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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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26th Jun 2021, 11:43 am | #41 | |||||||
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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Last edited by regenfreak; 26th Jun 2021 at 11:58 am. |
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26th Jun 2021, 12:14 pm | #42 | |
Dekatron
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Re: Relaxation oscillation in LC resonance/ Toko testers
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I'm not sure the nanovna saver software allows this feature yet although it really should have been there right from the start. Probably the first thing to do is check how good your cal kit and test fixture is organised for a simple 1 port measurement of s11. If you are using the bundled nanovna SMA cal kit then it will be very fiddly to set this up such that you can measure a resonator correctly on a test PCB. It can be done but it will involve some nerdy cal kit corrections. This is where the vast majority of nanovna users seem to slip up. They use the bundled SMA cal kit and end up with an incorrect test fixture calibration and they get duff results when they measure a real test network. They then condemn the nanovna hardware when really they haven't given it a chance. I can show you a basic cal kit and test fixture system but each connector costs about £4 and you will need three for the cal kit and then at least two more to use in a test fixture. Realistically you would be looking at about £25 in parts to make up a decent cal kit and fixture that would work extremely well up to 50MHz. Above 50MHz the nanovna starts to limit the performance although it can still work quite well up to about 300MHz.
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26th Jun 2021, 1:32 pm | #43 | |||
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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For bulky valve type IF transformers, i have to restore to croc clips(see photo) . In calibration, i used miniature non-inductive 50 ohms ceramic resistor as load. Once i tried this on 455khz and 10.7MHz IFs installed in the chassis of vintage radios, the results could be affected by movement of the cables. Once i built a test board using micro (expensive) silver component tester clips, they were very fiddly to use!!! Quote:
I managed to view the .sp1 file on my andoid phone using Touchstone Viewer. It only plots the graph automatically but does not show the raw data. What software do you use to view the .sp1 data and convert them to txt? Last edited by regenfreak; 26th Jun 2021 at 1:46 pm. |
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26th Jun 2021, 2:21 pm | #44 |
Heptode
Join Date: Nov 2018
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Re: Relaxation oscillation in LC resonance/ Toko testers
I have downloaded QucsStudio, it can open the HZ S RI R50 file, sweet!!!
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26th Jun 2021, 2:56 pm | #45 | |
Dekatron
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Re: Relaxation oscillation in LC resonance/ Toko testers
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For example, I think that an ideal 1 to 4 (impedance) transformer would have the following s-parameters (note this is the conventional mag angle MA format) # MHZ S MA R 50 freq s11 s21 s12 s22 10.7 0.6 -180 0.8 0 0.8 0 0.6 0 You can see that s21 and s12 are the same but the reflection coefficient for S11 and s22 are not the same. S11 has an angle of 180 degrees and s22 has an angle of 0 degrees. This is because this network is a transformer. Here's the nearest 10.8MHz s2p data line from a really good 4 to 1 transmission line transformer I designed a few years ago. 10.8 0.59759 174.07643 0.79433 -2.73648 0.79396 -2.67945 0.59604 0.45719 You can see it is pretty close to ideal, the 174 degrees angle of s11 is very close to the -180 of the ideal and the s22 angle is very close to 0 degrees. If I didn't measure s12 and s22 and just reused the same data as s11 and S21 I think the transformer model would misbehave quite badly in a simulator. It would probably mimic a fairly strange active device rather than a passive device.
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26th Jun 2021, 3:06 pm | #46 | |
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Re: Relaxation oscillation in LC resonance/ Toko testers
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One of them is the really nice (LF to 50MHz) 4 to 1 impedance transformer I designed a few years ago. It is designed for 50R at port 1 and 200R at port 2. The insertion loss is really low and the port match is excellent across a few MHz to about 50MHz. There are also a couple of files for the Toko 10.7MHz transformer from the CB radio. This transformer appears to work best (lowest insertion loss) when port 1 is about 50 ohms and port 2 is 370 ohms. Note that the data format is different as two of these files use the classic MA format but QUCS should cope OK I hope.
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26th Jun 2021, 3:17 pm | #47 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
I'll set up the nanovna to have a go at producing a valid s2p model of the CB transformer. To do this I have to swap the fixture around part way through the process.
I think that within a few years (at most?) there will be a full 2 port nanovna that can do this without having to do the annoying swap around. My first VNA was an old 3GHz HP8714B and like the nanovna this is classed as a T/R VNA because it can only measure S11 and S21. However, it did come with an external port switch box made by HP that permits full 2 port measurements. However, this is really fiddly to set up. I haven't used this old VNA for many years now.
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26th Jun 2021, 4:00 pm | #48 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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I have done a reflection measurement of a Toko FM using the test board i made a long time ago. I yet figure out a way of increasing the number of sweep points.... The Nano F can display Img S11(2nd photo) but not phase angle.But the spike of reflection S11 logmag is very sharp anyway. For a long time, i thought the frequency of spike was adversely shifted by the large impedance mismatch. The resonance frequency is around 10.826MHz. I really try to make sense of all the numbers at this stage. Last edited by regenfreak; 26th Jun 2021 at 4:11 pm. |
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26th Jun 2021, 4:48 pm | #49 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
I think that in the case of a conventional transformer with a tuned secondary the resonance frequency will also be affected by how you terminate the low impedance (untuned) winding.
You should be able to show this for the 10.7MHz CB transformer in QUCS if you mess with the port impedances. It looks like the resonant frequency with the input port unterminated is about 10.49MHz but if the input is left at a 50 ohm termination the resonant frequency goes up to about 11.00MHz. However, I don't think the transformer is used like that in the CB circuit. It is fed from a much higher impedance at port 1. Port 1 only has to be a few hundred ohms for the resonant frequency to stick close to 10.50MHz. I'm not sure how well adjusted this transformer is because it came from a 40 year old scrap CB board. It's L6 from an old mk1 Cobra 148GTL-DX CB and it may have been tweaked by whoever trashed the CB before I was given the salvaged PCB as a parts donor board way back in the 1980s.
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26th Jun 2021, 4:49 pm | #50 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
sorry the previous test was configured incorrectly in the test board. I forgot that the sockets were configured for transmission measurement and not reflection. So the results are invalid
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26th Jun 2021, 4:59 pm | #51 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
My cal kit for the nanovna is made usng these 18GHz SMA end launch connectors from Johnson. Sadly, the prices have gone up a bit recently.
I usually buy them in bags of 25 and they cost about £4 each (£100 for a bag of 25). However, Farnell seem to want about £7 for just one. I'm about to watch the football but later I'll show you how to make a cal kit using these connectors. It does require some careful work to make a decent open and short. Ideally, the difference in delay between the open and short should be less than 2ps if you want to make fairly critical measurements and this requires some care. Otherwise, the delay error can be corrected electronically using something like nanovna saver. Once you realise how critical the delay accuracy is you will realise how poor your current cal kit system is. The short looks like you have used a link wire for example. When full s2p measurements are made the through cal needs to be corrected for delay offsets/errors as well.
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26th Jun 2021, 5:00 pm | #52 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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26th Jun 2021, 5:08 pm | #53 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
Note that you don't 'have' to use these expensive connectors for doing stuff at 10.7MHz. There are cheaper SMA end launch connectors that can be used for making a decent cal kit. However, the delay issue is still relevant.
These connectors have something like 53ps delay (from memory) and they are only about 13mm long. So to get a delay error down to less than 5ps the open and short have to be within 13mm/10 = 1.3mm of each other. However, I would recommend trying much harder than this. It is possible to make the cal kit such that the residual delay error is much less than 2ps and the rest can be cal'd out electronically for the most critical measurements. Many people will probably dismiss this as 'phoolery' at just 10.7MHz but it depends on how accurately you might want to measure the ESR of an inductor or capacitor at 10.7MHz. A 5ps error becomes a big deal here even at these frequencies.
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26th Jun 2021, 5:09 pm | #54 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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26th Jun 2021, 5:16 pm | #55 | |
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Re: Relaxation oscillation in LC resonance/ Toko testers
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If you make a two port measurement of the transformer you can add whatever load you like in a simulator at a later time. You could put the 2 port transformer model into LTSpice along with a model of the rest of the IF circuitry. eg resistors caps and transistors and model the whole thing. That is the primary function of a 2 port VNA. It exports a valid network model of whatever is between the two test ports. This can then be used as a component in a simulator.
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Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 26th Jun 2021 at 5:22 pm. |
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26th Jun 2021, 5:20 pm | #56 | |
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Re: Relaxation oscillation in LC resonance/ Toko testers
[QUOTE=regenfreak;1385574]
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26th Jun 2021, 7:24 pm | #57 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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Here i tested a valve double tuned FM 10.7MHz transformer using S21 with each legs to ground. The minimum of logmag S21 is at 13.8MHz. The use of crocodile clips are unavoidable in real applications involving bulky components in vintage radios. So the question is: can the measurements be trusted or it is meaningless on its own? There is always uncertainty putting aside the issue of calibration kit. |
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26th Jun 2021, 8:01 pm | #58 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
Here is the S21 sweep of a DIY 2nd-order Butterworth filter designed for 40m band. It uses transformer coupling for both 50 ohms input and output impedance matching. So I am confident this is is correct as I did hand calculations using the method described in Zverev's book and simulation using Elsie. The results are fairly close.
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26th Jun 2021, 10:18 pm | #59 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
The double tuned transformer innards are unknown to me and I'm not sure how you have arranged the connections but I did try sweeping through a parallel resonator on a simulator with 50 ohm ports. In order to get the same response and selectivity as your plot I ended up with 11.2uH and 12pF.
This only needs an external 8pF to tune it down to 10.7MHz. The plot below is just a single resonator being swept so probably nothing like what is inside your IF can.
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26th Jun 2021, 10:23 pm | #60 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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I have done a measurement of series and parallel resonance frequencies of a 10.7mHz crystal. Since the impedance of a crystal may be the order of several hundred ohms, the impedance mismatch is not bad, therefore the S21 measurements can be useful in the characterization. The full steps for the crystal parameter measurements here: https://www.qsl.net/w2aew/youtube/cr...ts_nanovna.pdf |
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