Sunday, November 13, 2022

Video: transmitter IMD of (tr)uSDX

Updated 14 Nov (see bottom of page)

 

Kevin KB9RLW has posted an interesting series of videos to YouTube recently, exploring the poor SSB performance of his (tr)uSDX transceiver (beginning with this one).

I'm primarily a CW operator and wasn't too concerned about my (tr)uSDX's SSB performance but Kevin's video's piqued my interest and I decided have a look with my SA and an Elecraft Two-Tone Generator feeding a 700 Hz and a 1900 Hz tone into the mic input of the (tr)uSDX.

The purpose of this test is to measure IMD due to non-linearities in the transmitter. In a perfect world, only those two tones would be transmitted.

In our real world, the tones get mixed together and produce new frequencies that also get get transmitted. The amplitude of these frequencies, compared to the two originals, are how IMD is determined.

Typically, 3rd order IMD - those closest to the two originals - are 30 dB weaker than the injected tones. 5th order and 7th order IMD products are even weaker. I was hoping to post screenshots from the analyzer but saved them in Rigol's proprietary format rather than jpgs. But 3rd order IMD products are only 8-10 dB down from the intended sidebands.

In the (tr)uSDX - at least in mine, and quite likely in KB9RLW's and many others - there are numerous IMD products, and they are quite strong. This accounts for the splatter seen in Kevin's videos.


UPDATE:

A comment from Manual DL2MAN, one of the (tr)uSDX designers, mentions that the IMD of the radio will improve if a narrower receive bandwidth is selected since receive bandwidth affects transmit bandwidth.

My original IMD tests were performed with the (tr)uSDX set at its maximum bandwidth of 4.0 kHz.

The screenshot below shows the difference in IMD at the rig's maximum and minimum (1.8 kHz) bandwidths:

 

As can be seen, IMD is indeed improved when the rig is set to minimum bandwidth, however these improvements are mainly 5th order and beyond. Third order IMD is essentially the same but the overall transmitted audio quality is improved by selecting a narrow bandwidth.

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9 comments:

  1. I checked my trusdx and the 3rd order IMD is down about 10 db, lsb, 7.150 MHz. It is a kit build by me and sourced from newdiytech. I also have a classic kit from Amazon that displays similar results. Thanks for posting your test results. My external mic connection is to a 500, 1900, two tone from my Siglent freq generator providing. The IMD doesn't change much even at 50 mv p-p still is using about 6kc of bandwidth. Sure hope they can recommend a fix, I like the radio other than Tx problems.

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    1. My (tr)uSDX is also from newdiytech and was pre-built by them. I don't think they have done anything wrong - the issues we are seeing are inherent in the design and the method by which SSB is produced...although I hope I'm wrong and that a firmware update can fix the issue. This doesn't seem likely though, based on the most recent video from the designer:
      https://www.youtube.com/watch?v=RsZIJtpEM64

      The (tr)uSDX is best thought of as an experimental rig, which Manuel alludes to in the video. It is an $86, 5-band rig - there will be issues and it is unrealistic to think otherwise. I'm okay with that but will not be putting my (tr)uSDX on the air in SSB mode now that I know how broadbanded it is in that mode.

      I've received several emails asking me about parameters I didn't test - I'll get to those in mid-December.

      73,
      John

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  2. Near as I can tell the mic is directly connected to the processor. So i guess firmware is required, if possible, as a fix, but I believe the designers are mostly done with investing large amounts of time to trusdx project, Great CW radio. Thanks DL2MAN, & PE1NNZ

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    1. It should also be fine on FT8, WSPR, etc as only one tone at a time is transmitted in those modes.

      73,
      John AE5X

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  3. I see the same bahavior if I overload the Input, which is very easy to do.... The Audio Input Signal needs to be set -starting at zero- until the point of max output power (SSB). Not further. Once you go beyond that point, everything goes south. However: We found some firmware issues, and are working on a fix. But this is not related to overdriving input. More related to behavior when injecting certain frequencies, that will result in white noise like splatter on HF..... Try injecting a single tone below 500Hz and you will see, what I mean. 73 Manuel; DL2MAN

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    1. When the receive band with is set at 1.8kHz, 1) what is the center frequency for the transmit audio bandwidth and 2) is the transmit audio bandwidth also 1.8kHz? And if not, then what is it?

      I wonder if one can one create a filter between the mic capsule and the rig that limits the supplied audio to that same bandwidth, and cuts off below 500 Hz. And would that help limit the intermodal products and splatter.

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    2. What is the transmit audio bandwidth when the receive bandwidth is set to 1.8kHz? And, when so set, what is the center frequency of the transmit audio bandwidth?

      What I am thinking is I f we limited the audio coming out of the mic element to that bandwidth and centered it so that below 500 Hz was cut off, would it help recuse the imd products and the splatter problem?

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    3. Thanks
      Jerry Terres aa1of

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    4. Interesting...so the question is, is the excessive bandwidth created within the (tr)uSDX or is it allowing excessive audio bandwidth into the audio input? If the latter, your solution might (probably would) work.

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