AVcc noise

[G4IVV]

In a posting on the sideband suppression thread viewtopic.php?p=1302#p1302 I observed that I had a approx 78kHz noise on my IQ signals.

I haven't identified the source of that yet, but there is some present on Vcc. Looking at the Vcc side of L2 I have about 12mV of noise with some switching pulses around that frequency:

pic_28_2.png (18.45 KiB) Viewed 2238 times

Now the interesting thing is if we look at the other side of L2 on AVcc, we see a much bigger 78kHz sine wave of about 60mV pk-pk (change of 'scope vertical gain) :

pic_28_1.png (21.94 KiB) Viewed 2238 times

I think what is happening here is that the L2+C11 is resonant close to this frequency and its ringing badly. In the ATmega368 datasheet it recommends a 10uH inductor rather than the 100uH used here, which would move the resonant frequency away from this specific noise. This LC filter I think sees a fairly high impedance load so isn't damped very much.

The irony here is that the LC filter has made the noise on AVcc about x5 greater than if it had just been strapped to Vcc!

At this stage I'm in a bit of qandry - I will carry on experimenting to clean up Avcc, but I think the CC-BY-ND license means I cannot describe here any changes I make to improve the circuit as that would be creating a derivative?

73 Jonathan

[DL2MAN]

The source is the ADC itself. It´s the sampling frequency.
But why are you so focused on that ? Does it cause any problems ?

Don´t fix what ain´t broke, man....

And don´t look at the Audio Signal with your scope. This would be really ugly
Yet it works.....

73 Manuel; DL2MAN

[G4IVV]

But mine is broken - I have only about 20dB sideband suppression and I see non-equal noise on the IQ channels modulating the rx signal which led me down this path. Any imbalance between the I & Q channels will degrade a SDRs performance.

Currently my radio's receive performance is not good enough to be used on the air. The real world symptoms are that I hear a lot of false QRM from stations that are being received on the wrong sideband, this isn't just worrying about a 'scope trace.

My board does use a 6622 but I cannot buy a LT6231 so that will have to wait till later in the year when they are available (I did find one source in the EU but they refused to sell to anyone from the UK because of the bloody stupid Brexit). My 6622 doesn't exhibit the more catastrophic failure that you identified. I do have a 0603 sample book on its way and I will mod as per your recommendation, but I did try an experiment with patching with leaded components however it did not help and so I'm not hopeful that will fix my failure.

73 Jonathan

[DL2MAN]

No Offense.
You might want to try alternative OpAmps like LM833.
I was positively surprised about it´s performance today.... More to come, when I have more confirmations from other testers.

73 Manuel; DL2MAN

[mm0gyx]

No Offense.
You might want to try alternative OpAmps like LM833.
I was positively surprised about it´s performance today.... More to come, when I have more confirmations from other testers.

73 Manuel; DL2MAN

The entire first uk group have lm833 fitted, it’s what I picked, I have no complaints.

One of our group bought some 6231 from aliexpress, I don’t know if they are genuine. Anyway, I fitted one of these to my board and could not tell a difference, it was like an 833. There could be two reasons for that, 833 is adequate, or these 6231 from aliexpress are not really 6231.

Anyway, I like the 833, they do the job okay. There should be many from our group who could express their opinion too.

I didn’t make any quantitative tests, it’s just by ear.

Ian

[G4IVV]

Thanks Manuel and Ian,

I have ordered some LM833, they are cheap and readily available so I'm happy to run with that until the supply of LT6231 eases. I'll let you know how I get on with it, I don't mind doing this - its all part of the fun!

73 Jonathan

[G4IVV]

Just a quick heads-up: today Mouser now have nearly 2000 units of LT6231CS8 in stock.

Update 26/2/22: well that didn't take long - now they have all gone. Stock was there when my order was accepted, but my order status still shows it as being picked, so not sure I'm going to get any now.

73 Jonathan

[mm0gyx]

Thanks Manuel and Ian,

I have ordered some LM833, they are cheap and readily available so I'm happy to run with that until the supply of LT6231 eases. I'll let you know how I get on with it, I don't mind doing this - its all part of the fun!

73 Jonathan

Hi Jonathan,

I don’t know anything about sideband suppression values for the 833, it might not be any better, I didn’t measure it.

Ian

[G4IVV]

No problem Ian. It looks like I'll be receiving a few 6231's from Mouser sooner than expected, so I'll just wait and fit one of those to get back in sync with Manuel's original BOM.

73 Jonathan