Preamp for a Red Pitaya SDR? - Page 1
Forum section: electronics / rf-microwave

author: elroy
author_rank: Regular Contributor
timestamp: June 22, 2019, 05:13:07 pm
content: I recently bought a Red Pitaya 122.8-16 and have been using it as a multi-band skimmer for FT8 as well as WSPR.Overall it's working well, but the sensitivity isn't as great as I expected. A German ham who is getting excellent results with his told me I should look into a preamp. He said he is using a 10 db push-pull amp that had been sold by the magazine Funkamateur as a Red Pitaya add-on. However, the magazine is no longer selling that amp.So I'm looking around for something, either pre-built, kit or plans. Any suggestions would be greatly appreciated.P.S. I understand that Red Pitaya's prior SDR model, the 125-14, required a physical jumper to be set so that it would be sensitive to the voltage range from an antenna. As best I can determine, there is no such jumper on the newer model that I just bought.

author: _Wim_
author_rank: Super Contributor
timestamp: June 24, 2019, 08:59:58 pm
content: What low frequency response do you want? Some of the low cost LNA's on Ali-express have quite good performance (actually measured performance, not only published specs)Example:https://www.aliexpress.com/item/32861323230.html?spm=2114.search0104.3.16.3de53d6crvZiXV&ws_ab_test=searchweb0_0%2Csearchweb201602_10_10065_10068_319_317_10696_10084_453_10083_454_10618_10304_10307_10820_10821_537_10302_536_10843_10059_10884_10887_321_322_10103%2Csearchweb201603_53%2CppcSwitch_0&algo_expid=892855d9-50f2-4f61-9f6d-9e08fecc01a1-2&algo_pvid=892855d9-50f2-4f61-9f6d-9e08fecc01a1&transAbTest=ae803_3I have one of these, -3dB at 200kHz (so not the advertised 50k), from there quite flat (+-0.75dB) to around 800Mhz with around 32dB of gain, and a NF of max 2.2dB to 1 GHz (measured on my HP 8970A). For the price not bad at all.

author: _Wim_
author_rank: Super Contributor
timestamp: June 24, 2019, 09:04:37 pm
content: Found an XY plot made with the HP 8970A (the XY-pico scope does not have a Z-input, so retrace is visible).Lowest curve: 0 -10 dB full scale NFHighest curve: 0-40 dB gainX-axis: 0-1600MhzThe low frequency behaviour was tested with my picoscope directly

author: elroy
author_rank: Regular Contributor
timestamp: June 24, 2019, 09:24:44 pm
content: Thanks! For that price, it's definitely worth checking out. I might be using it from 135 kHz to 54 MHz, but it seems as though its performance is pretty good across just about all of that range.

author: erikka
author_rank: Regular Contributor
timestamp: June 25, 2019, 01:58:50 pm
content: Be sure to check on the intermodulation distortion (IIP3) as with only 35mA current and 30dB gain it may be a problem if you are receiving strong transmitters without band filter before the amp

author: elroy
author_rank: Regular Contributor
timestamp: June 25, 2019, 02:40:36 pm
content: Quote from: erikka on June 25, 2019, 01:58:50 pmWe do have some very strong medium-wave AM broadcasters nearby. It would be great to find a filter that specifically blocks, say, 500 to 1500 kHz, as there are times that it would be desirable for the Red Pitaya to be able to receive both the 135 to 479 kHz range as well as 1.8 to 54 MHz at the same time.

author: _Wim_
author_rank: Super Contributor
timestamp: June 27, 2019, 07:19:20 pm
content: Here are some plots of the distortion spectrum (measured with an Analog Discovery). I can't do the intermodulation distortion measurement. Signal generator was attenuated passively with 30db, attenuated signal was again amplified by the Chinese LNA.

author: _Wim_
author_rank: Super Contributor
timestamp: June 27, 2019, 07:28:19 pm
content: Quote from: elroy on June 25, 2019, 02:40:36 pmWhy not build one yourself. Here is a good calculator I already used a couple of times:https://rf-tools.com/lc-filter/

author: elroy
author_rank: Regular Contributor
timestamp: June 27, 2019, 09:05:26 pm
content: Quote from: _Wim_ on June 27, 2019, 07:28:19 pmGood idea. I've gotten a couple of suggestions of online calculators from friends, but this one looks particularly good. I'll have to break out the toroids and wire.

author: chris_leyson
author_rank: Super Contributor
timestamp: June 29, 2019, 09:54:54 am
content: You could try a lossless feedback amplifier they've got excellent spurious free dynamic range and low noise figure but I think it's going to be difficult to cover 135kHz to 54MHz with a silngle amplifier. A few examples for HF:-'http://www.phys.unm.edu/~lwa/memos/memo/lwa0071.pdfhttps://www.okdxf.eu/lankford/Norton-2N5109.pdf

author: elroy
author_rank: Regular Contributor
timestamp: June 29, 2019, 03:36:40 pm
content: Quote from: chris_leyson on June 29, 2019, 09:54:54 amThanks! I'd been wishing I could get my hands on a Clifton Labs preamp, but the owner/designer passed away some time back, and the business was sold to DX Engineering which isn't producing those. However, the manual at your link seems to have very complete schematics and instructions (if I can ever get around to building one from scratch). The other link isn't opening for me right now, but I'll retry it.