Advice needed for experimenting with simple RF circuits. - Page 1
Forum section: electronics / rf-microwave

author: apblog
author_rank: Regular Contributor
timestamp: March 20, 2019, 07:51:20 am
content: Hi everyone. I used to work with RF stuff a long time ago, writing firmware for RF test equipment for the cellular phone industry.But I never have built my own circuits or learned the basics. I would like to rectify that (haha). I do have some exprience with high speed (1.2Ghz) digital PCB design, and transmision line theory.My project is to characterize a set of temperature/humidity sensors and their base station display, which operate at 433 MHz. I would also like to design some rx and tx circuits to interoperate with this system.The only relevant test equipment that I have is a newish 1GHz scope (keysight MSOX3104T) and a really old 2GHz scope (lecroy wavepro 960).This is my plan:1. build a 433MHz CW transmitter that can be turned on and off rapidly (square wave AM).2. use the scope to tune the transmitter.3. build a 433MHz receiver that can output a square wave to my logic analyzer. Use the tx I just built to tune the receiver (the sensors only transmit once per minute).4. use the receiver and the logic analyzer to analyze the sensor's data protocol.So far, I have been able to use the scope to view the 433MHz pulses, and I was able to use the "max" math function to recover a square wave signal.I am hoping to get some advice on what types of circuits to use, and some starting points for reading up on simple circuit design. Most of the beginner info is for much lower frequencies.One stumbling block so far is that coils and caps appropriate for 433MHz seem to be impractically low valued. Another is selecting components for the diode detector and the associated filter that follows it.Some questions that come to mind are:1. what kind of high speed transistor should I use?2. what kind of variable cap to use? Or should I just tune my homemade coil with a slug or by spreading the windings?3. what sort of breadboard construction is most practical? (manhattan?)I know there are 433 MHz modules available, but the point of this project is to get some experience with RF circuits.FWIW, I will be operating at very low power initially - just with with stuff on my desk.Thanks!

author: bd139
author_rank: Super Contributor
timestamp: March 20, 2019, 08:40:20 am
content: 1. Look at MMICs. Much easier than discrete2. I would avoid trimmer capacitors at that frequency. Inductors are easier to tune. You can get 433 MHz coils and filters from toko still3. Take a look at this:https://pdfs.semanticscholar.org/2403/db840e6874e29f55ad9a4599481738773b57.pdf

author: apblog
author_rank: Regular Contributor
timestamp: March 20, 2019, 09:50:26 am
content: Thanks for the PDF, it was enlightening.MMICs look very cool, but for my first experiments I want to stick with discrete.By the way, it seems that I posted this in the wrong forum. Couod someone with the appropriate access please move it to the RFforum? Thanks.

author: bd139
author_rank: Super Contributor
timestamp: March 20, 2019, 09:59:20 am
content: If you're using discrete it might be easiest to build a lower frequency crystal oscillator that has a harmonic at 433MHz and select that with a tuned circuit / multipliers and amplify it. This is a strictly 1970s way of doing it but it does work all the way up to 1.2GHz quite nicely.Edit: have a look at the converter here:http://mirror.thelifeofkenneth.com/lib/ham_radio_magazine/Ham%20Radio%20Magazine%201978/10%20October%201978.pdf(page 42). That goes up to 432MHz which is pretty close.

author: apblog
author_rank: Regular Contributor
timestamp: March 21, 2019, 11:51:12 am
content: Quote from: bd139 on March 20, 2019, 09:59:20 amThat was an interesting article. It's probably a bit beyond my immediate capability. However it was neat to see that such things (pcb trace filters, transformers, etc.) can be done by amateurs.One thing I noticed was that he was talking about it being for 432MHz in the text, but the diagrams all showed a lower frequency of 384 MHz. I assume (and he implies) that the component values and trace dimensions produce 384MHz and that is to be used as the input into a mixer circuit elsewhere.Also I assume that the circuit could be pushed to an actual 432MHz.

author: bd139
author_rank: Super Contributor
timestamp: March 21, 2019, 11:59:11 am
content: Oh yes - you can go right up to 10GHz with amatuer kit no problemsI think you'd just have to retune it for a different harmonic to get 432 out. I wouldn't use the actual design there, just the basis of it for frequency multiplication for example.

author: apblog
author_rank: Regular Contributor
timestamp: March 21, 2019, 02:06:07 pm
content: I made an attempt to build a simple demodulator this morning... It was a total failure.I could see the 433 MHz signal, pulsing on the output. But no rectification or envelope following. The schottky diode that I used has a Vf of about 200mV at low current, so I feel like it should have worked.I figure that I don't need a tuning tank circuit if my antenna is right next to the transmitter. I did however make the antenna 1/4 wavelength -- 17.3 cm.Pictures attached if anyone wants to offer suggestions for improvement.

author: bd139
author_rank: Super Contributor
timestamp: March 21, 2019, 02:34:26 pm
content: The signal is going to be uV or very low mV range potentially at that level even with antenna close at hand so that won't bias the diode into conduction. Also that diode has 15pF of capacitance which is pretty high. Reactance of about 24 ohms. 1N5711 better at 2pF (184 ohms).Typically you need a some gain before using a diode as a demodulator. A simple receiver architecture for this stuff would be a bandpass filter, then multiple tuned gain stages, then a detector like that.I would start at a much much much lower frequency. Think AM broadcast. When you're building crystal sets etc, the LC tank on the front end actually provides gain.Really if I was doing this, I'd build a "direct conversion receiver" where you mix the incoming signal with a local oscillator, which results in the difference product being selected, then demodulate that. For example, if you had a 433MHz signal and 432MHz local oscillator, then that would result in a 1MHz signal out. Much easier to process! Downconvert ASAP. Much easier.You can probably build one of these with an SA602 mixer IC easy enough.

author: bsfeechannel
author_rank: Super Contributor
timestamp: March 21, 2019, 02:48:12 pm
content: Quote from: apblog on March 20, 2019, 09:50:26 amThe person with that power is precisely yourself. There's a button at the bottom left of this page called MOVE TOPIC. It only appears to you, because you are the OP of this thread.For further detail, seethis.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2019, 02:59:34 pm
content: Quote from: apblog on March 20, 2019, 07:51:20 amA simple question for you to self-check your level of understanding: do you understand IP3 and P1dB specifications and their relevance to RF Tx/Rx design?If not, whatever you do, you shoulddemonstratethat you are only transmitting power within the specified frequency band - not in adjacent bands, not at 2nd/3rd harmonics. If you demonstrate it to yourself, you stand less chance of having to demonstrate it to an irate neighbour, man in a black suit, etcN.B. you cannot use a scope to do that demonstration.

author: bd139
author_rank: Super Contributor
timestamp: March 21, 2019, 03:03:19 pm
content: You'll only get the men in black suits if you transmit 431-432 within 100km of charing cross.Otherwise you'll get the men in a jumper their wife bought for Christmas (ofcom).Or god forbid some RDF equipped hedgehog with some redneck hams in it.Most likely someone's garage door will be doing a jig.

author: apblog
author_rank: Regular Contributor
timestamp: March 21, 2019, 05:23:44 pm
content: Quote from: bd139 on March 21, 2019, 02:34:26 pmSo the RF is not only not turning on my diode, but just jumping straight through it due to the capacitance! That sounds like what I am seeing.I'm going to try to move this topic to RF, wish me luck...

author: RoGeorge
author_rank: Super Contributor
timestamp: March 21, 2019, 05:47:14 pm
content: Quote from: apblog on March 20, 2019, 07:51:20 am

author: biastee
author_rank: Regular Contributor
timestamp: March 23, 2019, 12:16:05 pm
content: Quote from: apblog on March 21, 2019, 02:06:07 pmQuick fixes to improve the sensitivity:1. add an inductor from the diode's anode to ground2. either remove the output resistor or replace it with something in the megaOhm range.Further refinement: match the input impedance of the diode detector.

author: Kalvin
author_rank: Super Contributor
timestamp: March 23, 2019, 12:57:56 pm
content: As you do not have a spectrum analyzer, you can use a cheap $15 RTL-SDR dongle as a poor man's substitute for frequency range 27 MHz - 1.5 GHz or so. The dynamic range is a bit limited (around 40 dB) and they tend to have some frequency offset and drift, but it is better than nothing when getting started. You can always check the actual frequency by using the known beacons in the VHF/UHF bands anyway. There are improved versions available with temperature stabilized oscillators and improved dynamic range. There are also some versions available which will tune to 500 kHz - 24 MHz with a separate antenna/signal input. Just buy 10 dB and 20 dB attenuators with SMA-connectors so you do not burn the dongle's front-end. You may also prefer a dongle with the SMA input connectors for easier attenuator connection. For more information, just visit the sitehttps://www.rtl-sdr.comYou may want to take a look at the Quick Start Guide to get started.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 23, 2019, 01:14:11 pm
content: Quote from: apblog on March 21, 2019, 02:06:07 pmWhat you are seeing there is not the output of your detector, but the "parasitic" voltage induced directly into the oscilloscope's probe ground clip.If you don't believe, just remove your circuit, and simply clip the ground alligator of the probe to the tip of the probe. Now, place the "shorted" oscilloscope's probe in the same place and position on the bench. See? Same signal. The ground piece of wire and the alligator clip are acting like an antenna. In order to avoid this, you need very short connection. Here's an example of how to do it properly:

author: apblog
author_rank: Regular Contributor
timestamp: March 23, 2019, 09:34:50 pm
content: This morning I had an inspiration and tried forward biasing the diode a little bit by connecting a current limited DC supply to the anode. 1ma to 20ma tried.,No luck. I was able to shift the DC level of the output up and down by doing this, but I still didn't see any envelope action.I also changed to using a BAT41 schottky, which has a lower spec'd capacitance of 2pf.I am thinking about winding a transformer to up the voltage coming off of the antenna. I have a few small toroid cores here somewhere. I think this is basically what biastee was suggesting.Right now I'm still just working with stuff I have on hand. Next week I will probably order a few specialty components.Quote from: RoGeorge on March 23, 2019, 01:14:11 pmDarn it. I know better too.I just tried making a ground wire loop and that's exactly what I've been seeing. This has invalidated all of my observations -- there could have been a subtle shift in DC level that I wouldn't have noticed.Also, thanks for posting the W2AEW videos. I like his videos but I've never gone down the radio rabbit-hole with him before. I actually want to try making his diode-ring mixer. But before that I need to make an oscillator.Quote from: biastee on March 23, 2019, 12:16:05 pmAn inductor from the anode to ground will serve as a low pass filter, correct? Am I just sizing it to start rejecting stuff above 434MHz? I think if I google "antenna matching" that I might find some stuff related to your last sentence.Quote from: Kalvin on March 23, 2019, 12:57:56 pmI ordered one of these last week (RTL-SDR-V3) and it just came yesterday. I haven't figured out the software yet. It did come with a nifty antenna kit though.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 23, 2019, 11:05:08 pm
content: Quote from: apblog on March 23, 2019, 09:34:50 pmI'm not sure I understand. Did you tried to connected the oscilloscope's probe as in W2AEW's video? Any pictures with the board and the signal measured with the new probing technique?Quote from: apblog on March 23, 2019, 09:34:50 pmThat inductance would make aDC returnpath. Its role is to reject the carrier (here 434MHz) while letting the DC to pass.https://www.microwaves101.com/encyclopedias/detectors

author: apblog
author_rank: Regular Contributor
timestamp: March 23, 2019, 11:12:41 pm
content: Quote from: RoGeorge on March 23, 2019, 11:05:08 pmNo, I tried connecting the ground wire of the probe to the probe tip and placing it near the circuit. That demonstrated to me that all of the 434MHz signal "leakage" through the circuit that I was seeing was actually just pickup from bad probing technique. I haven't tried better probing technique yet.

author: apblog
author_rank: Regular Contributor
timestamp: March 24, 2019, 02:30:13 pm
content: This morning I tried the inductor from anode to ground (.25uH), and am also now using perfect probing technique.I am still not seeing any rectification. I checked the V-I curve on the BAT41, and it's not going to turn on at all until it reaches 200mV, so there is basically no hope of it working with this circuit.I got out my Agilent 1156A active probe that I use for special occasions, and hooked it up to the output without any long ground leads. It was also picking up the carrier, so I think there is still some carrier leaking through the circuitCurrently I am looking through my junk box to see if I have any high bandwidth op-amps or transistors. After that I might try to make a transformer to boost the rf voltage.

author: KJDS
author_rank: Super Contributor
timestamp: March 24, 2019, 07:18:09 pm
content: Use a BAT63, and a TLC2652

author: apblog
author_rank: Regular Contributor
timestamp: March 24, 2019, 08:27:47 pm
content: Quote from: KJDS on March 24, 2019, 07:18:09 pmAre you suggesting that I make a "perfect diode" circuit, with a diode in the feedback loop?That's a great idea, but the TLC2652 only has a gain bandwidth product of 1.9MHz. So I think it wouldn't be able to track a 434 MHz carrier.Or I am missing something?I'll add those to my list of parts to order.

author: KJDS
author_rank: Super Contributor
timestamp: March 25, 2019, 07:20:26 pm
content: BAT63 and TLC2652

author: Teledog
author_rank: Regular Contributor
timestamp: March 26, 2019, 05:22:59 am
content: Not sure if they're still available..but TI had MSP430 eZ430 dev kits around years ago (still have one... with a pound of dust on it)433Mhz TX/RX..no licence neededNot sure exactly what you want to do with "square waves"..but it's an easy interface to the PC for some wireless projects anyway...PS: if you're trying to do UHF circuits "dead bug" style..good luck on any sort of stability!Give it up! Really!

author: bd139
author_rank: Super Contributor
timestamp: March 26, 2019, 07:52:35 am
content: It’s possible to hit around 700-800MHz dead bug fine. Don’t expect the circuit to work twice though.

author: biastee
author_rank: Regular Contributor
timestamp: March 26, 2019, 08:39:09 am
content: Quote from: KJDS on March 25, 2019, 07:20:26 pmHi KJDS,What are the advantages (& disadvantages if possible) of this topology? Thanks.

author: apblog
author_rank: Regular Contributor
timestamp: March 26, 2019, 09:12:45 am
content: Quote from: KJDS on March 25, 2019, 07:20:26 pmThanks for the diagram. I took the liberty of checking your previous posts and that method of biasing a diode is really clever. I'm looking forward to trying this circuit, and also trying to understand the temperature compensation aspect.Quote from: Teledog on March 26, 2019, 05:22:59 amI think the technical term might be "on/off keying". It's how my weather sensors transmit data. It might be manchester encoded, not sure yet.Quote from: bd139 on March 26, 2019, 07:52:35 amI'll be happy enough to see it work once!

author: bd139
author_rank: Super Contributor
timestamp: March 26, 2019, 09:44:08 am
content: Got a few minutes to write this up now.I don't think this is a viable approach to building a receiver for the first time reciever builder. Basically this is building a 432MHz crystal set which is fraught with problems. The issue is that there's no selectivity, the detector isn't at all sensitive and there's no gain stage and everything is done at the carrier frequency which is extremely high. Even a basic direct conversion receiver needs a selective front end, quality balanced mixer and a massive amount of gain. I would start at a much lower frequency and build something to a block diagram. Try a simple HF band receiver. Also learn to work to block diagrams rather than schematics first. Entire RF systems are usually described as block diagrams before anyone even picks up the soldering iron.If you want to receive on 432MHz as a first time builder it's important that you keep the UHF portions COTS and 50 ohms throughout which is difficult without using commercial parts. Ergo use commerical parts.The minimal receiver I would build for 432MHz without using a COTS IC is as follows. That kills off a lot of problems like impedance matching, downconverts the signal to 4.915MHz (cheap crystals here if you need filters!) and will actually work.Edit: LO should be 427.085. I was clearly drunk in charge of the calculator earlier

author: apblog
author_rank: Regular Contributor
timestamp: March 26, 2019, 10:46:29 am
content: Thanks for taking the time to create that diagram and post a write up.I know that building a non-selective crystal set with makeshift wiring at 432MHz is pretty dumb. I am having a bit of fun with idea though, and it's nice to get a feel for the problems involved and also for the basic technology. I'll probably muck around with it a bit more and then move on to a more practical approach.The more I read, and the more I experiment, I am looking forward to doing correctly, with a mixer, LO, and all.

author: apblog
author_rank: Regular Contributor
timestamp: April 04, 2019, 09:30:53 pm
content: Quote from: bd139 on March 26, 2019, 09:44:08 amWhat do you think of the following RF parts list for implementing your diagram?Code:BFTC-415+ LTCC Band Pass Filter, 330-500 MHzGALI-74+ Low Noise MMIC Amplifier, DC to 1 GHz (both input and IF amp)ADE-751H+ Double Balanced MixerRDP-2150+ Lumped LC Diplexer, DC-10 /40-2150 MHzAnd of course the SIS-570 you mentioned. I'm still looking at the options for that part.I'm curious why you mentioned that 4.915MHz is a common crystal frequency. I don't understand why a crystal is needed as part of the IF amp and detector.

author: bd139
author_rank: Super Contributor
timestamp: April 04, 2019, 09:55:47 pm
content: I haven't reviewed the parts TBH. I would probably use Minicircuits parts and toko filters.You don't /need/ a crystal filter but you can build an extremely tight (down to 200Hz) bandpass filter with crystals. It depends what you intend to receive.4.915 because it's dead easy to build the IF stages at this frequency and build decent IF filters with crystals if you need to do that. Higher frequencies require specialist layout and parts. Lower frequencies are impractical. Some IFs are a PITA like 10 and 12MHz which seem to be sensitive to other things in the vicinity which are clocked at those frequencies.

author: radiolistener
author_rank: Super Contributor
timestamp: April 05, 2019, 05:09:11 am
content: Quote from: apblog on March 23, 2019, 09:34:50 pmCongrats! This is very nice SDR receiver for the price.It also can receive short wave 1-14.4 MHz with pretty good quality.You're just need to select in the ExtIO settings windowDirect Sampling: Q inputTo get back to usual VHF/UHF mode, select Direct Sampling: Disablethe best software is HDSDRhttp://www.hdsdr.de/You will need to install drivers:1) install libusb from zadig:https://zadig.akeo.ie/2) place ExtIO_RTL2832.dll into HDSDR folder:http://hdsdr.de/download/ExtIO/ExtIO_RTL2832.dllHere is instruction:http://hdsdr.de/RTLSDR_with_HDSDR.pdf

author: apblog
author_rank: Regular Contributor
timestamp: April 05, 2019, 07:01:01 pm
content: Quote from: bd139 on April 04, 2019, 09:55:47 pmThat's interesting. It occurs to me that with tight bandpass filters you could make a receiver that demodulates multiple channels at once within the 433 MHz band, while only having one VHF/mixer section. That's useful for me because my temperature sensors transmit on 3 different frequencies.Quote from: bd139 on April 04, 2019, 09:55:47 pmMore research to do...Quote from: radiolistener on April 05, 2019, 05:09:11 amThanks for the tip!