Hi all.
My newly constructed 12v QMX is running the latest 1_00_009 firmware and is generally working well, but has two possible receive issues:
Receive Appears Slightly Deaf
My QMX appears slightly deaf on every band compared with a Yaesu FT-817. I tested CW receive sensitivity by connecting the antenna connector to the output of my signal generator via a step attenuator. The signal generator was set to generate 10 mV pp into 50 ohms (-36dBm). I then increased the attenuation until the receive tone was only just audible. At this threshold level the audio is much poorer quality than the 12dB SINAD that would typically be used for sensitivity measurements. However, since I don’t have a SINAD meter, I used this test criteria as it is easily and reliably reproducible between measurements.? I measured a threshold attenuation of 70dB for 14MHz, and 80dB for the other 4 bands on the QMX.
However, when I run the identical test setup with my FT-817, the threshold attenuation is 100dB, 20-30dB better than the QMX.
I would not have expected such a large difference, as others have reported similar QMX sensitivities to their normal rig. Measurements on an ICOM IC-705 show a 10-20dB improvement over the FT-817, an even bigger disparity from the QMX.
The Low Pass Filters can be eliminated as the source of any problem since transmit power is fine at 4-6 watts depending on the band, with all distortion products 50-60 dB below the fundamental. Having said that, the 20m RF sweep shape has less attenuation at lower frequencies than other filter responses that have been posted here. Could that be a sign of a build problem?
The Audio Filter Sweep results and Image Sweep results show the expected shape, except the measured pass value of -13 on the vertical axis is lower than others posted here (e.g. /g/QRPLabs/message/105782 ).
Does anyone know whether the numbers on the vertical axis are absolute values or relative and are they approx. dB?
I have rechecked and resoldered all joints around L401, T401 and C401-C404. I have also triple-checked the number of windings on L401 and T401.
Is this an issue with my build or do I have inappropriate expectations of the receiver’s sensitivity? If it is a build issue, what could it be?
Lots of Birdies?
There are lots of birdies, with some very strong - e.g. at 14.074.80 MHz. They are present with just a dummy 50 ohm load connected, all test equipment turned off and running off battery. Other example birdies are at 14.075.60 MHz, 14.076.00 MHz, 14.077.70 MHz, 14.094.50 MHz, 14.115.10 MHz to name just a few. I can find no obvious pattern.
I wondered whether the birdies were related to the PSU pwm signals. I measured the pwm frequency to the 3.3V and 5V boards, and the frequency to both is 116.345 KHz. This pwm frequency doesn’t appear to change, as I would have expected it to, when I change bands or move within the band. Could Hans’ cunning Dynamic Noise Relocation have been accidentally disabled in more recent releases (I had the same birdie problem with version 1_00_007)??
Any thoughts or suggestions on either of these problems would be greatly appreciated.
Dave
M0JTS
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