Quiet switch mode power supply (SMPS) for KiwiSDR
[from Peter, ZL2LD]
I have spent many hours testing different power supplies for the Kiwisdr and the new Kiwisdr2 radios.
My reference power supply for tests is a 12V lead acid battery with a 7805 linear regulator. It is ultra low noise but that’s not practical long term. I experimented with different power supplies (SMPS). I tried several different 5V Switched Mode Power Supplies (SMPS) and also made up a quiet supply using a 9V SMPS with a common mode choke and 7805 regulator with 1000MFD capacitor. The 9V/7805 was a lot quieter than the 5V SMPS but still not as quiet as the 12V battery/7805.
The SMPS noise is mostly visible at LF and lower frequencies.
I read that Apple make low noise phone chargers which makes sense when you consider that an iPhone has multiple radio receivers (mobile phone, Bluetooth, GPS, WiFi and NFC) and they want all those radios to operate well even while the phone is being charged.
I found a teardown review of the 20W USB-C iPhone and iPad charger where the reviewer looked at the noise on the DC output on a spectrum analyser. He then compared that to a no name Apple clone. The difference was huge.
So I went and bought a new Apple 20W USB-C power adaptor to test.
To get the USB-C to a 5V 5.5mm/2.1mm DC plug is a bit tricky. USB-C power supplies can communicate with the device being powered to output multiple voltages but without any data comms they can be forced to output 5V with a sense resistor. I guess that is so the powered device can get 5V to be able to get started then communicate back to the power supply.
To make it work to power a Kiwisdr I bought a USB-C male to USB A female adaptor. It must have the right sense resistor built in. These are very common as they are sold so people can connect old legacy USB devices to their flash new computer. Mouser have them in stock and they are a couple of dollars on AliExpress.
Then I had to get a cable with USB A male on one end and 5V DC 5.5mm/2.1mm plug on the other. You can probably buy these from a specialty electronics store. I got a couple from AliExpress. They were cheap but took three weeks to arrive. You can easily make it yourself. Most likely you will have an old USB A cable laying around that you can chop one end off. If you don’t have a spare 5.5mm DC plug you can probably find a dead or surplus power supply that you can chop off the DC connector with a short cable and join it onto the chopped USB A cable.
The wire colours in the USB cable are normally Red for +5V and black for -VE/GND but it is a good idea to plug the chopped USB cable into a USB socket and use a multimeter to check the 5V and ground. You solder the +5V to the centre pin of the 5.5mm DC plug and the -VE to the outer of the plug.
I also found a few sellers on AliExpress of USB-C male to 5V DC 5.5mm/2.1mm cables that does the whole job.
But if you want to get your new Kiwi running today then it will be much quicker to build your own cable.
The results on the spectrum scope were amazing all the way down to VLF. It is as good as the 12V/7805 battery reference supply.
If you only listen to good signals on 80m and up you can probably get away with any old 5V SMPS but if you want a nice quiet supply 0-30MHz it is worth using the 20W Apple supply.
P.S. I could not get the Kiwi to start on my nice quiet Agilent bench power supply as the Beagle gave one blink of a blue LED then refused to start. I suspect that the Beagle draws a high inrush current and the bench supply doesn’t respond fast enough so the power management chip “protects” the Beagle. I even tried adding some large caps on the supply output but it still would not start. It is a bit too fussy in my opinion. Just thought I would mention that in case any of you have similar issues.
Peter Munn, ZL2LD
Here is my complete power supply setup
Here is the USB A plug wiring if you want to use an ohmmeter to trace the wires.