Impedance Transformers and tapping SDR - Page 1
Forum section: electronics / rf-microwave

author: metrologist
author_rank: Super Contributor
timestamp: April 09, 2021, 12:09:31 am
content: I was reading some articles about tapping into the I/Q pins of those cheap USB SDR radios and the topic of impedance transformation came up.One article mentioned that the impedance of the SDR I/Q pins is ~3300 ohms and the best suited commercial impedance transformer is a small minicircuits T16:1... or you can wind your own balun on a binocular ferrite with 20 turns secondary followed by 5 turns primary (4:1). Then I got confused because even though I've read that the ratio of turns to impedance transfer is a square function (4^2 = 16), it seems to test a 4:1 balun you load it with 200 ohms resistor (50 ohms : 200 ohms). Why is the load not matched with 800 ohms if the impedance relationship is supposed to be a square function?Anyway, the article mentioned the correct way to tap I/Q is to have a transformer between the I & Q pins to provide a balanced input, so I set off to make my balun and mod my SDR. I should note that I had already tapped the I pin with a long wire, and it works quite well on ~15 feet of wire strung inside, but there are a lot of images and aliasing, etc...Then it occurred to me that the SW only selects either the I or the Q pin as input, so now I don't even understand the purpose of having the transformer between the I/Q pins? Why could I not just run the transformer secondary to one of the pins and then the other side to ground, or even just leave it floating? All I really want to do is try some filtering and an antenna tuner, but I can't do that with just a single wire.BTW, I made a toroidal balun using the ferrite from a burned out CFL swirl lamp and wire from its transformer, which measures ~53, -0.1i @ 1MHz and swings to ~51, -0.4i @ 25MHz (~ 30 to 24dB return loss). see picLast, I'm curious what this commercial transformer is. It looks like a twisted trio with 5 turns on one side and 4 turns on the other. I suspect the primary is the two pads on the back side and the three pads on the front are the secondary with a center tap, so you can get 1:1 or 1:2.

author: coppercone2
author_rank: Super Contributor
timestamp: April 09, 2021, 12:48:02 am
content: also might be useful for small portable radios, i.e. the tecsun does not have a 50 ohm impedance as far as I know, its out there, I believe it is in the kOhm range.Interesting to see 3Kohm impedance for a changeTypically if you have a differential input ADC, and you ground the input, you get 50% of your input range, not sure what the radio you have is doing but you get the idea, you need a differential output amplifier if your input signal is single ended to a differential ADC for the best results.You might want to put diodes behind the transformer though, typically sensitive differential input RF stuff has a clamp on it with 2 diodes, but hopefully your circuit has them.

author: metrologist
author_rank: Super Contributor
timestamp: April 09, 2021, 01:35:33 am
content: This is the common USB SDR radio and the mod.https://www.rtl-sdr.com/rtl-sdr-direct-sampling-mode/This is the upgraded version, ~$28 on ebay, so I'm tempted to just get it but I already have two. The first I bought was an early clone that has a daughter board for quadrature injection and two SMA connectors. It's not quite as good as the second I have, which is a nooelect that I simply tapped the I pin.https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/

author: radiolistener
author_rank: Super Contributor
timestamp: April 09, 2021, 02:36:23 am
content: I think transformer for I/Q streams is a bad idea, because you will never get two transformers with exactly the same phase shift and frequency response.

author: metrologist
author_rank: Super Contributor
timestamp: April 09, 2021, 11:03:14 pm
content: Quote from: radiolistener on April 09, 2021, 02:36:23 amI'm not sure about two transformers. I redid the mod to the SDR and found out that the tap point was the differential analog input Positive (Q path) and saw there is a second tap for the Negative Q path. I put my high impedance side to those two pins and cabled up my antenna tuner and...Nada. No signal on AM broadcast. I guess the coil around the torroid just shorts the inputs and no difference is detected.