Advice needed for experimenting with simple RF circuits.

[apblog]

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!

[bd139]

1. Look at MMICs. Much easier than discrete

2. I would avoid trimmer capacitors at that frequency. Inductors are easier to tune. You can get 433 MHz coils and filters from toko still

3. Take a look at this: https://pdfs.semanticscholar.org/2403/db840e6874e29f55ad9a4599481738773b57.pdf

[apblog]

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.

[bd139]

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.

[apblog]

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.

That 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.

[bd139]

Oh yes - you can go right up to 10GHz with amatuer kit no problems

I 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.

[apblog]

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.

[bd139]

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.

[bsfeechannel]

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.

The 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, see this.

[tggzzz]

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).
...
This is my plan:
1. build  a 433MHz CW transmitter that can be turned on and off rapidly (square wave AM)
...
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.

A 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 should demonstrate that 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, etc

N.B. you cannot use a scope to do that demonstration.

[bd139]

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.

[apblog]

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).

So 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...

[RoGeorge]

3. what sort of breadboard construction is most practical?  (manhattan?)

[biastee]

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.

Quick fixes to improve the sensitivity:
1. add an inductor from the diode's anode to ground
2. either remove the output resistor or replace it with something in the megaOhm range.

Further refinement: match the input impedance of the diode detector.

[Kalvin]

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 site https://www.rtl-sdr.com You may want to take a look at the Quick Start Guide to get started.

[RoGeorge]

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.

What 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:



 

[apblog]

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.

What you are seeing there is not the output of your detector, but the "parasitic" voltage induced directly into the oscilloscope's probe ground clip.   

Darn 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.

Quick fixes to improve the sensitivity:
1. add an inductor from the diode's anode to ground
2. either remove the output resistor or replace it with something in the megaOhm range.

Further refinement: match the input impedance of the diode detector.

An 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.

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.

I 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. 

[RoGeorge]

Darn 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.

I'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?

Quick fixes to improve the sensitivity:
1. add an inductor from the diode's anode to ground
2. either remove the output resistor or replace it with something in the megaOhm range.

Further refinement: match the input impedance of the diode detector.

An 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?

That inductance would make a DC return path.  Its role is to reject the carrier (here 434MHz) while letting the DC to pass.
https://www.microwaves101.com/encyclopedias/detectors

[apblog]

Darn 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.

I'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?

That inductance would make a DC return path.  Its role is to reject the carrier (here 434MHz) while letting the DC to pass.
https://www.microwaves101.com/encyclopedias/detectors

No, 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.

[apblog]

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 circuit

Currently 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. 

[KJDS]

Use a BAT63, and a TLC2652

[apblog]

Use a BAT63, and a TLC2652

Are 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.

[KJDS]

BAT63 and TLC2652

[Teledog]

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 needed
Not 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!

[bd139]

It’s possible to hit around 700-800MHz dead bug fine. Don’t expect the circuit to work twice though.