High Tx IMD with new K3s/10

[Bill Leonard N0CU]

I recently got a new K3s/10 built up and running. This week I ran some tests to get familiar with it and one test that surprised me was the high Tx IMD levels. I first noticed that the two tone waveform on a Tek o'scope showed flat topping starting at about 4.5 W. As the power level was increased the flat topping got worse. I checked the spectrum with my HP spectrum analyzer (on 14 MHz) and measured the following 3rd order IMD levels feeding a 1.1 SWR:

1 W: -36 dBc
4 W: -31 dBc
4.5 W: -33 dBc
6 W: -34 dBc
8 W: -25 dBc
10 W: -21 dBc

It was interesting that as the power was increased from 4.5 to 6 watts, the increased distortion didn't show up in the 3 rd order IMD, but rather the 5 th and higher order products.
Since I am new to Elecraft radios, I don't know if this behavior is expected or if there is something in my transmitter that needs to be checked out.
I have built up a number of amps over the years and I am used to seeing flat topping start at, or a little below the maximum CW output power. My K3s puts out about 12 W max, so I was expecting the flat topping to start at about 11 W.

Bill  N0CU

[wayne burdick]

Bill,

This is not representative of K3S/10 performance. Either the LPA module is misadjusted or the spectrum analyzer is being overdriven. If you send the unit in we'll check it in our lab and make any adjustments required.

73,
Wayne
N6KR

On May 2, 2016, at 7:10 PM, Bill Leonard N0CU <[hidden email]> wrote:

> I recently got a new K3s/10 built up and running. This week I ran some tests
> to get familiar with it...


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[Vic Rosenthal]

Not normal. Make sure the power supply voltage is stable and do the testing at 13.8v.

Vic 4X6GP

> On 3 May 2016, at 5:10 AM, Bill Leonard N0CU <[hidden email]> wrote:
>
> I recently got a new K3s/10 built up and running. This week I ran some tests
> to get familiar with it and one test that surprised me was the high Tx IMD
> levels. I first noticed that the two tone waveform on a Tek o'scope showed
> flat topping starting at about 4.5 W. As the power level was increased the
> flat topping got worse. I checked the spectrum with my HP spectrum analyzer
> (on 14 MHz) and measured the following 3rd order IMD levels feeding a 1.1
> SWR:
>
> 1 W: -36 dBc
> 4 W: -31 dBc
> 4.5 W: -33 dBc
> 6 W: -34 dBc
> 8 W: -25 dBc
> 10 W: -21 dBc
>
> It was interesting that as the power was increased from 4.5 to 6 watts, the
> increased distortion didn't show up in the 3 rd order IMD, but rather the 5
> th and higher order products.
> Since I am new to Elecraft radios, I don't know if this behavior is expected
> or if there is something in my transmitter that needs to be checked out.
> I have built up a number of amps over the years and I am used to seeing flat
> topping start at, or a little below the maximum CW output power. My K3s puts
> out about 12 W max, so I was expecting the flat topping to start at about 11
> W.
>
> Bill  N0CU
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/High-Tx-IMD-with-new-K3s-10-tp7617100.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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>
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> Please help support this email list: http://www.qsl.net/donate.html
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[Jim Brown-10]

On Mon,5/2/2016 7:10 PM, Bill Leonard N0CU wrote:
> one test that surprised me was the high Tx IMD levels.

Some things to check.  What power supply voltage?  Was it a matched
load?  Distortion increases with most power amplifiers (including the
output stage of a rig) with low power supply voltage and with a
mis-matched output.

And, as was suggested, was the test gear overdriven?

73, Jim K9YC
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