USB Capture card for DIY SDR - Page 1
Forum section: electronics / rf-microwave

author: gcrcien
author_rank: Newbie
timestamp: November 17, 2023, 03:55:17 am
content: Hello everyone, im really new to the forum, i have been dabbling with electonics for a while now and my last projects have involved RF in the HF ranges, for example a few VFO's for old radios and a whole transceiver from scratch.My last project is a simple tayloe receiver driven by a si5351 plus a bit of amplification after the demodulation, it can produce audio and it works well enough for what it is.I was wondering if theres a product that behaves like a soundcard but with higher trhoughput, in my case i would want to feed the I and Q signals to SDR++ but with only 40khz the bandwidht would be limited.i know sdr's are cheap but to learn all this i think the best is to do it yourself, and actually i do own a rtl sdr

author: Andy Chee
author_rank: Super Contributor
timestamp: November 17, 2023, 04:49:00 am
content: There was this 192kHz capture card, but I think it might be obsolete:http://www.wb5rvz.com/usb2sdr/Other than that, I'd also be interested in alternatives to my cheapo eBay USB 48kHz sound card.

author: gcrcien
author_rank: Newbie
timestamp: November 17, 2023, 06:21:43 am
content: Thank you Andy for your suggestion, but to be honest 195 euros seems a bit expensive to me, im not sure why there's no such a product it seems simple enough to make at least from an ignorant person's perspective.By the way, is this behaviour normal? the sensitivity of the system seems to be a bit too when the input signals are too closeThese pictures show a linear frequency sweep and the resulting signals on the I and Q ports at the output of the mixer when the LO and RFin are matched.Also is really cool to see the phase inversion of signals during the sweep

author: Berni
author_rank: Super Contributor
timestamp: November 17, 2023, 06:36:14 am
content: Yeah this is why you tend to run SDRs with the carrier slightly offset to avoid weirdness around the 0Hz point.The easiest way is as people said, get a cheap sound card that can do 192KHz or 384KHz. If you want to go really cheep then look for noname chinese USB sound card dongles on AliExpress or similar:https://www.aliexpress.com/item/1005006168143401.htmlIt is a lottery what you are gonna get. Some of them are likely pretty crappy with being noisy, missing AC coupling, having poor antialiasing filters etc.. But some of them might actually be really good products that might perform just as well as the expensive fancy sound cards from name bands.For going any faster than that on the cheep, then you need to get a RTL SDR dongle and build a mixer to place the signal you want to listen to inside the range of frequencies it supports.

author: magic
author_rank: Super Contributor
timestamp: November 17, 2023, 06:45:30 am
content: It would be a bit of a DIY project, but one might try to buy a Cypress FX2 breakout board (not too expensive, they are also sold as poor man's logic analyzers) and connect it with some parallel output ADC. Unfortunately, I'm not aware of ready-made products or published designs of this type.FX2 (and its compatible successor FX2LP) can pass over 30MB/s in good conditions, so 30MSPS at 8 bit or 15MSPS at 16 bit should be doable. One would need to read the datasheet to connect all the signals and arrange for clocking somehow. IIRC, there are option to use an external clock or to output 12MHz/24MHz from the FX2.On the software side, programs like fx2pipe simply download data from the chp and write it straight to a file. In order to work, the program uploads a small firmware to the chip over USB, which could be customized to set a few registers on startup and reconfigure the I/O interface, if necessary.

author: iMo
author_rank: Super Contributor
timestamp: November 17, 2023, 07:24:12 am
content: That is not a simple nor a cheap project, as the OP is looking for a dual channel simultaneously sampling (I/Q) 12-16bit ADC (he wants to see a wide "waterfall" on the screen, imho), pushing the data via the usb continuously into a computer.The big players do it with direct sampling of the RF with a single channel 12-14bit ADC and FPGA (ie at 130MSa/s like the IC7300), or they do ADC with a cheapo 16-24bit stereo audio codec in the audio baseband (like those 40kHz OP is mentioning).

author: magic
author_rank: Super Contributor
timestamp: November 17, 2023, 07:36:04 am
content: BTW, does it really have to be USB?Computer and laptop built-in soundcards often have 192kHz recording, extending usable BW to 80kHz.384kHz seems to be where it gets exotic, with prices to match.

author: iMo
author_rank: Super Contributor
timestamp: November 17, 2023, 07:45:53 am
content: With 192KHz sampling the BW of an I/Q is 192kHz.PS: that 192KHz sampling is perfect for ham radio, as the ham radio bands with the most activity are about that wide..

author: gcrcien
author_rank: Newbie
timestamp: November 17, 2023, 01:20:00 pm
content: It doesnt need to be that wide, the thing with normal soundcards is they are too slow.For example a 41khz samplerate would give me just over 20khz bandwidth and that with only one channel since most computes use mono for the mic input.The cards mentioned previously with 384khz sample rate are actually in the range i want, a 192khz wide waterfall is actualy great since for example you can see most of the 40m band on a single screen, i just need to ensure these are stereo input to acommodate the I and Q signals

author: Berni
author_rank: Super Contributor
timestamp: November 17, 2023, 05:39:49 pm
content: Then you must be really unlucky because most sound cards that i came across are stereo input. Most cards have a line in jack (this is always stereo), and if they only have 2 jacks then the microphone one is usually in stereo too.It is pretty much only the mobile phone jacks that use the 4 pin combined headphones+mic jack that they have that 1 extra ring for a mono microphone input. I have an 4 pin to dual 3 pin adapter for those so that they can work with a normal sound card.

author: gcrcien
author_rank: Newbie
timestamp: November 17, 2023, 05:45:06 pm
content: Yup, im using a laptop wich has only one input, combining a single mic and headphones :/

author: iMo
author_rank: Super Contributor
timestamp: November 17, 2023, 06:36:55 pm
content: Yep, the 192kHz sampling with a cheapo stereo USB dongle gives you 192kHz wide waterfall in the PC (with I/Q and the sufficient BW of your opamp amps).I can confirm that, I did it with SoftRocks ~20y backFYI - The legendary Softrock SDR series (the same as your radio above)http://www.wb5rvz.com/and perhaps the first SDR sw (written for Softrock radios), works greathttp://www.dxatlas.com/rocky/

author: radiolistener
author_rank: Super Contributor
timestamp: November 18, 2023, 11:44:27 pm
content: Quote from: magic on November 17, 2023, 07:36:04 amI'm using CX31993 USB DAC (24 bit 192 kHz mono ADC / 32 bit 384 kHz stereo DAC), but it has very sharp filter above 20 kHz, so despite the fact that it works at 192 kHz it is impossible to see signals above 20 kHz.But it has very good dynamic range. Third picture shows about -145 dBFS noise floor for open input. And it is confirmed with very clean and detailed sound from mic.Note: I'm using custom modification of sdr++ with fixed unity gain (0 dB on the spectrum equals to 0 dBFS and don't depends on used window function and resolution like it happens with original sdr++) and fixed audio_source code to support mono audio source.