Relaxation oscillation in LC resonance/ Toko testers

[regenfreak]

The reflection coefficient measured by the VNA is typically 0.9969 across the HF band and this agrees with the above equation based on VSWR. This requires a properly calibrated test fixture and a decent VNA but I'd expect a nanoVNA to give a similar result up to about 50MHz. The traces will be a bit noisier though as the nanoVNA tends to struggle when the reflection coefficient gets above about 0.995. I don't really trust the nanoVNA to make critical measurements like this above about 50MHz.

With all due respect, I am not confident i can see the difference in readings between 1.000 and 0.9968 on the radius (magnitude of reflection coefficient) at zero degree in the Smith chart even at 10.7MHz. If someone knows how to do it, I would love to know the trick.

[regenfreak]

My old/early £35 (Hugen) nanovnaH is a very good instrument when used up to 50MHz and I'm basing that statement on over 30 years' RF design experience using lab VNAs and I've even spent time using the classic old HP 8405A vector voltmeter at work and here at home.

In the right hands and with a decent cal kit and test fixture this nanovnaH really is capable of making measurements that are close to that of some lab VNAs when used up to 50MHz. Above 50MHz my nanovna can't be used for critical measurements. However, at 10.7MHz it can deliver excellent performance.

Give me a bit of time to set something up for a 10.7MHz resonator measurement with my nanovnaH here. I'll compare it to an Agilent VNA and an old HP8405A vector voltmeter (currently warming up...)

I am eager to hear your experience I would be delighted if it is possible.

[G0HZU_JMR]

The first test is a fairly soft one but I think it is quite well controlled.

The inductor is a tight tolerance 3.3uH 1812CS SMD inductor from Coilcraft. This only has a Q of about 40 at 10.7MHz. See the datasheet chart below for the Q of the 3.9uH version. The cap is a rather nice ATC 800B 68pF porcelain SMD cap. The pair should resonate at about 10.62MHz. However, this assumes no self capacitance in the inductor. In reality the resonant freqeuncy will be slightly lower than 10.62MHz assuming both parts are accurate. I think the inductor is a 2% part so there is going to be some uncertainty as the the resonant freqeuncy anyway.

I measured the two parts together as a resonator on the nanovna and also an Agilent lab VNA and you can see the result below. The traces agree so well it's hard to tell which is which! There are four traces per plot so you have to look closely to try and tell them apart. Because the inductor only has a Q of 40 the reflection coefficient is only 0.989.

The reactance of 3.3uH at 10.6MHz is 220 ohms. The Q of the inductor is expected to be about 40.

Rs = X/Q = 220/40 = 5.5 ohms

Rp = Rs*(Q^2 +1) = 5.5 * 1601 = 8806 ohms.


So both VNAs should show about 8800 ohms at resonance and resonance is going to be somewhere just below 10.6MHz.

See the plots below and you can see that there is no difference between the lab VNA and the nanovna in this measurement. Both agree on resonance at 10.54MHz and both correctly show an Rp of 8800 ohms. Both show the same phase response and both show the expected 0.989 reflection coefficient.

I'll try and make up a resonator with much higher Q and repeat the tests. It probably isn't worth going above a Q of 200 though as I doubt a typical Toko
coil/resonator will have a Q of 200 at 10.7MHz.

[regenfreak]

I measured the two parts together as a resonator on the nanovna and also an Agilent lab VNA and you can see the result below. Because the inductor only has a Q of 40 the reflection coefficient is only 0.989.

The reactance of 3.3uH at 10.6MHz is 220 ohms. The Q of the inductor is expected to be about 40.

Rs = X/Q = 220/40 = 5.5 ohms

Rp = Rs*(Q^2 +1) = 5.5 * 1601 = 8806 ohms.


So both VNAs should show about 8800 ohms at resonance and resonance is going to be somewhere just below 10.6MHz.

See the plots below and you can see that there is no difference between the lab VNA and the nanovna in this measurement. Both agree on resonance at 10.54MHz and both correctly show an Rp of 8800 ohms. Both show the same phase response and both show the expected 0.989 reflection coefficient.

This is very impressive.

What software do you use to obtain the plots :

* magnitude of reflection coefficent (in very fine scale) vs frequency?

* Zcomplex vs frequnecy?

How do you create the markers?

How do you export the nanoVNA sweep data to this software? I have never used the NanonVNA with a PC and I am eager to repeat your experiment.

[G0HZU_JMR]

To give the nanovna a really tough test I just measured a 100k ohm 0805 chip resistor. This 100k ohm resistor has a VSWR of 2000:1 so the reflection coefficient should be 1999/2001 = 0.9990

I measured it across the same span of 9.5MHz to 11.5MHz with 100 data points and the plot is below.

You can see that the nanovna is right on the limit here but I am running old firmware and I'm not using any averaging. With averaging I think the trace would be cleaner but this is still an impressive result for a £35 VNA. The measured reflection coefficient was a very noisy 0.999 as expected.

The (measured) parallel capacitance for this chip resistor seems a bit high though. I was expecting 0.08pF for the resistor plus 0.05pF for the stray capacitance in the test fixture = 0.13pF. But the nanovna measured 0.18pF here. Maybe there's some flux or something that is adding a tiny bit of extra capacitance?

Anyway, hopefully you can see that the nanovna is quite capable although this last measurement really is stretching the limits I think.

[G0HZU_JMR]

[QUOTE=regenfreak;1385394]

This is very impressive.

What software do you use to obtain the plots :

* magnitude of reflection coefficent (in very fine scale) vs frequency?

* Zcomplex vs frequnecy?

How do you create the markers?

How do you export the nanoVNA sweep data to this software? I have never used the NanonVNA with a PC and I am eager to repeat your experiment.

Thanks. I'm using an old version of Eagleware Genesys for the plots but this could be plotted just as well using a simple graph in VBasic.

I wrote my own software to dump out the data but I think most people use the freebie nanovna saver software for this stuff. It should work just as well.

I am eager to repeat your experiment.

Be prepared for (initial) disappointment unless you use a decent cal kit and test fixture.
I think this is where many people go wrong with the nanovna.

[regenfreak]

thanks. I'm using an old version of Eagleware Genesys for the plots but this could be plotted just as well using a simple graph in VBasic.

I wrote my own software to dump out the data but I think most people use the freebie nanovna saver software for this stuff. It should work just as well.

I am not good at Window stuff or programming. I attempted to install NanoVNA saver a few time before but failed. Now i figure out i may to install the Service Pack first.

Be prepared for (initial) disappointment unless you use a decent cal kit and test fixture.
I think this is where many people go wrong with the nanovna.

Well, life is full of disappointment. I guess i have to fail many times before i get somewhere. I am interested in testing real applications; e.g. FM IF transformers removed or installed in very old radios. The stray capacitance and inductance of the test clips and leads have to be excluded in the calibration.

[G0HZU_JMR]

If you really want to test/model a classic 10.7MHz IF transformer that has a primary and secondary winding (rather than just a basic LC resonator) then the snazzy way to do it with the nanovna is to make a full two port measurement of the transformer and then export a genuine 2 port s-parameter file. This is quite fiddly to do though.

To complicate matters further, some transformers have a tap on the secondary and this would normally require an exotic 3 port VNA but a 3 port measurement can still be done with a 2 port VNA if you have a lot of patience.

Note that I don't think any of the freeware nanovna programs allow you to properly measure a full two port network for s11 s21 s12 s22. This is why I wrote my own program to do this. It may have changed now though.

Once you have a proper 2 port model you can then use this as a transformer component in programs like LTSpice. The nanovna should model the transformer primary and secondary behaviour quite well so the model should perform very well like this.

[G0HZU_JMR]

It's possible to just look at the third column in the dumped s-parameter data from the nanovna and the lab VNA to see where the resonance is because the sign of the angle changes negative at about 10.54MHz in both cases.

The snippet of s11 data below is from the nanovna and this can be dumped using the nanovna saver software.

# Hz S RI R 50
! Frequency Hz real(S11) imag(S11)
10420000 0.988573908 0.010341576
10440000 0.988372206 0.008625949
10460000 0.988739371 0.006885186
10480000 0.988614022 0.005169825
10500000 0.988744497 0.003320412
10520000 0.988633394 0.001779268
10540000 0.988805592 -0.000110200
10560000 0.988664507 -0.001608916
10580000 0.988673031 -0.003477841
10600000 0.988917648 -0.004789891

The next snippet is from the Agilent VNA and the results are very similar!


10500000 9.888580e-001 3.779487e-003
10512500 9.886181e-001 2.476336e-003
10525000 9.886138e-001 1.344887e-003
10537500 9.888957e-001 4.784818e-004
10550000 9.885880e-001 -7.739982e-004
10562500 9.888286e-001 -1.760291e-003
10575000 9.888918e-001 -2.781209e-003

It's easy to spot resonance occurs at 10.54MHz in both cases.

You can see that you don't really need exotic software to process this data as it already shows the complex reflection coefficient as mag and angle.

The nanovna data is much easier on the eye though because it doesn't use the e-00x format.

[regenfreak]

If you really want to test/model a classic 10.7MHz IF transformer that has a primary and secondary winding (rather than just a basic LC resonator) then the snazzy way to do it with the nanovna is to make a full two port measurement of the transformer and then export a genuine 2 port s-parameter file. This is quite fiddly to do though.

I guess you meant two port transmission measurement is to ground each legs of the primary and secondary coils of the IF transformer (like sweeping a 2nd order Butterworth bandpass filter). Now what plot would I look at the S12 transmission measurement? So only tiny fraction of the power is transmitted through the filter.

In a reflection measurement using your |Z|/ 0 degree method, I would "isolate" the secondary by connecting the secondary LC in parallel with a 4.7K ohms "damping resistor." So I can sweep the resonance of the primary without the influence of the secondary resonator.

Unlike transistor radios, most valve FM IFs has no tappings in the secondary.

None of the NanoVNA can do s12 s22.

It's possible to just look at the second column in the dumped s-parameter data from the nanovna and the lab VNA to see where the resonance is because the sign of the angle changes negative at about 10.54MHz in both cases.

The snippet of s11 data below is from the nanovna and this can be dumped using the nanovna saver software.

# Hz S RI R 50
! Frequency Hz real(S11) imag(S11)
10420000 0.988573908 0.010341576
10440000 0.988372206 0.008625949
10460000 0.988739371 0.006885186
10480000 0.988614022 0.005169825
10500000 0.988744497 0.003320412
10520000 0.988633394 0.001779268
10540000 0.988805592 -0.000110200
10560000 0.988664507 -0.001608916
10580000 0.988673031 -0.003477841
10600000 0.988917648 -0.004789891

Thanks your posts are very helpful and useful. I am taking notes of what you wrote with all the calculations. I can see the inflexion point of imagery part of S11. I hope i should get used to the idea that S11 is a complex number by now!

[G0HZU_JMR]

I'm glad my posts are useful, thanks.

If we can go back in time to 1966 then this stuff would probably have been done with a classic HP8405A vector voltmeter. I'm lucky to have one of these here along with a rather nice HP 778D directional coupler. At 10MHz this coupler has well over 40dB directivity so it can make fairly good reflection coefficient measurements even without the modern calibration corrections offered by a modern VNA.

If you look at figure 3 in the attached pdf this is the HP8405A + HP778D coupler setup I have here although I'm using a modern Agilent sig gen that offers 0.01dB amplitude adjustment and very stable amplitude control vs time.

The procedure is covered in more detail in the other attached PDF AN77-3.

I also include a decent lowpass filter and a 20dB attenuator at the output of the sig gen to make sure it doesn't get upset by load changes. I also have a 5 digit DVM connected to the recorder connection at the rear of the HP8405A and this lets me read the reflection coefficient to three decimal places and this is much better than the front panel analogue meter dial.

It really is pushing the limits of this setup but it did manage to report the reflection coefficient as 0.989 / 0 degrees at 10.54MHz. It agreed the resonance frequency was 10.54MHz as this is the frequency the phase dial went through zero on the +/- 6 degree scale.

The HP8405A VVM is a really good educational tool and I'll jump at any excuse to stick it on the bench even though it is outclassed by modern VNAs for most things now.

[Cruisin Marine]

Hi Regen,

There is loads of software for Nano VNA for many platforms available https://github.com/ttrftech/NanoVNA

Nano VNA saver may be what you are after http://nanovna.com/?page_id=90


PS: It takes many seconds to load on my win PC

[G0HZU_JMR]

To give another confidence check I soldered the very same 3.3uH inductor and 68p cap in parallel on a test fixture but without any connections.

I then probed the resonator remotely with a VNA s21 measurement using an H field probe and an E field probe to sniff the resonator with no loading.

I have a Q measurement routine that I can launch and this controls the VNA remotely with a netbook PC. This gives an continuous measurement update of resonator Q on screen.

The marker shows the peak of the unloaded resonator is at 10.565MHz and the unloaded Q is 40 as expected. The remote probes don't load the resonator and the VNA is making a very low level 'sniff' measurement of the resonator here.

the resonance isn't quite the same because the VNA predicted 10.54MHz and the remote method shows 10.565MHz but this could be down to heat ageing of the inductor or the cap from the soldering or maybe it could be just be a subtle layout issue.

Note that the Q measurement marker shows 10.585MHz but I think it has found a temporary noise peak here. The main marker 1 is set to search for the peak with 16x averaging.

The VNA Centre Frequency is 10.540MHz and the span is 5MHz so the scale is 500kHz/div.

However, I still think the result is very good!

[regenfreak]

If we can go back in time to 1966 then this stuff would probably have been done with a classic HP8405A vector voltmeter. I'm lucky to have one of these here along with a rather nice HP 778D directional coupler. At 10MHz this coupler has well over 40dB directivity so it can make fairly good reflection coefficient measurements even without the modern calibration corrections offered by a modern VNA.

If you look at figure 3 in the attached pdf this is the HP8405A + HP778D coupler setup I have here although I'm using a modern Agilent sig gen that offers 0.01dB amplitude adjustment and very stable amplitude control vs time.

The procedure is covered in more detail in the other attached PDF AN77-3.

That 1966 HP vector voltmeter is a collectable beauty. Those RF test
equipment oozes aura of bygone era. Those gears must cost an arm and a leg at that time. Now I can get a 800MHz-2.5GMHz directional coupling bridge in metal housing from Aliexpress for £6. I could get a TinySA for £35, a 80db step attenuator for £16, a white noise generator for £12, RF power meter for £15. One can build a cheap RF lab for self-learning easily.

I have been lusting after the Nanovna v2 plus 4 since last year. I wanted to wait for the price to come down but it keeps going up. Its 90db dynamic range is mind boggling at that price point though.

I have managed to dump a test data file from NanoVNA Saver but the Touchstone file viewer is not compatible with Window 7,,,, have to find a workaround solution.

There is loads of software for Nano VNA for many platforms available https://github.com/ttrftech/NanoVNA

Nano VNA saver may be what you are after http://nanovna.com/?page_id=90

I have got it working first time tonight after installing the C++ patch for Window 7. I am excited

I have not tried Joe Smith's Labview interface yet as I struggle with installation as usual.

[regenfreak]

I have a Q measurement routine that I can launch and this controls the VNA remotely with a netbook PC. This gives an continuous measurement update of resonator Q on screen.

The marker shows the peak of the unloaded resonator is at 10.565MHz and the unloaded Q is 40 as expected. The remote probes don't load the resonator and the VNA is making a very low level 'sniff' measurement of the resonator here.

the resonance isn't quite the same because the VNA predicted 10.54MHz and the remote method shows 10.565MHz but this could be down to heat ageing of the inductor or the cap from the soldering or maybe it could be just be a subtle layout issue.

Note that the Q measurement marker shows 10.585MHz but I think it has found a temporary noise peak here. The main marker 1 is set to search for the peak with 16x averaging.

The VNA Centre Frequency is 10.540MHz and the span is 5MHz so the scale is 500kHz/div.

However, I still think the result is very good!

wow the tip is very big. Your VNA is very sensitive. I have the full set of the near filed E and H probes (cheap type III with side shieldings) but my dip is tiny.

[G0HZU_JMR]

I couldn't resist putting the resonator back on the VNA fixture and this time I copied the parts placement as accurately as I could.

This meant the components were arranged and soldered exactly as per the probe sniff test.

This shifted the VNA result up to 10.560MHz so I'm a bit happier now as it agrees closer to the probe sniff method and it probably rules out my earlier heat ageing theory.

I think my VNA has 125dB dynamic range but the newer models from Keysight manage something like 135dB.

[G0HZU_JMR]

Now I can get a 800MHz-2.5GMHz directional coupling bridge in metal housing from Aliexpress for £6. I could get a TinySA for £35, a 80db step attenuator for £16, a white noise generator for £12, RF power meter for £15. One can build a cheap RF lab for self-learning easily.

Yes, it really is amazing what can be purchased for very little money and it can sometimes outclass the classic lab gear from the 1980s or 1990s.

When I was a spotty student in the early 1980s I remember visiting a G8 radio ham as he had a spectrum analyser. At the time this was the equivalent of someone having an exotic supercar and it was the first time I'd ever seen a proper spectrum analyser. I was quite excited to even be in the same room as that analyser and I felt like a privileged guest!

It was also slightly amusing because he seemed reluctant to switch it on to demo it. I did wonder if it was broken. Either that or he only wanted to power it up when it was really needed. I can't remember what model it was but it was something clunky from the 1960s and probably worthless to anyone but a collector today.

I just wonder what will be available in another 5-10 years' time from the far east for £100 or so...

[G0HZU_JMR]

I guess you meant two port transmission measurement is to ground each legs of the primary and secondary coils of the IF transformer (like sweeping a 2nd order Butterworth bandpass filter). Now what plot would I look at the S12 transmission measurement? So only tiny fraction of the power is transmitted through the filter.

I'll demo this tomorrow and hopefully you will be impressed with the results the nanovna can deliver when it produces a transformer model.

I've just removed a 10.7MHz IF coil from an old CB radio and I'll do a 2 port measurement using both the lab VNA and the nanovna tomorrow. This is one place where the lab VNA will be a bit better because I can apply full 10 or 12 term error correction during the calibration process. However, the nanovna should still produce a highly useable transformer model even if the full s2p file shows some obvious (but small) errors.

[G0HZU_JMR]

Actually I just remembered I measured a full 2 port s-parameter model of a small (receive only) 49:1 EFHW transformer with my nanovna a while back.

This transformer is supposed to match 50R to 2450R and it is wound on a small ferrite toroid. Radio hams use transformers like this to match to end fed half wave antennas.

It may seem to be completely daft to try to measure this directly with a 50R VNA as the high Z secondary is hopelessly mismatched away from the intended 2450R when both primary and secondary are connected to the 50R ports of a VNA.

However, because the VNA has measured all 4 s-parameters s11 s21 s12 s22 then the mismatch loss in the S21 measurement is kind of captured (and stored) in the highly reflective s11 and s22 measurements.

This means that if you take the 2 port model of the transformer and load it into a simulator (eg LTSpice) and change the port impedance of the secondary to 2450R then the transformer should then show the correct (low) insertion loss. This probably sounds too good to be true but this is correct.

I demo'd it with a youtube video a while back (sadly no sound) and this compares the insertion loss against the classic method where a 2400R resistor is placed in series with the secondary and a basic s21 sweep is made with a lab VNA. This will add 16.9dB of error in the insertion loss but this is corrected in the simulator. This shows the correct insertion loss in the purple trace in the top RH corner of the video.

To cut a long story short, if the nanovna derived 49:1 transformer model is any good it should show the same insertion loss as the top purple trace when the secondary port is set to 2450R in the simulator. You can see that the nanovna model nails this test really well at about 53 seconds into the video at 0:53 where the port is tweaked up to 2450 ohms and the insertion loss at about 20MHz is about 1dB.

https://www.youtube.com/watch?v=1xUe16ex7aU

The video then goes on to show how the insertion loss can be improved even further (at about 1.8MHz) if the input port 1 is moved away from 50R.

This shows what can be done with a £35 nanovna if you explore its potential a bit further than most people.

[G0HZU_JMR]

Whoops! I just had a look at the s-parameter data again and I forgot that my nanovna exports the data as # Hz S RI R 50 rather than the classic # HZ S MA R 50.

Therefore, the exported data in the s1p file isn't the classic mag and angle reflection coefficient data. However, the sign still changes when the 1 port measurement goes through resonance at 10.54MHz. Sorry for any confusion caused! I should have remembered this. I thought I had changed the data format to S MA R 50 in my software but it looks like I haven't.

I've attached the nanovna derived s2p data file from that 49:1 transformer video below and you could have a play with it in LTSPice or QUCS or maybe RFSIM99 if you can run it on your computer.

This is a genuine full 2 port s11 s21 s12 s22 data file taken with a nanovna. I've had to change the file extension to .txt to get it to upload but you can change it back to .s2p once downloaded. This should capture the behaviour of the 49:1 transformer and port 1 is the low Z (50R) primary and port 2 is the high Z (2450R) secondary.