When I am home growing a new rig I like to first "noodle" the expected result. When I use one of the standard FOAM modules (Filter Oscillator Amplifier Mixer), in advance I have a calibration that a properly operating IRF510 RF Final should produce more than 100 milliwatts on 40M. This process works pretty well for non-SDR radios. Throw that out the door when you add a single board computer to the hardware set.
From time to time, I drag out my home grown 20M SDR transceiver. There was such an event yesterday. The main SDR board can be interfaced with a variety of SBC's including an RPi3, 4 and 5 as well as a Windows 11 machine and the ASUS Tinker Board. All of these computers are running QUISK from N2ADR. This is where the mystery starts.
Home Grown SDR
R Pi 5
No contest as the Linux based machines performs much better using QUISK than anything Windows. The only reason I have QUISK on the Windows 11 Machine is that I can now operate the Hermes Lite 2.0 which is 100 feet away from the shop computer with five different software programs including QUISK.
Prior to Python 3, the ASUS boards were better than the R Pi Boards. But I have been unable to update the Tinker Boards with Python 3. The latest versions of QUISK require Python 3. So as a class the R Pi Boards work with Python 3.
Changing nothing with the Analog hardware, the results with the various R Pi's is not predictable and therein lies the mystery. I have always had difficulty with the R Pi 4 to work with QUISK, and my home grown SDR. No problem with the R PI 3 or R PI5. The issue with the R PI 4 is the external USB sound card. Remove that device and it works FB but install it and the spectrum literally bounces all over the screen.
I thought I might have a faulty R Pi 4 and a 2nd purchase resulted in the same issue. The R Pi 4 and R Pi 5 are running in the 64-Bit Mode. Taking the SD Card out of the R Pi 4 and swapping into an R Pi 5 and it runs without the jumping of the Spectrum. So, the problem must exist with the R Pi 4 system architecture.
I have two R Pi 5's and swapping the same SD Card between the two results in different performance. The problem du jour is that I see a trace of a recurrent spike on the received spectrum.
Noise Spikes
Now, I have a desktop machine running Linux Mint 20 and I spotted the same type of spike, even with the machine off. As I found out many of the desktop machines even though off have a Wake on LAN capabilities, so, they are not really off. When I unplugged the Mint 20 machine the spike at that time disappeared as you could hear the spike in an adjacent KWM-2. So, I am suspecting something new in my home is the culprit. I replaced the Mint 20 Computer power supply with a quality low noise unit and that seemed to fix that problem.
[I seem to recall friend N2CQR had a recurrent noise spike issue traced to a new tread mill.]
But using different SD Cards in the R Pi 5 has variability in the presence of the recurrent spike with one SD Card only showing a slight trace and the other is much more apparent. The awful truth just like blondes versus redheads, all SD Cards are not the same. Plus, we all know blondes are "spikey". Likely you really need a premium SD card to exact the best performance overall. It follows there are blondes and there are blondes!
Them that know at times may not know.
73's
Pete N6QW
