I've used one of these cheap switch-mode DC-DC converters (available on eBay for a few bucks) for my WSPR U3S transmitter. Checking the transmitted signal reveals strong spurious emissions. Hints from the QRPLabs group leads me to the cause - the DC-DC converter. Replacing it with a linear LM317 one solves the problem.

Spectrum analysis

To get an idea on the output quality of this switch-mode DC-DC converter, I hocked it up to an SIGLENT SSA3021X spectrum analyzer. The converter was fed with 13.9VDC from a linear regulated bench power supply. As load I've used Re:load Pro I bought some years ago on KickStarter.

To protect the input of the spectrum analyzer, I've put an 1.5nF foil capacitor in series with the analyzer input for some measurements.

Load Scan width 1MHz Scan width 100MHz Scan width 1MHz Scan width 100MHz
  1.5nF capacitor in series 1.5nF capacitor in series direct connect direct connect
0mA

1MHz NoLoad

Peak voltage ~2.5mV

 

1MHz NoLoad

 

100MHz NoLoad

 

100mA

1MHz 100mA

Peak voltage ~ 10mV

100MHz 100mA

 

1MHz 100mA

 

100MHz 100mA

 

1.000mA

1MHz 1A

Peak voltage ~35mV

100MHz 1A

 

1MHz 1A

 

100MHz 1A

 

Now the question is, why do these 10mV riples (you can see the switch-mode converters frequency 56kHz) have such big effects on the output of the DDS on the U3S main board?

Scope measurements

Next round - connect a digital oscilloscope with a 1:10 test head to the switch-mode DC-DC converter. The channel is bandwidth-limited to 20MHz with AC coupling.

load input terminal output terminal input terminal output terminal
  Bench power supply Bench power supply LiFePo accu LiFePo accu
100mA  

Output 20MHz 100mA

Here we now have ~200mV peaks

Input 20MHz 100mA

 

Output 20MHz 100mA

 Even a little bit more ~230mV

1.000mA

Input 20MHz 1A

Is this load regulation of the

bench power supply?

Output 20MHz 1A

More load things get worse ~430mV

 

Input 20MHz 1A

 Hm - even with thick cables

and big power pack!?

Output 20MHz 1A

 A litle bit less ~400mV

 

 

 

2014 CW (Morse Telegraphy) was added to the German Nationwide Inventory of Intangible Cultural Heritage.

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