How to direct sample (HF and MW reception) with a ~$10 RTLSDR (RTL2832) dongle! - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: October 25, 2017, 07:25:27 pm
content: As they say a picture tells a thousand words.This is less of a how-to - more of a collection of data points. But it definitely can be done and one then has a very affordable shortwave radio (if you already have a computer).To be honest, it will work with much less than these people have done but will be quite non-optimal in crowded RF environments because of all the noise from VHF/UHF bleeding in. But these approaches deal with it.Direct sampling works extremely well with a magnetic loop because of the loop's natural signal pre-selection ability.The dongles pictured are mostly the cheap midsized dongle with the SMD crystal (its the one with the elliptical row of holes on one side) thats used in the Ba5SBA kit - or based on the popular Newsky TV28T v2 (the core that many of the remarketers use)Both can be had for around $7-12 on ebay.Some of the photos are from a long running thread on the Hellocq web site in China that I attempted to read a while back using Google translate.

author: cdev
author_rank: Super Contributor
timestamp: October 25, 2017, 07:47:22 pm
content: The unmodified dongles have too much USB noise computer noise and switching power supply noise below 24 MHz to work well withoutextensiveadditional bypassing and filtering of their power supplies - as well as common mode chokes on USB cables.. so thts basically what a RTLSDR direct sampling setup needs to accomplish.Also, in addition to ESD protection, low pass filtering of the HF input needs to be done so that they can receive HF signal and less problematic alias-causing VHF and UHF noise.

author: metrologist
author_rank: Super Contributor
timestamp: October 25, 2017, 08:31:36 pm
content: I see, the schema's are a bit small, but I think I can make it out. You are saying the first image is the one to implement, and then some power supply filtering...?

author: cdev
author_rank: Super Contributor
timestamp: October 25, 2017, 10:50:28 pm
content: Its a good start...You need low pass filtering and it makes sense to also filter out AM BCB sometimes, especially if you have very strong AM broadcast stations in your vicinity.You can get a good start on that with a 5 pole filter.. RFSim99 can design that kind of filter pretty easily.Total cost can be very little if you have some junk lying around, some generic parts.

author: cdev
author_rank: Super Contributor
timestamp: October 26, 2017, 06:53:26 pm
content: Quote from: metrologist on October 25, 2017, 08:31:36 pmThe original instructions for the original board can be found athttp://115.28.16.44:81/file/3156.doc(alsohttps://kechuang.org/t/62913)Youtube videos on direct sampling (all of them)https://www.youtube.com/results?search_query=direct+samplingThe Ba5SBA 'mother board" uses a single filter designed to cut off at 28 MHz, but really, two separate filters - 0-14 MHz and 14-28 Mhz would likely work much better.Also, the input impedance of the RTL chip input is likely much higher than the several hundred ohms suggested by the use of a 4:1 impedance transformation. from 50 .. So a higher ratio of turns in the transformer might be helpful.. if the antenna was 50 ohms..See discussion athttp://radiochief.ru/kv-ukv/sobiraem-shirokopolosnyj-sdr-priemnik-na-baze-rtl2832u/Maybe an antenna matching circuit would be good there?It would likely be worth it to see if that 1:2 ratio was wrong.. at least for your antenna.Also, I would try other transformer materials than 43 - Others would be better for much of HF..I would also try a binocular core or perhaps multiple balun cores..This is an investigation which other more knowledgeable people here could add a lot more to it than I can.

author: ebclr
author_rank: Super Contributor
timestamp: October 27, 2017, 12:21:38 am
content: Complementary informationhttps://www.dropbox.com/s/icue2bsqw1jmgt5/RTL-SDR%20DIY%20Kit%20Installation%20Instructions.pdf?dl=0