Home made HF upconverter - Page 1
Forum section: electronics / rf-microwave

author: zimm
author_rank: Newbie
timestamp: July 01, 2020, 05:10:04 pm
content: Hi.I've been playing with one of those RTL-SDR dongles with a range between 25MHz to 1.7-ish GHz and I wanted to listen to stuff below the device's lower limit, which means I need an upconverter. I know I can buy one, but I'd like to build one myself just for a bit of practice and to hopefully get a better understanding of the theory behind RF mixers.I foundwhich explains how to build a diode ring mixer (and aon how to use it as an upconverter) but I have a few questions:for the LO, is it necessary to use fairly high frequencies (100MHz used in the tutorial, 125MHz used in commercial models) or can I just use a 30MHz crystal likethis one? I'm thinking that shifting the RF signal up by 30MHz will bring it above the lower limit of the SDR dongle, so I should be able to listen to HF, shortwave etc, no?I have a few toroids salvaged from power supplies, they look just about the right size as the ones used in that video, but I have no idea what material they're made out of. Is the material important or can I just use them for the RF mixer transformers without issues? Is there a way to determine their characteristics without complicated/expensive equipment?

author: radiolistener
author_rank: Super Contributor
timestamp: July 01, 2020, 07:23:40 pm
content: you can just buy some cheap RF mixer module on ali or ebay. Something like that:https://www.aliexpress.com/item/32798750251.htmlJust connect sine generator on LOin, antenna to RFin and attach IFout to RTLSDR. Works ok.You can use NWT as LO generator.The normal level for LO input is about -30..-40 dBm, so if you use NWT with 10 dBm output, just connect it to LOin through 40 dB attenuator.On the other hand, you can connect HF RF directly to the Q branch of RTL chip on RTLSDR dongle and use direct sampling mode. It has more good quality. Usually this input has a small pin near the chip, so it's not a problem to solder it. Just add some LPF on the input to reduce aliases from high frequencies.

author: radiolistener
author_rank: Super Contributor
timestamp: July 01, 2020, 07:31:14 pm
content: Quote from: zimm on July 01, 2020, 05:10:04 pmit's better to be able to change LO frequency, because some frequency may have issue from station on a mirror channel. You can use NWT7 as LO source.

author: Bud
author_rank: Super Contributor
timestamp: July 01, 2020, 09:39:45 pm
content: You will need to determine if with your selected LO frequency you can do effective filtering on the RF and IF path of the mixer. The second video talks about that at 3:30 but he did not show the filters. Basically, the lower your LO is the more difficult it will be to filter out the unwanted combination frequencies and image frequencies. By difficult i mean building a filter with high out of band attenuation , especially for frequencies adjacent (close) to the LO and its harmonics. When the LO is high relative to the band being upconverted it is much easier to realize effective filtering because the sum and difference products are farther apart from each other on the frequency scale.Also the resulting IF frequencies should not fall into an existing occupied frequency band, i.e. your local FM broadcast band.

author: fmkit
author_rank: Contributor
timestamp: July 15, 2020, 07:50:40 pm
content: Getting good performance out of 77.76 - 240MHz tcxo + IAM81008

author: vk6zgo
author_rank: Super Contributor
timestamp: July 16, 2020, 12:42:22 am
content: You could do it "old school" & use a frequency multiplier to give an output at 120MHz, or alternately an "overtone" oscillator which operates on a multiple of the crystal frequency.The latter may be a little "touchy" to get going, as usually the "overtone" crystals are optimised for such use, but it should be useable.The bonus is that you will learn something---- of course, if you just use the 30MHz crystal direct, you will learn about "image frequencies"!PS:- The thing you linked to is not a "crystal", it is an "oscillator module", so the "overtone oscillator" I suggested is ruled out.Sometimes (often?) the output waveform from such devices is rich in harmonics, so your "frequency multiplier" may need to be no more than a resonant circuit at the required harmonic.Note that if it is a good square wave, they theoretically don't have even harmonics, so you may be stuck with 90MHz, 150MHz, & so on...

author: cdev
author_rank: Super Contributor
timestamp: July 16, 2020, 03:25:02 am
content: You can tap into the RTL chip itself, many dongles even have the pads exposed for you to tap into. You could destroy it, but its fun and they are cheap. (you can likely try both methods, setting up direct sampling and an upconverter will let you compare the two.Plus you'll learn a lot doing it that way too. In some ways the direct sampling is even more useful. If you can look at the inside of your dongle, assuming you have a Rafael Micro tuner.. can you see what the situation is with pins 4+5 ? Is it easy to get to them, or hard? If its easy, its a no brainer. You can likely find the stuff around your house to get it working kind of just to start. If you need to connect directly to the chip you'll likely have the best luck with extremely thin magnet wire and lots of flux. How are your soldering skills, and do you have some nail polish or glue handy? (to glue the two tiny wires in place after you successfully connect them up.)Some dongles (Newsky TV-28T variants) even have added pads to tap into pins 4+5. It is a differential pair that on the R820T and R820T2 tuner is accessible as the Q input under direct sampling. Watch out for ESD. If its a humid day or environment, ESD is unlikely to be much of a problem, but still observe ESD precautions.but if its dry, observe strict - ESD precautions. You'll need to build a simple band pass filter to exclude other signals outside of the range you want to hear. (Band pass not low pass, also you need to make a small RF transformer that ideally can fit inside of the dongle.The easiest and in some ways the best setup uses a magnetic loop. Which is just a loop of wire and a variable capacitor. (but dont leave out the trransformer to prevent ESD frying your chip.)Quote from: zimm on July 01, 2020, 05:10:04 pmToroids are not likely what you want if they are from power supplies, but since we are at the low part of the sunspot cycle and most of the stuff now at night is near the bottom of the HF bands they might do for tonight. It depends. You'll never know unless you try it out.

author: fourfathom
author_rank: Super Contributor
timestamp: July 16, 2020, 04:13:23 am
content: You might be interested with a design I did using the direct-sampling mode of an SDR to make a 10MHz ham receiver gateway station. The details start on page 10 of this presentation:http://wb6cxc.com/wp-content/uploads/2019/04/Presentation-1-0.pdfIn this I touch on the filtering required to deal with the aliasing caused by the fairly low sampling frequency of the direct-input SDR. Since I gave this presentation to my ham club, I have built a few more of these, operating at 7MHz. I have also fabbed a small circuit board for the input filter and preamp so the rig needs fewer coax interconnect cables (and it's cheaper by a couple of $$$).I use the "RTL-SDR-Blog V3" which costs about $22. It has the direct-node enabled via the command-line interface so you don't need to do any soldering. It also has a more stable oscillator than some of the cheaper SDRs.