I think you might be over thinking this, Mike.
Once you have the rig working its easy, at least it was for me.
If you have ever had a rig with adjustable BFO or Passband tuning, its obvious from the sound of the background noise
where your BFO needs to be.
Then tune some weakish signals and see if the peak is at the pitch you prefer.
Then hit transmit and adjust the TX offset to the same tone. Listening to your own signal for the sidetone is one
of the great features of the NC40 I think, same thing I have always done with my big rigs.
With a matched set of Chuck's crystals and the specified circuit values, I can (just) get my desired 400Hz.
If you like a higher tone, you probably need to change the osc. parts or the crystals.
If you can get your TX offset to go through zero beat, you can confirm you are on the right sideband, I had to do this
with my other rig.
73,
Gary
WB6OGD
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On 11/26/2017 8:37 AM, Mike WA8BXN wrote:
Thinking about this a bit more ... once C17 is properly set it may be that the setting of C34 is trivial (well more or less). Since the transmit sidetone is generated by listening to the transmitted signal in the receiver, C34 may simply be adjusted to get the same offset tone used in setting? C17 when transmitting (into a dummy load of courI thise).
73/72 - Mike WA8BXN
From: [email protected] <[email protected]> on behalf of Mike WA8BXN <hubby2k@...>
Sent: Sunday, November 26, 2017 9:51 AM
To: [email protected]
Subject: Re: [qrp-tech] Things to watch out for building crystal oscillators
Hi Doug ---
The other two crystals both have trimmers associated with them. Should be able to adjust them a few hundred cycles.
The real questions: Would we actually prefer X5 or X6 to be some amount above or below the filter crystal frequency? If so, by how much? That is to say for both those crystals (and I would expect different answers for each) what would be the optimal frequency to have with their associated trimmers set to mid range?
And a related question: For those with minimal test equipment, what is the best way to adjust C34 and C17?
For C17 we would want to place a test signal at the center of the passband of the crystal filter and adjust C17 for the desired tone at the headphone output (600 Hz for example). The test signal could be at the IF or 7 MHz frequency range.? Knowing when it is centered in the crystal filter passband is the tricky part. Just looking for max audio output may not be best due to audio filtering effects. A scope looking at AF1 is probably best.
Once C17 is set properly C34 should be adjusted so we get the same offset between transmit and receive. That is for a station we are talking to if they are "zero beat" to our frequency we will hear them where we should. Its not clear what easy way will accomplish that.
Other related questions: If the crystal filter crystals are not well matched, will we hear any difference in received audio? I suspect the filter will be a little wider. Build on board #2 here will use sockets for X1-X4 and try to investigate that. It may well be we are getting excited about perfect matching when it doesn't make that much difference to the average user of the rig.
73/72 - Mike WA8BXN
wb6ogd