Note: groups.io will be down for maintenance this evening, starting at 2AM Pacific Time (9AM Friday 10/17/2019 UTC), for up to two hours.
Re: Digital BFO Mod: Terrible Audio! #bitx40help
Crystal ladder filters tend to have a steeper skirt on the upper freq side than on the lower freq side as Tim suggests.toggle quoted messageShow quoted text
See fig 3 here: https://www.arrl.org/files/file/QEX_Next_Issue/Nov-Dec_2009/QEX_Nov-Dec_09_Feature.pdf
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.
On Fri, Mar 16, 2018 at 01:00 am, Tim Gorman wrote: