Re: Digital BFO Mod: Terrible Audio! #bitx40help

Jerry Gaffke

Crystal ladder filters tend to have a steeper skirt on the upper freq side than on the lower freq side as Tim suggests.
See fig 3 here:
Other than that, I disagree.

I'd say that unless other factors override this, the BFO frequency is better off above the crystal filter
to get maximum opposite sideband rejection, using the sharper skirt on the upper side. 
This also maximizes carrier suppression, though with a balanced modulator this may not be
a primary consideration.  Once you get rid of the opposite sideband, you can shape the signal
further with audio filters, though this generally isn't needed.

With the 6 pole crystal filter on the uBitx, both skirts are fairly sharp, much better than the Bitx40.
So having the BFO above or below the crystal filter works well enough.

A second consideration is that the local oscillators should be above the intermediate frequency.
On the uBitx, we have the VFO above the 45mhz first IF, for a 7.2mhz signal the VFO is 7.2mhz above 45mhz.
With the high side VFO, all VFO harmonics are up in VHF and beyond, and products of those harmonics are
more easily gotten rid of. 

On the uBitx, clk1 would best be 12mhz above the 45mhz first IF, to create a 12mhz
signal into the second IF.  However, on the uBitx we have the further issue of Nano 16mhz and 12mhz oscillator
harmonics mixing with BFO harmonics, creating those audio tones the some posts here have complained about,
so we leave the BFO below the 12mhz crystal filter and use have clk1 above 45mhz for USB, below 45mhz or LSB.

Jerry, KE7ER

On Fri, Mar 16, 2018 at 01:00 am, Tim Gorman wrote:

One thing to remember is that crystal filters have always been
considered to be *upper* sideband filters primarily.

They don't have infinitely steep firewalls at each end of the bandpass.
Think of a crystal filter having a slope of 60deg on the low frequency
side and a 45deg slope on the upper frequency side. It's not that
bad but it illustrates the point.

The filter works best when the carrier is on the low frequency side of
the crystal and the audio is above the carrier. The high frequencies in
the voice are usually lower in power anyway so the increased slope on
the upper frequency side of the filter still provides adequate

If you switch and put the carrier on the upper frequency side of the
filter and the audio lower in frequency than the carrier then you are
using your filter where it works the worst. It's not so bad for
transmitting other then it doesn't suppress the carrier as well but for
receiving it tends to let low frequency noise *and* signal through more
than if you use it as an upper sideband filter.

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