Re: Raduino oscilators. 33mhz and 57mhz. Documentation says one thinng, but this is what I measured.

Jerry Gaffke
 

Here's a current and more complete summary of what's going on with the uBitx.
For the few who really really want to know.
Not many, judging from the response to post 44278.


Actual frequencies used in the original uBitx code are to have clk0 (bfo) fixed at 11996500 hz,
maybe 500hz below the 12mhz filter's 2000 hz wide 3dB passband.
Oscillator clk1 (second local oscillator) is fixed at 56995000 hz for USB and 32995000 hz for LSB.
You can find those three numbers in file ubitx_20.ino at lines 166, 163, 164 respectively.
I'm looking at the version dated Dec 6, 2017:      https://github.com/afarhan/ubitx

The vfo is used to select the operating frequency Fop according to these formulas.
For USB:    Fop  = vfo - (clk1-bfo)       so    vfo = Fop + (clk1-bfo)       where clk1 is around 45mhz+12mhz
For LSB:    Fop  = vfo - (clk1+bfo)       so    vfo = Fop + (clk1+bfo)      where clk1 is around 45mhz-12mhz

To receive a 7.2mhz LSB signal (where 7.2mhz is the frequency of the suppressed carrier),
the VFO gets set to   7200000+(32995000+11996500) = 52191500 hz.
That formula gives an exact result, not an approximation.

Regarding USB vs LSB:
The BFO corresponds to the carrier frequency of the station being received or transmitted.
The 12mhz filter is always above the BFO, so within the 12mhz IF it allows through only the upper sideband.
The VFO is always above the 45mhz first intermediate frequency, and so always inverts the sidebands:
    A carrier at 7200000 would get translated to vfo-Fop = 52191500 - 7200000 = 44991500 hz
    A lower sideband at 7198500 would get translated to 52191500 - 7198500 = 44993000 hz

In this example we assume the lower sideband is generated from a single audio tone into the mike of 1500 hz.
I have chosen 1500 hz because it will land in the middle of the 12mhz filter's passband, assuming the filter
has a 3 dB passband that's 2000 hz wide and the BFO is 500 hz below that passband.
The actual range of frequencies passed will be 500 to 2500 hz.
Those assumptions of 2000 hz and 500 hz might be off by a couple hundred hz.

Likewise, a high side clk1 of 56995000 hz  for USB always flips the sidebands when translating to 12mhz,
however the low side clk1 of 32995000 hz we use to receive the 7.2mhz LSB signal does not:
    Our 7.2mhz carrier:    44991500 - 32995000 =  11996500 hz    (exactly equal to our BFO frequency)
    Our 7.2mhz lower sideband:  44993000 - 32995000 = 11998000 hz  (in the middle of the crystal filter passband)

That's how the original uBitx code works.
I believe there is a problem because where an LSB signal hits the 45mhz filter will be 4khz removed from
where a USB signal hits it, resulting in a different audio quality between the two.
A solution to this is found in post 44278, review the previous posts in that thread to see why.
Even with the fix, there will be differences in the audio unless the 45mhz filter response is flat
across the 2000 hz at the center.

Jerry, KE7ER


On Wed, Mar 14, 2018 at 09:07 am, Michael Shreeve wrote:
Thankyou Jerry for trying and fairly successfully explaining what Ashhar is doing by using the ACTUAL frequencies used in the uBITX.

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