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 Post subject: 2nd harmonic
PostPosted: Mon Jan 21, 2019 9:12 am 
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Joined: Mon Jan 21, 2019 8:54 am
Posts: 5
I purchased a used HR50 with qsk and atu. It has sn 13xx with the RevF board, Firmware is 3.0d. When I test into a dummy load with a directional coupler in line, I see the second harmonic about 37 db down on 40M 36 db down on 80M and 43 db down on 20M. I re did the bias alignment which got me to the current readings, about 4 db better then originally tested prior to the bias adjustment. Any ideas on what to look for ? The previous owner was using this for 20M cw without any issues.


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 Post subject: Re: 2nd harmonic
PostPosted: Mon Jan 21, 2019 11:41 am 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
The measurements should have been made with the amp running on a 13.8V supply capable of 15 amps or more, sagging voltage or current limiting could cause higher than expected harmonics. Also whatever you are driving the amp with should meet or exceed -43 dBc or the amp may not. You also need to factor in the frequency response/linearty of your directional coupler. An attenuator is a better choice if you have one. Measurements should be taken at 50W output, max.

The ATU should be set to BYPASS otherwise and stored tuning solutions might be applied presenting a non 50 ohm impedance to the amp.

If you are confident that your test conditions are correct and the 2nd harmonic is higher that expected it may be that the push-pull aspect of the amp is out of balance. Since re-biasing had an effect on the harmonic level this tends to support this. I would recommend a new set of MOSFETs and re-biasing the amp. If one of the MOSFETs has more Rds-on than the others the 2nd harmonic would increase.

Another culprit might be the 1N4007 diodes in the QSK circuit, one of those was bad you could see some interesting harmonics as well.

The good news is that a new set of MOSFETs is only $20 and a new round of diodes uder $5.

73,
Jim WA2EUJ


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 Post subject: Re: 2nd harmonic
PostPosted: Mon Jan 21, 2019 12:30 pm 
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Joined: Mon Jan 21, 2019 8:54 am
Posts: 5
Power supply is 30A,14.2V with about 5 ft #12 wire feeding the HR50. I was using a K3s at low power to run the test. I verified that it was not producing the 2nd harmonic. I did verify the frequency response of the directional coupler at low power using the tracking generator and it is within about 1 db across the frequency range of interest. I also used the same set up to measure the harmonic content of the K3s at 50 watts and it was as expected. ATU was in bypass for the test and swr reported on the HR50 was 1.

Is there an easy way to bypass the QSK board to rule out a bad diode ?


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 Post subject: Re: 2nd harmonic
PostPosted: Tue Jan 22, 2019 7:05 am 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
OK it sounds like you have a solid measurement system. I've spent a lot of time assuming that there was something wrong with an amp only to find issues with the testing environment. I guess the last question is whether the harmonics are generated in the front end of the spectrum analyzer and the only way to tell for sure is to insert a fixed attenuator between the output of your coupler and the input of the SA and see if the measurement is the same (adjusting for the additional loss).

It's easy enough to remove the QSK board and apply the RF to the AMP_IN terminal block. It's more complicated to pick-up the output. You could make a jumper that connects to QSK-C or move W1 to the RLY position jumper across RLY-1 NO connections and pick the RF up at RF_OUT. That's probably the easiest since you could use the existing coax jumper to connect to RF_OUT.

At the bottom of this page:

https://sites.google.com/site/hardrock50beta/home/barebones-kit-information

You can see the spectrum analyzer measurements that were made for the FCC Type Acceptance and compare them to your results.

73,
Jim WA2EUJ


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 Post subject: Re: 2nd harmonic
PostPosted: Tue Jan 22, 2019 9:32 am 
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Joined: Mon Jan 21, 2019 8:54 am
Posts: 5
I have a 0 - 120 db step attenuator in line, currently set to 40db. I will try increasing the attenuation. The directional coupler is -29.5 db, so the carrier is below -20 dbm going into the spectrum analyzer.

Looking at the schematic, It looks like I would connect the output to qsk-b. I have some adapters that go from a 2 or 3 pin header to sma. Can this test be done with the bottom cover off ? Since I have 1N4007 diodes on hand, it may be easier to just replace the two in the transmit path and retest.

I also have a number of the RD16HHF1 MOSFETs on hand, but not sure I have enough to come up with a matched set. Do you typically match these prior to sending them out ?

Thanks for your help
joe


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 Post subject: Re: 2nd harmonic
PostPosted: Tue Jan 22, 2019 3:59 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Hi Joe,

It may be easier just to replace the diodes but yes the bottom cover shouldn't effect the harmonics. We don't match the MOSFETs since you'll set the bias individually they are generally close enough. When we create our sets of four they are all from the same manufacturing lot to insure that they are as close as possible. Occasionally Mitsubishi sends us a notice that they are moving the RD16HHF1's to a different foundry and there will be some slight differences in the process.

One of our CM's tried to source them but ended up with counterfeits which were terrible. Our come in a box that says 'Mitsubishi' on the side.

73,
Jim WA2EUJ


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 Post subject: Re: 2nd harmonic
PostPosted: Wed Jan 23, 2019 12:47 pm 
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Joined: Mon Jan 21, 2019 8:54 am
Posts: 5
Jim,

You have very good instincts in suspecting the test setup. When I reran the test with the bottom cover off the 2nd harmonic increased about 3db. I then proceeded to test with an additional 10db of attenuation to rule out an over loaded spectrum analyzer. The carrier was reduced by 10 db but the 2nd harmonic was unchanged. After swapping out the cable from the attenuator to the spectrum analyzer the 2nd harmonic was 50+ db down. I'm not sure why this did issues did not show up when I ran the same test with the K3s. I tested the cable on the VNA and could see that it was bad. flexing one of the two ends caused intermittent results. After dissecting it I could see a bad crimp on the shielding on one end.

Prior to putting the bottom cover back on I noticed the shield from the input coax was not connected at the RF board. This may have contributed to the erroneous readings.

The amplifier is now ready for some on the air testing.

Joe


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 Post subject: Re: 2nd harmonic
PostPosted: Thu Jan 24, 2019 9:18 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Hi Joe,

I'm glad that you got your issue resolved and that the HR50 is working OK. I always want to make sure of the test fixture before diving into an equipment repair and thank you for being willing to investigate your setup.

73,
Jim WA2EUJ


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