Fix for bad sideband suppression on 30 and 20m Band

DL2MAN
 

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Fix for bad sideband suppression on 30 and 20m Band

Post by DL2MAN »

Some users have reported bad side Sideband suppression, especially on 20m.

Guido and me investigated a little and could duplicate the issue on 20m and 30m Band, but only with LMH6622 OpAmp installed (substitutes for LT6231).

The recommended fix would be: Change your OpAmp to LT6231. However, there was a reason why it was substituted in the first place, and that is: LT6231 are very hard to get at the moment.
So in case you already have LMH6622 installed, cannot get LT6231 anywhere or don´t want to unsolder SMT IC: There´s an alternative Fix: Solder another 82k Resistor parallel to R15 and R24 each. This attenuates Amplification factor of the OpAmp by 6dB and seems to take care of the issue.

See pic: Original 82k next to another 82k Resistor (soldered on Top of Capacitor)
Unbenannt.JPG
Unbenannt.JPG (82.11 KiB) Viewed 11371 times
While we do not KNOW exactly what´s going on, we have a few theories....
Working range of LMH6622 is smaller, because it's not a rail to rail opamp.
Different Impedance might also be part of the Issue (hard to determine, because Datasheets don´t provide comparable data)
Long story short:
Tiny varations in opamp input cause a bigger error on opamp output that in might steer one of the opamps out of it's working range, when tuned to 20m, resulting in degraded side band rejection on receive.


Wrap up:
Only Rigs with alternative OpAmp LMH6622 are affected by this issue !

Best Recommondation: Substitute OpAmp for Original Type LT6231
Second Best: Substitute OpAmp for LM833
OR
Expedient Solution: Solder additional 82k Resistors parallel to existing 82k resistors R15 and R24 while keep using LMH6622

And in any case: Build new RIgs only with LT6231 !!!

73
Manuel; DL2MAN and Guido; PE1NNZ
G4IVV
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by G4IVV »

Thanks Manuel for the update.

I've been struggling with poor sideband suppression and have spent several hours today investigating and had found I have IQ imbalance. My board has a LMH6622 and I had just decided to try and get a LT6231 as one my first steps to try to solve it!

One of the symptoms I found on my board is that there is (superimposed on the audio) a 78kHz square wave of about 7mV pk-pk at the junction of R12,R14 and a much smaller square wave on the other channel at R21,R22. The 5v supply to the opamp is a little noisy but the the 6622 spec sheet implies that the power supply rejection should be good enough.

73 Jonathan
DL2MAN
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL2MAN »

When overloading the Opamp, it showed I and Q in my case:
WhatsApp Image 2022-02-20 at 15.10.57.jpeg
WhatsApp Image 2022-02-20 at 15.10.57.jpeg (83.43 KiB) Viewed 11341 times
But when presented normal RX Signal levels, one of the channels would no longer work with LMH6622:
WhatsApp Image 2022-02-20 at 15.18.58.jpeg
WhatsApp Image 2022-02-20 at 15.18.58.jpeg (93.44 KiB) Viewed 11341 times
This was how I measured:
WhatsApp Image 2022-02-20 at 15.11.12.jpeg
WhatsApp Image 2022-02-20 at 15.11.12.jpeg (243.15 KiB) Viewed 11341 times
G4IVV
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by G4IVV »

Interesting, mine doesn't exhibit that behaviour. I do get some sideband rejection but its not that good (even say a S5 signal can be heard on the other sideband). I have a 78kHz square wave noise superimposed more on one channel than the other:
i_q_output.png
i_q_output.png (21.97 KiB) Viewed 11333 times
I wish I had 3 hands when trying to hold 2 probes and hit the 'scope button :)
Last edited by G4IVV on 20 Feb 2022, 20:24, edited 1 time in total.
G4IVV
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by G4IVV »

And with no signal, this is the 78kHz noise on each channel (scope probes the same place as yours):
i_q_noise.png
i_q_noise.png (25.54 KiB) Viewed 11326 times
I haven't spotted any obvious assembly issues. I have had a quick look and it doesn't look promising on finding a LT6231 unfortunately.
storyofyouandme
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by storyofyouandme »

Because I can't buy the lt6231, I'm going to try with the lm4562. The lm4562 is easier to buy, and the parameters are better than the lm833. Dl2man has recommended using the lm833 to replace the lt6231. If it is feasible to use the lm4562, please give a suggestion when dl2man has time.
73.
DL2MAN
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL2MAN »

The recommondation for LM833 (including warning for bad performance) has been withdrawn after discovering the sideband suppression issues.
LM4562 is a 12V OpAmp, and can be used, if you change the solder jumper: However, there is no guarantee, it will behave better than LMH6622.

The only recommondation I give at the moment is: Use LT6231, as it was designed around that OpAMp. And it is still available, I just ordered 10x from AliExpress from a Seller that a Buddy told me was legit, and doesn´t sell fake....
storyofyouandme
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by storyofyouandme »

Thank you very much for dl2man's reply. Lm4562 can be used directly without changing the circuit. Is my understanding correct? Of course, if I can buy the lt6231, I will first consider using it. I have asked the dealers who often buy, but they are temporarily out of stock, and have been reminded that the refurbished chips are fake.
seventy-three!
DL2MAN
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL2MAN »

Try it, and tell us how it works ;)
We have used the LM4562 in the Past, but not on this (tr)uSDX Board yet.
73 Manuel, DL2MAN
wa2t
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by wa2t »

DL2MAN wrote: 22 Feb 2022, 20:23The only recommondation I give at the moment is: Use LT6231, as it was designed around that OpAMp. And it is still available, I just ordered 10x from AliExpress from a Seller that a Buddy told me was legit, and doesn´t sell fake....
Got a link? I got what I am fairly sure are fakes from JSSD Technology on AE, and AE rejected my counterfeit dispute.

Thanks and 73,

Robert, WA2T
G4IVV
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by G4IVV »

Got a link?
Try Mouser, they seem to keep coming in and out of stock the last few days, for example as I type this they have 239 in stock: https://www2.mouser.com/ProductDetail/5 ... S8%23TRPBF

Set up a stock alert and order as soon as you get the email.

73 Jonathan
G4IVV
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by G4IVV »

Just to confirm that Mouser came through OK with my LT6231 order and that has resolved my sideband suppression issue :)

73 Jonathan
DM3FAM
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DM3FAM »

Hello Jonathan.

Thanks for your feedback and see you soon in a QSO :)

73 Marcel DM3FAM
G4IVV
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by G4IVV »

Hello Marcel,

Next time I chase you on a summit I'll try and use my (tr)uSDX :)

73 Jonathan G4IVV
g7jur
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by g7jur »

Would using a LM833, work better than a LMH6622 with the extra resistors.
I cannot find a LT6231 anywhere.
Philip
G7JUR
DL2MAN
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL2MAN »

LM833 seems to work.
But I don´t have enough data to be sure (sample size 2)....
So keep in mind, this recommandation might be withdrawn.

LT6231 can be purchased from mouser or digikey. They seem to get in stock from time to time, but you gotta be fast. They also can be bought from more doubtful sources on ebay and aliexpress....

73 Manuel; DL2MAN
DL2MAN
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL2MAN »

wa2t wrote: 28 Feb 2022, 23:41 Got a link? I got what I am fairly sure are fakes from JSSD Technology on AE, and AE rejected my counterfeit dispute.

Thanks and 73,

Robert, WA2T
https://de.aliexpress.com/item/10050024 ... 5c5fhHQ31f
g7jur
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by g7jur »

G4IVV wrote: 01 Mar 2022, 10:36
Got a link?
Try Mouser, they seem to keep coming in and out of stock the last few days, for example as I type this they have 239 in stock: https://www2.mouser.com/ProductDetail/5 ... S8%23TRPBF

Set up a stock alert and order as soon as you get the email.

73 Jonathan
Just ordered one from Mouser. They have 47 in stock.
Thanks Philip G7JUR
DL3XAC
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL3XAC »

Hello to all,
I finished today the kit, which I bought from the approved supplier on eBay. Everything works fine. But the SSB modulation is horrible. I checked the mainboard and can see that LT6231 is in use. Now I would like to know the experiences of the guys, who changed to LT6231. Is SSB working now. And has anybody an idea what my problem could be?

73 de DL3XAC Carsten
dl6sez
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by dl6sez »

Hello Carsten,

the SSB Modulation has nothing to do with Receiver component LT6231 in (tr)uSDX....
and LT6231 is the best OpAmp for the receiver so be lucky with it :) some others have non ideal replacement OpAmps due to LT6231 supply shortage because of Covid problems in China.

According to SSB Modulation: It is not as good as other nearly HiFi rigs but it is okay and think about to test it with about 90dB attenuator between tx and rx and don't forget about self oscillation when you have your microphone near your speaker where you check the sound of modulation.
Simply record it with an inbuilt recorder, today included in many new rigs (IC7300 for example), with volume at 0 on your rx.
Don't forget the attenuation and you will hear a good SSB sound of your voice on the record.
Believe me i have a (tr)uSDX and a uSDX built by myself here and both perform well.
The SSB is not perfect but good if one has the understanding of how it is generated with a 8-Bit Microcontroller... it can't be better ;)

73 de Chris DL6SEZ
Chris DL6SEZ, JN48XL near Ulm, Southern Germany
DL3XAC
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL3XAC »

Hello Chris,
Thanks for your fast reply and clarification. So, I misunderstood the comments before regarding LT6231. My receiver is working really well in CW, SSB and also AM. I am just at the beginning of checking everything out and because of my SSB issue maybe you are right. I will check it out carefully again and play a little bit with the settings. Except my SSB modulation, it seems all is working. This is fine and I am happy about it. I start now with the fine tuning :-)

Thanks again es 73 de Carsten DL3XAC
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DH2LAB
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DH2LAB »

If the conditions and the distances are right, you can also listen to yourself in the WebSDR.
This is how your radio sounds on the other HAM.

http://websdr.org/
73 de Matt, DH2LAB
DL3XAC
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DL3XAC »

Dear Chris and Matt,
I checked my SSB signal now via WEBSDR and it is interesting, the modulation is much better than what I hear with my own radio, when I am transmitting with the (tr)uSDX into a dummy load. I remember now, that I had this effect already in the past, with another radio.. so thanks to you and it seems my problem is solved.

:-) 73 de Carsten DL3XAC
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DH2LAB
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by DH2LAB »

DL3XAC wrote: 27 May 2022, 19:05
I checked my SSB signal now via WEBSDR and it is interesting, the modulation is much better than what I hear with my own radio, when I am transmitting with the (tr)uSDX into a dummy load.
Glad you are happy. :!:
Yes, that's also my experience over many years of amateur radio. Even without an antenna on the other receiver, the signal is too strong to make good qualitative statements without other measuring devices.
Today WebSDR really helps a lot and you can also test your propagation easy and hear everything with your own ears.
I got good modulation reports with the (tr)uSDX test QSOs in some 40 meter groups. Not exuberant, but good and satisfying. So I always recommend checking yourself with WebSDR.
73 de Matt, DH2LAB
Magnus
 

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Re: Fix for bad sideband suppression on 30 and 20m Band

Post by Magnus »

I just got my assembled (tr) usdx, om 20 m there is a small carrier wave transmitted even though the unit is om recieve. Is this what this thread is referring to?
Kind regards
Magnus de LB9YI
Image of carrier signal
Image of carrier signal
Screenshot_20220528-003728_Gallery~2.jpg (28.66 KiB) Viewed 8641 times
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