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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, 10:40 pm | #61 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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26th Jun 2021, 11:12 pm | #62 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
I wonder if the vna approach may not be the answer, perhaps a noise genny (or sweeper) and Spectrum analyser in linear mode could be better?
I swear my old HP 141T/855X mainframe analyser unit, I could detect a gnat passing wind in central Africa I got so used to using that setup. As loosely coupled in and loosely coupled out as possible of course- letting the resonant cct. absorb just enough to show an effect, the sweep gen could be turned up and the SA turned down to accommodate (within reason and applied attenuators being used of course). The cct loading may be the big factor, but certainly not the only one under applied supply power conditions. Experimentation needed of course to get the desired result (if poss) This is a form of experimentation I have not really attempted to do in seriousness, as usually one can guess what the cct. resonance is designed to do when powered up especially. We usually know the common IF frequencies used, and if not, a rough measurement will usually lead to the eventual answer we seek. There may always be far too much difference between non powered and powered up equipment to give a "spot on" answer. Peaking powered up ccts. in the real world will be the only definite answer anyway. But, as the company motto always said "Close enough is good enough" well, normally in my case anyway. Last edited by Cruisin Marine; 26th Jun 2021 at 11:37 pm. |
26th Jun 2021, 11:45 pm | #63 | ||
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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Indeed maybe we are going down a rabbit hole. But there are always hidden secrets and knowledge that are not written in books and web sites. When i make a valve Fm radio from scratch, i always try to figure out why the manufacturers designed components in certain ways and not other...there were tricks that are intriguing. My natural tendency is asking lots of questions and driving myself nut. The other approach is to forget about it and move on, then i may take up another hobby instead. |
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26th Jun 2021, 11:51 pm | #64 |
Dekatron
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Re: Relaxation oscillation in LC resonance/ Toko testers
For sure it's always worth experimenting with other methods. If the double tuned transformer (or the radio) comes with a manual I'd expect it to show or state the source and load impedances required to deliver the 100kHz BW at 10.7MHz.
Looking at the circuit values this will probably mean the source and load terminations need to be many tens of k ohms in parallel with about 10pF. The coupling between the tuned circuits is also probably going to be very light so I'd expect a basic through measurement with a 50R VNA to show very little response at all. Having seen that this must be a lightly coupled double tuned circuit I can see why you want to match up to a higher impedance to test it. Does the manual state anywhere that there needs to be additional capacitance provided by the valve and maybe even another padding capacitor to tune the circuit down to 10.7MHz? Without this, the component values don't look right for 10.7MHz. It does look like it will be closer to 13-14MHz.
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26th Jun 2021, 11:51 pm | #65 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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26th Jun 2021, 11:58 pm | #66 | ||
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Re: Relaxation oscillation in LC resonance/ Toko testers
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Last edited by regenfreak; 27th Jun 2021 at 12:06 am. |
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27th Jun 2021, 12:07 am | #67 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
I wonder if you had your VNA croc lead setup like the image below for your S21 measurement? This does seem to give the same response that you were seeing? Can you check your connections to see if this is what you had?
If so, does the response change to look like the second image if you sweep the other resonator as per the revised connections?
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27th Jun 2021, 12:10 am | #68 | |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
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27th Jun 2021, 12:21 am | #69 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
Indeed maybe we are going down a rabbit hole. But there are always hidden secrets and knowledge that are not written in books and web sites. When i make a valve Fm radio from scratch, i always try to figure out why the manufacturers designed components in certain ways and not other...there were tricks that are intriguing. My natural tendency is asking lots of questions and driving myself nut. The other approach is to forget about it and move on, then i may take up another hobby instead.
We all do the same! To a lesser or greater degree. It is not a hobby, it is either an annoyance we are inflicted with, or a pure gift we are blessed with! I prefer to think it is the latter. It is wonderful to question EVERYTHING- that is what Mind Magic truly is! |
27th Jun 2021, 12:21 am | #70 | |
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Re: Relaxation oscillation in LC resonance/ Toko testers
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It could just be a lucky combo of parameters though. I don't know much about valve IF filters so that is my first go at trying to get one simulated.
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27th Jun 2021, 12:31 am | #71 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
I have wired exactly like yours:
primary dip at 14.4MHz -54.4db seconday dip at 13.4MHz, -51 db So the secondary is closer to yours. I was wiring like what i attached in the drawing; Shunt-primary and shunt-secondary with common ground |
27th Jun 2021, 12:35 am | #72 | |
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Re: Relaxation oscillation in LC resonance/ Toko testers
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Last edited by regenfreak; 27th Jun 2021 at 12:41 am. |
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27th Jun 2021, 12:43 am | #73 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
If you were to add 9pF across the input side then does this move the null fairly close to 10.7MHz in each case? See the plot below.
This indicates the resonator on the left is now correctly tuned to 10.7MHz when the external 9pF cap is added.
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27th Jun 2021, 12:45 am | #74 | |
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Re: Relaxation oscillation in LC resonance/ Toko testers
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However, someone else with more experience of LTSpice might be able to set up the simulation better than I can. I've only used LTSpice a few times.
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27th Jun 2021, 12:47 am | #75 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
Yes the trim caps can be regarded as the dynamic inter-electrodes capacitance ( and Miller capacitance) of the grid and the plate. It is a function of the gain and operating conditions of the IF amplifier valve.
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27th Jun 2021, 12:48 am | #76 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
Thanks for all this. Goodnight I am off to bed too
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27th Jun 2021, 12:37 pm | #77 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
The Japanese FM IF comes in a box set ( two IF transformers and a ratio detector transformer). The user manual provides the sweep response curves and recommended IF valve 6BA6 (see photos) which is variable-mu pentode. Usually it uses a plate voltage of about 75V.
From the 6BA6 datasheet: Anode impedance ra = 250k input impedance 4.5K Inter-electrodes input capacitance = 5.5pF Inter-electrodes output capacitance = 5pF anode resistor 10k (for 75V plate voltage) qm = 4.3mA/V Cin=Cgk+Cga(1+A) and A = Vout/Vin A= -gm X [ra/(ra +RL)] |
27th Jun 2021, 3:06 pm | #78 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
Thanks. I think there is enough info now to have a go at making a similar filter on a bare piece of PCB.
The VNA plot below is a first attempt but I made this with inductors that probably only have a Qu of 200 at 10.7MHz. However, I set the source impedance to 220k ohm and the load impedance was 68k ohm. I used the same inductances as the original and adjusted the coupling to get a 100kHz bandwidth. I padded the capacitances with the extra 9pF and 13pF as in the previous posts. I've used series resistive (very lossy) matching to set up the port impedances to make the plot below so that is why it shows about -65dB on the response. However, the passband looks about right. My nanovna struggles to make the same plot. I think I'll have to upgrade the nanovna firmware to allow a lower resolution bandwidth and a higher source power. I'll have a go at this in a bit and post up a nanovna plot of the filter. It might be best to try and repeat what I'm doing with the real IF filter as it only took a few minutes to set up the test below and that included soldering the parts together on a test board. There aren't many parts involved. The second plot is the Genesys model of the filter and it seems to agree quite well with the filter I just built using the same components and transformer coupling factor.
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Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 27th Jun 2021 at 3:18 pm. |
27th Jun 2021, 3:32 pm | #79 |
Heptode
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Re: Relaxation oscillation in LC resonance/ Toko testers
Thanks. I have tried to replicate your early reflection experiment on a Toko10k series FM black 10.7MHz IF. I have added reflected test sockets on the board.
The S11 test does not work, the Img S11 is a flat line; no sign change throughout. Then i have done S21, please attached. The Toko FM 10.7MHz has L = 4.1mircro H. I dont know C, may be it is 50pF. Data sheet does not state it. Q may be 50. |
27th Jun 2021, 3:34 pm | #80 |
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Re: Relaxation oscillation in LC resonance/ Toko testers
What are the two RF chokes for that are trying to hide in the green Trio box?
Could these be external matching components? Could you take a nice image of the Trio datasheet on its own and then pdf or zip it up and post it onto the forum? Otherwise the image quality will be too poor as the forum limits the size and quality of jpgs. It would be nice to see the plots on the datasheet.
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