Re: Digital BFO Mod: Terrible Audio! #bitx40help

Tim Gorman


Of course you are correct. I plead sleep deprivation at 3am and a
faulty memory. The low side is the one with the worst slope.

I have had a chance to go back and look at my notes since then.
According to my notes when I was building crystal filters show that
having the steepest slope on the high frequency side of the audio
provides the best bandwidth restriction in the received signal, ie.
cutting interference and noise. The low audio frequencies are typically
attenuated in the transmitter mic circuits by using lower value coupling
capacitors, i.e. high pass filtering. So the crystal filter is not as
important for attenuating the low frequencies in the received signal.

And you are correct about balanced modulators today. The filter doesn't
need to provide nearly the amount of carrier suppression as in the
"old" days.

I still believe Ashar has made the best overall choices in the way this
rig is designed.

tim ab0wr

On Fri, 16 Mar 2018 11:22:45 -0700
"Jerry Gaffke via Groups.Io" <> wrote:

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
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 attenuation.

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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