poor IMD on barefoot K2, even at low power

[dl0tuh]

Hi,

I gave a test to my PSK31 setup today - barefoot K2 #5932 with M-Audio sound card in my desktop pc. My laptop pc was connected to a FT-840 and listening. Both used old good DigiPan.

I let my laptop measure IMD and also asked for this value during QSOs. In all cases it's about -16dB which is very poor I beleive.

The power level is set to 5W. PC sound output level was decreased so that only the first ALC bar blinks.

I also tried power levels setpoint from 0.1W to 15W. IMD value changed continuosly from -19dB to -12dB.

Anything I could try/check to figure out the reason of destortion?

vy 73 de DL2IPU Oleg

[Don Wilhelm-4]

Oleg,

With the K2, you should have zero ALC bars illuminated - reduce your
audio input level until the ALC bar does not illuminate.
Are you using SSB mode?  If so, have you set the menu SSBC parameter to1:1?
You can set up the RTTY filter set (turn RTTY=ON in the secondary menu),
and you will have a dedicated mode for soundcard data modes.  The
advantage is that the compression setting is independent of that used
for SSB.

73,
Don W3FPR

dl0tuh wrote:

> Hi,
>
> I gave a test to my PSK31 setup today - barefoot K2 #5932 with M-Audio sound
> card in my desktop pc. My laptop pc was connected to a FT-840 and listening.
> Both used old good DigiPan.
>
> I let my laptop measure IMD and also asked for this value during QSOs. In
> all cases it's about -16dB which is very poor I beleive.
>
> The power level is set to 5W. PC sound output level was decreased so that
> only the first ALC bar blinks.
>
> I also tried power levels setpoint from 0.1W to 15W. IMD value changed
> continuosly from -19dB to -12dB.
>
> Anything I could try/check to figure out the reason of destortion?
>
> vy 73 de DL2IPU Oleg
>  
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>  

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[Julian, G4ILO]

Don Wilhelm-4 wrote

Oleg,

With the K2, you should have zero ALC bars illuminated - reduce your
audio input level until the ALC bar does not illuminate.
Are you using SSB mode?  If so, have you set the menu SSBC parameter to1:1?
You can set up the RTTY filter set (turn RTTY=ON in the secondary menu),
and you will have a dedicated mode for soundcard data modes.  The
advantage is that the compression setting is independent of that used
for SSB.

In DATA (RTTY) mode it is perfectly OK to have one ALC bar illuminated. I believe that the K2s use a "soft" ALC in this mode, similar to the K3. When set up this way my 10W K2 actually produces lower IMD (-34dB at 5W) than my K3 at the same power level, and the ALC maintains the power level to that requested across a wide range of audio frequencies.

On the other hand, I have found it almost impossible to measure IMD reliably using sound card software. I measured it using the KK7UQ IMD meter.

[dl0tuh]

Don, Julian, thank you for your replies!

I forgot to mention - RTTY mode is activated and selected, SSBCr is 1:1. SSBA is 3.

The real IMD value may be far from what DigiPan tells me, but I see a very well known picture - two solid lines on the waterfall for two main tones and several minors for intermodulation products when sending idle.

I was not sure if the distortion happens in FT840 or PC on receiver side, but with an antenna I can see strong but clean signals on the air. I also used my SoftRock and recorded the PSK sound - same distorted picture.

Can it be something very stupid, like one dead transistor (Q7 or Q8) in the PA?

[Don Wilhelm-4]

Oleg,

It sounds like you have most everything right, so you may indeed be
getting a false indication.

A quick check on the PA transistors is to connect a dummy load, set the
power control to maximum and press TUNE.  If you develop over 14 watts
on 40 and 30 meters, then the PA transistors are likely working just
fine, but if the actual maximum power is only 6 to 10 watts, then you
can suspect a bad PA transistor.

You could be overloading the receiver - run the K2 into a dummy load and
put a short wire on the receiver - adjust the wire length so you obtain
an S-9 or less signal into the receiver.

As Julian mentioned, the IMD report on many PSK-31 applications is not
to be trusted for absolute values, but it can still be used for relative
comparisons.

You might want to check the output of your soundcard for "cleanliness"
too.  A bit of distortion in the soundcard output would create a bad
IMD, but the M-Audio soundcard is usually clean.

Try swapping the laptop and the desktop PCs to see if that makes any
difference (drive the K2 from the laptop).

73,
Don W3FPR

dl0tuh wrote:

> Don, Julian, thank you for your replies!
>
> I forgot to mention - RTTY mode is activated and selected, SSBCr is 1:1.
> SSBA is 3.
>
> The real IMD value may be far from what DigiPan tells me, but I see a very
> well known picture - two solid lines on the waterfall for two main tones and
> several minors for intermodulation products when sending idle.
>
> I was not sure if the distortion happens in FT840 or PC on receiver side,
> but with an antenna I can see strong but clean signals on the air. I also
> used my SoftRock and recorded the PSK sound - same distorted picture.
>
> Can it be something very stupid, like one dead transistor (Q7 or Q8) in the
> PA?
>  
>  

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