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Tim? AB0WR 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 measurement. 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 such problem. 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 |
