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Regarding the BFO, after the master oscillator is calibrated to an accurate frequency, the BFO MUST be calibrated to the individual uBITX crystal filter as no two of them are exactly the same. Such is the nature of filters built using bulk crystals, matched or not. Matching them is never an exact science as I've been finding out here trying to match crystals to come up with an add-on, plug-in crystal filter using the headers supplied on a V4 board. The BFO also cannot be adjusted to zero beat or you lose the ability of the filter to separate USB and LSB as well as CWU and CWL signals to give you what us ancient CW operators used to call "Single Signal" CW receive.
So far I have not found an easy way to do this except trial and error "guesstimation". I'm seriously hoping someone can come up with a really simple way to get this right.
Jim Sheldon, W0EB
------ Original Message ------
Sent: 7/20/2018 10:03:10 AM
Subject: Re: [BITX20] calibrate v4 ubitx
No, adjusting the BFO frequency is a bad idea.
That will change the audio characteristics on both transmit and receive.
And it will only "calibrate" the rig when operating at the frequency of the reference oscillator you happen to be using.
The si5351's reference oscillator is nominally 25mhz, but might be off by several ppm.
When using the si5351bx routines, the vco inside the si5351 is operating at a nominal frequency of 35*25mhz = 875 mhz.
The value of the variable si5351bx_vcoa has a nominal value of 875000000, what we want to do is adjust that value
until it is exactly equal to the true vco frequency.
So one way to do this is to measure the 25mhz oscillator of the si5351 reference,
perhaps using a separate well calibrated receiver instead of a frequency counter to avoid loading that crystal.
Let's call that value fxtal25mhz.
Then force si5351bx_vcoa to be equal to 35*fxtal25mhz.
Now the si5351bx routines use this new si5351bx_vcoa value when computing the output divider values
used to create clk0, clk1, and clk2. All three should be dead on.
We need to calibrate all three clocks simultaneously, not just the BFO.
Was more complicated on the Bitx40, where we had a separate crystal oscillator for the BFO.
On the uBitx, we only have one reference oscillator.
An easy way to calibrate using a 50mhz reference might be to divide it by two using a flip-flop,
and calibrate using the resultant 25mhz signal using whatever calibration software is in
the firmware you are using. I have not looked over the various firmware releases, but
given some of the forum posts my impression is that is is not being done right.
On Fri, Jul 20, 2018 at 07:11 AM, Joel Caulkins wrote:
I agree that calibration is a 'exercise', why does it need to be this hard. I'm no coder, but Don ND6T's calibration routine worked really good for me when I was calibrating my Bitx 60, http://www.nd6t.com/bitx/Calibration.htm All we need to know is what our actual 25Mhz crystal frequency is and change that line in the sketch, then all we need to adjust is the BFO frequency which is easy when the rig is on frequency. I don't know how to code so I'm not sure what is involved in adding something like this to the existing sketch but it sure beats zero beating.