Thursday, October 23, 2014

Minima controller shield - Using second Si570 as BFO.

I have implemented changes to the Minima controller code developed by Eldon Brown (WA0UWH) and others to use the second Si570 on my controller shield as the BFO oscillator.  This change allows easy setting of your Minima BFO oscillator frequency to the desired point on your crystal filter passband curve, be that at the -3 dB point or further down the skirt.

The IF edit mode of the controller firmware will allow the adjustment of the BFO frequency.  Separate frequencies are maintained for USB and LSB.

A separate issue is regarding the calibration of both of the Si570 devices on the shield against some frequency standard.  For most folks that have a crystal oscillator for the BFO, the IF edit mode has probably been used to calibrate the dial to some frequency standard and this is fine.  However adjusting the IF frequency offset from the displayed frequency only allows you to match the value that the controller uses to whatever frequency your crystal oscillator BFO is tuned to whereas with the second Si570, you can actually move the BFO frequency in software which is not the same thing as calibrating the dial.

The calibration of the Si570 devices as I see it is a separate matter from setting the desired BFO frequency as you certainly don't want to change where the BFO frequency sits on the IF passband just to calibrate the dial.  So, I anticipate providing a different mechanism that will allow moving the VFO frequency (and BFO frequency for those using a second Si570 as the BFO oscillator) without changing the displayed frequency in order to determine the error offset for each Si570.  This error offset can then be stored in eePROM and applied automatically from that point forward to the appropriate Si570.

More to come on this topic...

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