Re: #ubitx-help Strong Audio Tones in and outside audio receive passband #ubitx-help

Ashhar Farhan

Since we noticed these problems, we do check for the presence of these tones. Our pass test includes viewing the audio spectrum at 5v/db, 625 hz per division setting of FFT on rigol. We set the carrier so that the audio passband is inside two cursors at 300 hz and 3000 hz. Then we check for any spurs upto 15 khz that are above the noise floor. If these tests pass, we move to the next test of transmit outputs on 3.5, 7, 14 and 28 mhz. These tests are a part of the firmware being shipped.

I suspect the 16 mhz crystal must have aged. There are two things you can try. First is go into the bfo settings and move it slightly so the spurs go away. Second is to add some capacitance to the 25 mhz crystal, about 10 pf. There is a third way too, which is to program the si5352 initialization code to enable a different load capacitance on oscillator. I havent tried the third method myself.

Very often, closed commercial designs tend to ignore or bury design faults. Open source proceeds with acknowledgment and fixing these to make the system better. As the saying goes, given enough eyeballs, all bugs are shallow.

- f

On 3 Feb 2018 4:19 am, "Jerry Gaffke via Groups.Io" <> wrote:
HFO's being off suggests the 25mhz crystal at the si5351 might be the problem on this one.
Regardless, it is probably a matter of 3'rd harmonic of the 16mhz nano oscillator
beating with the fourth harmonic 1.9985mhz bfo.
Tone should change when you bring your finger close to the very very dinky crystal near the nano's processor.
Will likely change when you bring your finger near the 25mhz ref crystal near the si5351 also.

I'd take a calibrated SW receiver, listen for the 16 and 25mhz oscillators, see if one is off frequency
then maybe try swapping out just that crystal.    Especially if that signal seems somewhat unstable.
A crystal in an entirely different package will probably work  if you keep the wires short.

If the 25mhz reference oscillator were unstable, that would make the ubitx receiver unstable also,
which I assume is not the case.  So best guess is John's 16mhz crystal near the Nano.

What exactly do you mean by "HFOs were off far enough to effect the signal by 8dB."
So, perhaps your BFO as shipped from HFSignals was far enough from the 12mhz crystal ladder filter
that signals were down by 8dB?  Or perhaps you updated your firmware, and lost the HFSignals calibration?


On Fri, Feb 2, 2018 at 02:04 pm, Dave Bottom wrote:
Thanks for your hard work.  My uBITX has the same exact issue with the "squeal".  Mine did not originally have this issue until a couple of weeks ago (received Dec 22) and now it has gotten worse over that time.
It is interesting that the HFOs were off far enough to effect the signal by 8dB. Something to check and maybe this should be part of alignment for each radio?  At least with a spectrum analyzer this won't be too hard.
Yes interested in your code for this test.  
So now when I have time I know what to try, although it doesn't sound like much fun changing the Nano.
Dave WI6R

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