Posted by
Elecraft mailing list on
Jan 01, 2021; 8:56pm
URL: http://elecraft.85.s1.nabble.com/K3-Tx-Spurious-on-Specific-Frequencies-on-160-80-40-tp7667119p7667136.html
I think I have worked it out.
Taking the lid off and poking about showed that if I touched the crystals at
the DDS end of the filter on the synthesiser board, the spurious levels
rose. Upon taking the lid off the other (newer) radio, I realised that the
KSYN3 in the new radio (SN 8529) had a shield installed that neither of the
KSYN3s in the older radio (SN 4198). Looking at both construction manuals -
somewhere between 2010 an 2013 a shield was added to the KSYN3, so this
looks like leakage from the DDS pre-filtering, and some aluminium is the
pragmatic answer to get the issue down to acceptable levels.
As an experiment, I have just grounded the 4 crystal cases (recognising this
may change the characteristics of the passband a little) - and it has
improved the performance by 15dB - putting the two radios in line with each
other.
73,
Andy, G4PIQ
From:
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Sent: 01 January 2021 18:31
To: '
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Subject: RE: K3 Tx Spurious on Specific Frequencies on 160/80/40
Thanks Rick. To answer those questions.
I'm measuring this with the radio into a power attenuator and then with
additional attenuation into a Perseus SDR operating as a spectrum analyser -
well below its overload threshold and with plenty good enough phase noise
and dynamic range to see this. I've also confirmed the signals are there on
the second K3.
I ripped out all the normal station cabling as part of the testing - so -
yes everything is tight.
I suspect I haven't noticed this before for two reasons. Firstly - the level
of the spurii are very variable by specific frequency in the band - so you
have to be a bit unlucky - and unless you're listening for it yourself on
site - most times it would be too weak for people to comment / skimmers to
pick it up. Also - the times I have run this radio with a second receive in
band while transmitting it's normally been with the run radio above 7025
where the spurii are at a much lower / no existent level.
Secondly - I wonder if something is degrading in that radio. The fact that
the spurious level changes significantly with drive levels suggests this is
being generated by a non-linearity in the PA chain, and I wonder if I'm
short of gain somewhere in the driver strip so the ALC is forcing the low
levels in the strip to be driven harder.
It is present on both radios, but is 15dB worse on one radio than the other.
Andy, G4PIQ
-------------------------
NK7O wrote :-
I don't have an answer, I have no test gear that would even allow a look
(other than another radio). But I'm curious how you're measuring and
observing (not doubting) these results.
Except for the reports seen on the air to validate your observations; if
the radio change barely affects the results and the condition does not,
it's either a failure of both radios or the testing environment (i.e.
transmitting into the same antenna).
What happens with changes in power output, different (dummy) loads?
Then the eternal question; since this hasn't been noted before at your
station; what else is different or has changed? Have you tightened your
antenna connectors lately?
HNY,
Rick NK7I
On 1/1/2021 9:30 AM, g4piq--- via Elecraft wrote
In general I've found the spurious performance of the K3 to be pretty good,
but I've noticed something today that I thought it would be worth asking
more widely if anyone else has seen and fixed.
I was calling CQ on 40m around 7014 kHz and I noted that several skimmers
also reported me weakly on c. 600 Hz either side of this frequency.
Sometimes skimmers have bad spurious responses, but seeing this on more than
one skimmer caused me to look more closely at my transmit spectrum. I
discovered that - particularly on 40m - there were specific frequency ranges
where a pair of sidebands appeared that move at 150% of the change in
carrier frequency. So on 7012 kHz they are at roughly +/- 500 Hz, at 7011
kHz they are at roughly +/- 1 kHz, at 7010 kHz they are at roughly +/- 1.5
kHz. They cross over with the carrier just below 7013 kHz.
Similar patterns recur in other parts of 40m, but they are strongest near
the bottom of the band. There are dramatic step changes in the level of the
spuria at the boundaries between synthesiser segments - So I see spurii from
7000-7004, none from 7004-7009, spurii 7009 -7015, etc. They peak in
strength when they are about +/- 500 Hz from the carrier, but drop
significantly in level as you get further from the carrier.
The level of these spurii is what was concerning - at their worst they were
less than 30dB down on the transmit signal. They grow somewhat non-linearly
with power. At 5W they are -40dBc, at 12W they are -32dBc, at 13W they are
-35dBc and at 100W they are a little less than -30dBc. I can see the same
style of spuria in some segments on 80m and 160m - but at lower levels.
I assumed this was a fault with that specific radio - and in some ways it is
- but the same spurii are present on both K3s that I have here - but on the
second radio they are no worse than -45dBc.
I've disconnected everything connected externally from the radio, tried
different power supplies, and recalibrated the synthesiser and Tx gain with
no significant change.
Both radios have the original synthesisers - not the KSYN3A.
Any leads from anyone on this one?
73,
Andy, G4PIQ
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