More adventures in filtering the power supply for an AFE-822 SDR

[ChrisSmolinski]

I ended up building an LC filter to put on the output of a wall transformer: http://www.radiohobbyist.org/blog/?p=2027&preview=true


Before:


And after:

[TheRelayStation]

as a quick test to see its effectiveness, can you switch between the wall wart and two 9V batteries in parallel while in use ?
like a temporary switch in place that you can switch to the battery and then unplug the wall wart, you could observe any changes in real time.

[ChrisSmolinski]

I'd have to stop and re-start the SDR when switching power supply sources.  A battery would a good test, though, as the lowest possible source of noise. I've got a large 12V battery I could use.

[Pigmeat]

If you only installed a potentiometer with a sturdy, insulated easy to grip surface to adjust it, like a knob, between the power source and the SDR you wouldn't be having these problems.

[skeezix]

If you can find it, try a 5V battery into the USB port. That doesn't use the internal DC/DC converter.

[ChrisSmolinski]

I took a look at the AFE822 schematic. All the power supply circuits seem to be on the same page, I didn't see a DC/DC converter, unless it is lurking elsewhere.

The incoming DC is run through an LM7805, that 5 volts is fed to three linear regulators that produce 1.8 volts, and two 3.3 volt supplies, one for analog and one for digital circuits. The 5 volts from the USB connector is also fed to those regulators, but through a P-channel MOSFET, which is switched off if 5 volts is present from the 7805. So there's some protection in case you apply power to both the DC jack and USB.

There's a fuse and 18V TVS diode on the DC jack, so any voltage above that will be clamped, and eventually you'll open the fuse. There is also a fuse on the USB port power pin.

[TheRelayStation]

I took a look at the AFE822 schematic. All the power supply circuits seem to be on the same page, I didn't see a DC/DC converter, unless it is lurking elsewhere.

The incoming DC is run through an LM7805, that 5 volts is fed to three linear regulators that produce 1.8 volts, and two 3.3 volt supplies, one for analog and one for digital circuits. The 5 volts from the USB connector is also fed to those regulators, but through a P-channel MOSFET, which is switched off if 5 volts is present from the 7805. So there's some protection in case you apply power to both the DC jack and USB.

There's a fuse and 18V TVS diode on the DC jack, so any voltage above that will be clamped, and eventually you'll open the fuse. There is also a fuse on the USB port power pin.

i assume youre using the USB connection from the AFE822 to a PC ?
i sliced open my USB cable and applied small chokes on each wire (D+, D-) at both ends of the cable (except i disconnected V+ at both ends and tied it into GND and assuming GND is equal) and it significantly reduced all kinds of RF and images generated from the PC (RF generated by a PC could potentially travel through the USB directly into the unit as well as the USB cable itself acting like an antenna from the PC and as itself, hence the use of chokes on both ends)

[skeezix]

Never mind... the older AFEDRI's have that. The newer ones have a linear in 'em, as well as the AFE822x.

From the manual:

Attention!!! AFEDRI SDR-Net 2.1 and down have switched DC/DC converter to connect to external power supply (this was done to decrease power dissipation). Because there is switched DC/DC converter that is working on 930kHz frequency, so there interference is possible on this frequency or its harmonics. Beginning AFEDRI SDR-Net rev.2.2 it has linear power supply it does not have interference from embedded power supply.

[ChrisSmolinski]

i assume youre using the USB connection from the AFE822 to a PC ?
i sliced open my USB cable and applied small chokes on each wire (D+, D-) at both ends of the cable (except i disconnected V+ at both ends and tied it into GND and assuming GND is equal) and it significantly reduced all kinds of RF and images generated from the PC (RF generated by a PC could potentially travel through the USB directly into the unit as well as the USB cable itself acting like an antenna from the PC and as itself, hence the use of chokes on both ends)

Sounds like a good idea. You disconnected V+, so you're powering the SDR via the external DC jack instead of via USB?  I have seen some USB data cable that have built in chokes, I am not sure how effective they are at reducing RFI.

I'm using the ethernet connection for data to the computer, I'm not a fan of USB for that. I was using the USB just for power, but now I'm not using it for anything at all