Re: uBitX assembled: bit of a challenge with alignment/bfo adj - Volume/switch knob fix

Arv Evans


It seems that a number of new users of uBITX are not familiar with the "zero-beat" 
process, and not familiar with BFO offset relative to crystal filter bandwidth.  

If doing a zero-beat and your BFO is not properly aligned then the received signal 
will not be placed properly inside the filter passband.  This Allows the opposite 
sideband to be heard over top of the zero-beat.  

Technical info..... When setting the BFO frequency relative to received frequency, 
the BFO should be set approximately 12 db down one side of the crystal filter 
skirt.  Which skirt is determined by which sideband is going through the filter.
You have a DSB signal prior to the filter and setting the BFO 12 db down the 
filter skirt further reduces the unwanted sideband.  Doing this also rolls-off low
frequency audio tones to make your sideband signal more efficient.
The uBITX assembly instructions do include a block diagram that should make 
this easier to understand.  


On Wed, May 27, 2020 at 10:14 PM <pulco.ant@...> wrote:
Hello. Got my full kit (board v6.2 / firmware v6.1) the other day and assembled it this morning.Easy as pie.

I managed to puzzle out (I think) how to get into alignment: hold tuning knob in while powering up. The screen calibrate _everytime_ is annoying. Zerobeating is challenging because there is a second hetrodyne that becomes audible and climbs in frequency *just* as I'm approaching zerobeat. So far, when I tune WWV afterwards, the display read 10,000.05, and I've done it a number of times. Setting the BFO - what's that about? I used an audio spectrum analyzer as 1 post suggested, but - is that all there is to it?

Volume/switch knob: I dealt with that by grinding down about 3/32" of the knob from the radio side of the knob. This allows that setscrew to move further down the switch shaft and it gets much better purchase on the shaft.It just doesn't have the same flare now as the tuning knob.

Please ring in with suggestions.

Thanks and 73
Dan D'Eon

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