Re: K3 Tx Spurious on Specific Frequencies on 160/80/40

Posted by Rick Bates, NK7I on Jan 01, 2021; 5:55pm
URL: http://elecraft.85.s1.nabble.com/K3-Tx-Spurious-on-Specific-Frequencies-on-160-80-40-tp7667119p7667123.html

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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