Re: calibrate v4 ubitx

Bill Cromwell

Hi Jim,

The easiest way I have found to set the BFO is to use something like the graphical display in fldigi or Spectran - I prefer spectran. Set the display to show the audio frequency as opposed to showing you the radio frequency. The waterfall or spectral displays of the background noise will show a 'shadow' of your "system" pass band and that will be largely determined by the xtal filter passband. Be sure to disable or bypass any audio filters. My passband with my uBitx is about 1900 Hz wide. I see a broad, mostly flat curve with a slope at either end. Changing the BFO up and down will move the slope up and down the frequency scale. I adjust the BFO so that the low frequency slope starts to roll off at about 300 Hz. Thereafter, tuning SSB and AM signals (using synchronous AM detection) gives me clear voices. Music is not going to be good through that filter no matter what we do with the BFO. The master oscillator (25 MHz) must be calibrated first.

I hope this helps.


Bill KU8H

On 07/20/2018 11:32 AM, Jim Sheldon wrote:
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
bark less - wag more

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