[hpsdr] HUM on piHPSDR SSB TX signal

[Mike Schwendeman]

Good day, all!

This is my first posting on this system; -Hopefully, I will be successful.

I have just finished v.2 of my two piHPSDR units.  Both are running the 
latest version, v.1.1.1-Beta.  What WONDERFUL software this is!!!  The 
advancements from my past v.1.0.7 to 1.1.1 is so dramatically advanced!  
At first, I had some operator issues, but after looking in the vastly 
changed menus, I quickly saw my resolutions.

In both of my units, I have some HUM when I transmit in SSB.  It was a 
lot worse at first, with my v.1, but after some changes, it is now 
tolerable, mostly.  It is now low to the point that some do not notice 
it, unless I mention it...

I will list some "points of interest" or observations that have been 
looked into or simply observed, as I have experimented:

- If I simply TOUCH the metal of the USB and Ethernet connectors in my 
Raspberry PI 3, the hum almost completely goes away!  (I am standing at 
the bench, insulated sandals, not touching any other thing but the metal 
connector shells.)

- Armed with this finding that is I simply touch the metal shell of the 
'Pi's connectors, I tried touching other parts of the assemblies, no 
change or similar improvement.  (!!)

- I added a jumper wire between both ends of the short USB cable that 
allows me to provide a possible added/needed ground between the 
USB-to-RX-and TX audio dongle connectors; -No change at all.

- I have run with three small 5 volt power supplies, all have the same 
AC hum in SSB transmit.

- The above-mentioned 5 volt supplies are two units that are rated at 
2.5 amp, and one that is "rated" at 4 amp.  All have the same HUM on SSB 
transmit.  (The 4 amp one that I referred to as "rated" is a bad unit, I 
believe; -The piHPSDR causes this unit to sag to 4.8 volts!)

- I have tried first a 150, then a 1100 uF capacitor across the piHPSDR 
unit's DC input connectors, no change at all, when I clip them in and 
out with sturdy jumper-clips.

- When I feed the units with my Astron RS-35 (set to 5.00 volts at the 5 
VDC input port on the units), I have no to very low hum!!

- I have tried REVERSING the plug's polarity on the 2.5 amp 5VDC 
supplies; -No help, no change.

- I see no change to the transmit hum, when I change from v.1.0.7 to 1.1.1.

- I do have a 3 amp voltage regulator (LM-323?) that has not yet been 
pressed into service due to time limitations.

- I have tried USB-headsets, both of these have the same HUM. (These 
headsets were plugged directly into the 'PI's USB ports, no USB 
extenders were used.)

- The power level settings (0 to 100%) make NO difference in the hum 
level, which was surprising to me.

- The MIC LEVEL settings effect the hum as well as the transmitted 
signal, of course...

- I have eliminated the USB extender cable(s) by plugging the USB "sound 
card" dongle directly into the 'PI.  (As now set up, the extender cable 
is needed, as the USB sound-card is inside the piHPSDR's bases.)

- I have tried three USB audio in-out dongles, some are worse than 
others!  My point: they are indeed DIFFERENT.

So, I am curious as to what I am missing.  I have considered a more 
proper 5 amp power supply, but from my past experiences with the three 
now used, I am not hopeful.  My best results to reduce or get rid of the 
hum have been to
(1) touch the connector shells in the 'PI or
(2) to use the Astron RS-35 power supply.

Thank you in advance for any assistance.  Other than the TX-HUM issue, 
both of my units are sure nice.  (Great job, MELTON!!!) Your latest 
version (1.1.1-Beta) is very nicely advanced, and I appreciate the logic 
of the new menu layouts.

Thank you;   Mike, K0JTA