Re: Simple spur fix

Warren Allgyer


We may be talking "past" each other or I may not understand the intent. But as I see it the added filter is indeed a passband filter for the 45 MHz LSB/USB signal.

Referring to the attached scan: Blue trace is the filter passband measured out of the circuit with a 50 ohm in/out. Purple is measured with a 10x scope probe on the S/A input and attached to the same filter in place in the circuit. Purple is the input side of the filter. Yellow is the output side of the filter. So the screen reads top to bottom: Purple input, Blue filter, yellow filter output.

The two "humps" on the purple and yellow traces are the LSB and USB signals respectively with the audio input being swept from 0 - 5 kHz. Due to the way the 45 MHz SSB signal is generated and later heterodyned to operating frequency, the right side of both humps is the low audio frequency and the left is the high frequency. While the sweep was done from 0 - 5 kHz, the humps themselves represent only about 300 Hz (on the right side) to 2.7 kHz on the left. The 'scalloping' evident on the purple trace comes from the response curve of the 12 MHz crystal lattice filter in the SSB generator.

My comment about the shape of the newly added filter is evident by comparing the yellow output humps to the purple input. The right side of the leftmost hump and the left side of the rightmost hump are attenuated by the filter shape. This will have the effect of attenuating the low audio frequencies on LSB and the high frequencies on USB.

Some good news is the eBay filters appear to be a good match for the SSB generator as both humps are centered in the filter passband. So I think I am in good shape to do some testing on the effect the added filter has on the spurs. That will have to wait until the afternoon as our club technical breakfast is in about an hour.

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