Best USB RtL or similar SDR stick for a TXCVR IF 68,985 - Page 1
Forum section: electronics / rf-microwave

author: DC1MC
author_rank: Super Contributor
timestamp: October 08, 2020, 06:56:00 pm
content: Hello everybody, as nobody wants to sell me the necessary crystals to clone the unobtanium FM board of my YAESU FT-920, or do some other form of modulator/demodulator for 8.215MHz, I've decided fro another approach, and that is to add a "waterfall adapter" to the device, it has a nice tapping point on the first IF of 68,985MHz and I'll either plug it into my PC or put some RPi and just output the audio. I will have at least FM reception plus the other SDR goodies.Now, I've got some early SDR sticks, even some that were claiming low freq front-end and they were mediocre at best under 100MHz, my question to the community is:Is there a not too expensive modern device that could be used around 70MHz with good performance ?Bonus points if is Linux supported, extra bonus points if available in EU.EXTRA, SUPER, DOUBLE PLUS bonus points if someone could come or point me to a schematic for such module ( the FM mod/demod, not SDR) or 8.215MHz, doable with reasonably costing and available components. I will still install the SDR tap but I'll try to design a replacement FM board as well.Thanks,DC1MC

author: radiolistener
author_rank: Super Contributor
timestamp: October 10, 2020, 12:13:27 am
content: For a low cost, I think RTLSDRv3 is the best choice. But look for original rtlsdr blog version, because there are a lot of fakes which looks similar, but actually worse quality rtlsdr with no direct sampling branch.Bellow 300-600 USD it will be to hard to find something better.

author: cdev
author_rank: Super Contributor
timestamp: October 18, 2020, 06:33:02 pm
content: I personally think the HackRF has better receive than the RTL-SDR. Plus it has both a built in LNAThey both have the ability to power an external one.Both are 8 bit but the HackRF has a lot fewer spurs, (than my RTLSDRs, I dont have an rtlsdr-blog one, however)Plus the sample rate can be adjusted over a much wider range.There are - it seems affordable VHF-UHF SDRs and affordable HF SDRs, vbut ery few cover both, and 70 MHz is right at the cusp where neither category is really that applicable..

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 18, 2020, 08:06:17 pm
content: Quote from: cdev on October 18, 2020, 06:33:02 pmThe genuine RTLSDR-blog units are a lot better than the cheap ones. Not sure if it would make that much a difference with a fairly strong signal at 70MHz, however.For 8.215MHz, the V3 with direct sampling can work with it, or put it into a quadrature demodulator run at 8.192MHz which will yield an low IF of 23kHz, which will work with audio ADCs that support 96kHz sampling rate or higher.For transmit, a $5 fl2k will be able to generate either IF frequency.

author: DC1MC
author_rank: Super Contributor
timestamp: October 18, 2020, 08:53:28 pm
content: Many thanks to everyone that took the time reply, very interesting stuff, especially the fl2k thingie that is really new to me. Holly crap, this thing can emulate a GSM tower !!!I was wondering about SDRPlay RSPdx, does anyone have experience with it, it claims to have 14bits and while not as cheap as the RTL-SDR, I'll only buy one and once.Now if someone know a non PC/Computer assisted method for a FM mod/demod running at 8.192MHz WITHOUT requiring a quartz of this frequency, (I was desperately asking the manufacturers, both small and big, no chance to get any if there is no order of tens of thousands), I'm all willing to give it a try.Cheers,DC1MC

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 19, 2020, 03:51:57 am
content: Quote from: DC1MC on October 18, 2020, 08:53:28 pmA high end microcontroller/mid range DSP or low to mid range FPGA will do it. In particular, the FPGA will give you quite a few PLLs that can be used to generate all sorts of frequencies from a common reference frequency like 10MHz.If you take the microcontroller route with something like a dsPIC or STM32, you can use the PWM module to generate a set of quadrature clocks that can drive the quadrature demodulator in order to bring the signal frequencies down to within the bandwidth of the internal ADCs.The FPGA route is a lot more difficult and somewhat more expensive, but you'll get a lot more capabilities with it. You can drive a quadrature demodulator with it just like with the microcontroller but that's a bit silly given any reasonable FPGA for that task will have no problems handling the 20Msps or so required to sample the 8MHz IF.