)
will be around 30 Watts then the IMD of the KPA3 (and the rest of
the signal chain) will be acceptable.  If you press the K3 to 100
W *average* output in PSK31 the PA will saturate, you will cause
about 4 dB of compression (peak clipping) of the PSK waveform and
PSK IMD will be around -15 dB or worse.

Non-linearities (compression) destroy BPSK and QPSK signals.


73,

   ... Joe, W4TV


On 2015-05-11 11:12 PM, Jim Brown wrote:
> On Mon,5/11/2015 6:45 PM, Joe Subich, W4TV wrote:
>> The K3 may be cleaner than low end 12V/100W rigs like the Icom 706mkIIg
>> 7000, 7100, Yaesu FT-450/991, etc. but it is certainly *not* as clean
>> as the other top of the line contest rigs like the Icom 7600/7800,
>> Yaesu 2000/5000, Kenwood 990, etc.  For example, ARRL measured the K3
>> at -29/-43/-46/-51 dB vs. -32/-49/-52/-54 dB for the Icom 7800 and
>> -31/-46/-54/-57 dB for the TS-990.  That makes the K3 3 to 6 dB *worse*
>> than other top of the line rigs at rated output.
>
> But Joe, you are fixating on a single specification, which is only one
> part of what establishes the cleanliness of a radio. On CW and FSK,
> there is the keying waveform, and on all modes there is phase noise. The
> K3 rise time for both modes is quite well controlled, so the keying
> transients that excite IM in those other radios don't excite IM in the
> K3. And phase noise in the K3, even before the new synth boards is close
> to the best of any modern radio.
>
> The occupied bandwidth measurements show the SUM of all these
> mechanisms.  I've been using pink noise for SSB spectrum measurements
> because the signal excites IMD in a manner similar to what a voice
> would, and I've made measurements with me talking into a live mic.
> There's no significant difference between the live mic and the pink noise.
>
> The FTDX5000 report to which I posted a link shows that the K3 and
> FTDX5000 have almost the same occupied bandwidth on SSB, but the K3 has
> 10 dB of signal processing. AND -- my K3 is even narrower because I've
> cut everything below about 400 Hz with TXEQ. If you missed the link,
> it's on my website.
>
> In many contests, I've got a CQ machine 8 miles from me with a 7600 and
> a legal limit amp who regularly wipes out 5-10 kHz on CW, but K6XX, only
> 3 miles away with a K3 and ancient tube amps is only 700 Hz wide at -50
> dBC. The 7600 and 7800 tested quite similarly in bandwidth tests by ARRL.
>
> Six months ago, when I began doing my own measurements with a P3/SVGA,
> you insisted that the IMD in a K3 would make a mess of PSK31, so I
> measured PSK31 signals. They are in the set of P3 measurements I have
> posted here several times, and they clearly show that you were wrong!  :)
>
> For those who missed the link, the measurements, as well as a
> description of my test setup, are on my website. k9yc.com/publish.htm
>
> 73, Jim K9YC
>
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