Re: ubitx circuit, wiring, source code on github

Ashhar Farhan

Earlier, the calibration routine put out a 10 MHz signal out the antenna (by unbalancing the diode mixer and setting the first oscillator to 10 MHz). I chucked it because it assumed that everybody had a dummy load or a 10 MHz antenna. 

On the other hand, it is easier for a ham to call up someone and ask them to call CQ on a particular frequency. The way the present calibration works is like this. I call up  friend to call CQ from his calibrated radio exactly at 7050 KHz. I set the dial to 7050 KHz, Then you switch on the calibration and tune until it goes dead zero beat.  If only life was that simple. 

First, the crystal filter has a steep skirt. We would ordinarily celebrate this. But in our case, that means, unless the signal is a 30 db over S9, you are not going to hear the zero beat, much less the swing to the other sideband.

Second issue with this is that as we recalibrate the VCO and reset the oscillators, the BFO too is corrected by an equal amount and the zero-beat will go away. To compensate for this, this is what I came up with, bear with me...

The VCO frequency is at 875 MHz. We generate any needed frequency by dividing the PLL by a fraction. So, to generate 1 Mhz, it is divided by 875. Now, suppose our reference that we are trying to align with is at 1 MHz, we need to nudge the 875 MHz VCO. If the VCO moves by 875 Hz, we can see the 1 MHz move by 1 Hz. To move the 1 MHz by 10 Hz at  time, we have to move the VCO by 8750 Hz at a time. Our BFO is at 12 MHz, if we push the VCO up or down by 875 Hz, the 12 Mhz will be  moved by 12 Hz. If we push the VCO around by 8750 Hz, the 12 Mhz BFO has to be corrected by 120 Hz. This is what the code does. 

- f

On Fri, Dec 8, 2017 at 1:14 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
To scan the 12mhz filter passband, I'm thinking we drive CW-KEY high to unbalance the first mixer,
freeze clk1 at 57mhz, then scan clk2 and clk0 around 45mhz and 12mhz respectively,
monitoring the resultant audio beat note with scope or AC voltage meter.

Since the first mixer is unbalanced, anything on clk2 will continue on through to the second mixer
where it mixes with the 57mhz clk1 to create something near 12mhz. 
If clk0 varies from 12.000mhz down to 11.990mhz while clk2 varies from 45.000mhz to 45.010mhz, we should hear a constant
1khz audio tone that varies in intensity according to the 12mhz crystal filter's response to the 57mhz-clk2 frequency that
is passing through it.  This could be plotted on graph paper while reading the AC audio voltage and the frequency
of 57mhz-clk2 from the display, 

At any rate, nothing I am bringing up today should slow hfsigs down in building those rigs.
They will likely work fine as shipped, and all these minor adjustments will give the tinkerer's among us something to do.

Jerry, KE7ER

On Thu, Dec 7, 2017 at 10:09 am, Jerry Gaffke wrote:
An alternative would be to scan clk2 and clk0
through the crystal filter passband such that a constant frequency audio beat note is heard, and have the
user identify where the amplitude falls off at each edge of the 12mhz crystal filter passband.

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