Re: calibrate v4 ubitx

Jerry Gaffke
 

Yes, calibratoin should be easier.
Just needs appropriate firmware for the Raduino.
Should not require anything else.
No frequency counter.
No scope.
No host computer hooked up through the usb port.

Just tune the radio to an AM station such as WWV, enter the calibration routine,
turn the encoder knob till zero beat of the carrier is achieved, then hit PTT.
If you can't hear WWV, a shortwave broadcast station is almost always on an even 5khz frequency, within a few Hz.
The calibration accuracy will be proportional to the frequency of the station you calibrate to,
so WWV at 15mhz will give results 15 times more accurate than an AM broadcast station at 1mhz.

I haven't tried the various firmware releases, but from posts in the forum it seems none quite have it right.
Something patterned after the code in post  54501 (or post 35235 of last December) should just work.
My posts on how to do this correctly go back even further, into the Bitx40 days.

The only issue is detecting exactly where zero beat is, given the capacitive coupling
through the audio amps which limit the low freq response.
But as is, accurate enough to get within 10 hz.
Given that inherent uncertainty, may as well multiply the encoder reading by 100, not by 10, so you can find zero-beat more quickly.
That gives a granularity of 1e6 * 100/875e6 = 0.114 ppm.
Still plenty accurate, accurate enough that temp changes in the 25mhz crystal will be more of an issue.

Jerry, KE7ER

 

On Fri, Jul 20, 2018 at 08:15 AM, Joel Caulkins wrote:
You are right, I did use my calibrated HP 5384A frequency counter to determine the crystal frequency. I had forgot that. I found calibrating the hardest part of building my last uBitx transceiver, it just seems like it should be easier, but what do I know, I'm strictly a hardware man. I'm all for using WWV when the bands are good enough to actually hear it.

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