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QMX+ All LPF Sweeps Are Bad #lpf #QMXplus

 

I have assembled and have working a QMX+, with one caveat;? all of the LPF sweeps are bad.
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The audio filter sweeps, RF filter sweeps, image sweeps, and SWR sweeps look reasonable, although on a couple of bands the SWR sweeps aren't flat across the entire bandwidth swept, but were always good in the ham band frequencies.? The I/Q sweeps all looked good except for 160 Meters where the phases were in sync for some reason.? Not really a problem for me, I don't own that much wire.
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Here's the real problem.? All of the LPF sweeps are bad.? For example:
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0 ?| ? ? ? ? ?* .* ?* ?* ?* .* ?* ?* ? ? . ? ? ? * ?* .* ?* ?* ? ?. ?* ?* ?* ? .
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? ?| ** ** ** **.* ?* ?* ?* ** ** ** ** ** ** ** ** * .* ?* ** ** ** * ?* ?* * *
? ?| ** ** ** **.** * ** ** ** ** ** ** ** ** ** ** **.* ** ** ** ** ** * ** * *
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-8 |* ? ? ? ? ? . ? | ? ? ? . ? ? ? ? ? ?. ? ? ? ? ? ?. ? ? ? ? ? . ? ? ? ? ? ?.
? ? ----------------------------------------------------------------------------
? ? ? ? ? ? ? ? 6 ? ? ? ? ? 9 ? ? ? ? ? 12 ? ? ? ? ? 15 ? ? ? ? ?18 ? ? ? ? ? 21
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-7 |. ? ? ? ? . | ? ? ? ?.* ? ? ? ?. ? ? ? ? . ? ? * ? ?. ? ? ? ? . ? ? ?* ?.
? ? ----------------------------------------------------------------------------
? ?16 ? ? ? ?24 ? ? ? ? 32 ? ? ? ?40 ? ? ? ?48 ? ? ? ? 56 ? ? ? ?64 ? ? ? ?72
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All of the other bands exhibit a similar pattern.
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I tried looking for something common to all of the filters, but nothing I came up with made any sense given that the other sweeps were 'normal'.
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Since everything looked OK except for the LPF sweeps, I decided to perform a real-world test.? I hooked it up to my EFHW at 20 feet and worked Oklahoma on 40 Meters FT8, so the transceiver is working.? I don't have the necessary test equipment to check the spectral purity of the output signal so I'm hesitant to continue using it on the air until I resolve this problem.
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I tried searching for a solution both here and with Google and read a lot about fixing issues I don't have, but nothing that pointed me in the right direction.
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Thanks all for the great ideas I'm about to receive.
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Andy
WB9YXA
 

The LPF graphs give the impression that the LPFs aren't working, that there is a short bypassing all the LPFs regardless of the band selected.

One check would be to look for the 3rd harmonic being transmitted.? Do you have a receiver with a waterfall display? A SDR perhaps?? Try a test transmission into a dummy load and see if you can receive it at the fundamental frequency and 3 times that, for example 7.074 MHz and 21.222 MHz.? The dummy load will probably radiate enough that you can receive the signal locally.? If you get a significant signl on the 3rd harmonic then the LPFs aren't working.
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Difficult to see why, other than some major miswiring.
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Chris, G5CTH
 

I found the problem.? I was curious about the output power and turned on the power display.? When the display didn't show power or swr, I started troubleshooting that symptom.? It didn't take me long to to track the problem down to a bad solder joint on T507.? Repairing that connection corrected the LPF sweep issue and the swr display.? It makes sense, the measuring circuit was common to all of the LPF paths, thus the across the board impact on the sweeps.
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Thanks.
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Andy
WB9YXA?
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