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Some earlier BITX transceivers did use a ring mixer but the carrier suppression was
not noticeably better than with the present design. Allison's comments are valid and
reflect a good place to start looking.
Placing the BFO down the edge of the crystal filter passband helps improve carrier
rejection and improves audio by filtering off unneeded lower frequency voice products.
You can tailor the lower frequency speech response by how far down the filter skirt
you place the BFO.
If you have means to do spectrum analysis you can measure crystal filter response
and use that information to determine where the BFO should be set. If you do not
have spectrum analysis tools, it is still possible to do a manual sweep by adjusting the
BFO in small (20 to 100 Hz) steps across the filter passband and plot the filter output
using a diode detector and graph paper. Note that linear graph paper will show an
expanded curve where log graph paper will show a more conventional decibel curve.
If noise is being injected into the balanced modulator via the microphone or microphone
amplifier it will not be possible to get a deep null in carrier balance. It is conventional
practice to short the microphone input while doing carrier balance adjustments and
Allison's comment regarding possibility of undetected ultra-sonic tones present that
can cause unwanted carrier insertion is something that we do not usually look for.
A quick look at modulator AF input with an oscilloscope would probably show any
Typical crystal ladder filters usually show a steeper curve on its lower sideband than
on the upper sideband. Some of the high-IF designs ignore that and as a result will
On Fri, Apr 27, 2018 at 8:50 PM, Tim Gorman <tgorman2@...> wrote:
I've never been able to get over 25db of carrier rejection according to