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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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5th Dec 2020, 7:10 pm | #21 | ||
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
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Re: Stagger-tuning of FM stereo valve IF stages
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I thought "monitoring the anode of the following pentode" would include the effect of the primary IF of the following IF stage. I would draw picture to see if we are thinking on the same page |
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5th Dec 2020, 8:38 pm | #22 |
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Re: Stagger-tuning of FM stereo valve IF stages
Valves tend to act as current sources, so the signal voltage on the anode of the following stage is a strong function of the resonance of the next IFT. So if you want to see the response of one IFT, then the grid of the following stage is the place to look.
David
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5th Dec 2020, 9:04 pm | #23 | |
Heptode
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Location: London SW16, UK.
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Re: Stagger-tuning of FM stereo valve IF stages
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6th Dec 2020, 9:47 am | #24 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
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Re: Stagger-tuning of FM stereo valve IF stages
I'm withdrawing from this discussion as you're now receiving inconsistent advice, and that can only add to the confusion.
John |
6th Dec 2020, 12:07 pm | #25 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
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Re: Stagger-tuning of FM stereo valve IF stages
I will try probing both grid and anode of the following stage to see what the differences they will make on the visual sweep. I always learn something new by digging deeper.
The temptation of any project is that once it is working after the construction/repair, we tend to get done with it. The fun part is when probing technique looks simple on the surface, it isn't simple underneath. When the measured waveforms look weird on the scope and then wait a minute...Houston we have a problem. For sure, the the stage gain can only be measured by the grid of the following stage. Otherwise you include or multiply the gain of valve amplification of the following stage. Last edited by regenfreak; 6th Dec 2020 at 12:18 pm. |
6th Dec 2020, 1:11 pm | #26 |
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Re: Stagger-tuning of FM stereo valve IF stages
What you can di is to fit a resistor load for the anode of the following stage then that following stage acts as a buffer for your probing and you don't therefore detune the IFT you're interested in because the load pulling it is the grid of the stage after, which even with the resistive anode load fitter will be close to what the IFT has to handle in proper use.
There are many ways of doing these things, but on the whole it saves trouble to look at things in as close to their natural environment as you can. Always be aware that filtery things are heavily dependent on the loads applied to their input and output. For example, if someone gave you a box of ideal lossless capacitors and inductors, you'd think you could make some pretty spiffing filters, right? Right! Now, that energy you put into the finished ideal filter, there's stuff on the wanted frequency and on unwanted frequencies. Where does the unwanted frequency energy go? It isn't allowed out of the output, and all the filter components are lossless, so there is only one place for it to go - reflected back out the input. Even with lossless components your filter will have a transition region where the reflection complements the passband. For coupled resonator filters, what about Q? it's kind of important for positioning your poles to get a chaosen response, but your lossless parts all have infinite Q. B*gger! your filter is going to be infinitely narrow. Nope, it relies on the resistive component of both the source and load impedance to bring down the Q of the first and last resonators respectively. The inner resonators have their Qs set by coupling to the outer ones. So source and load impedance are critically important. David
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6th Dec 2020, 3:59 pm | #27 | ||
Heptode
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Location: London SW16, UK.
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Re: Stagger-tuning of FM stereo valve IF stages
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Instead of building a test jig with LC input and output impedance matching networks, a magnetic sniffer probe ( see attached photos)can be used to measure the resonance frequencies of the primary and secondary IF. This contactless measurement gives more accurate resonance frequencies of double-tuned IF LC filter eliminating the detuning effect, impedance mismatch etc with S21 measurement. As you move the probe along the longitudinal axis of the IF coils, you can see the shift of dominance of the dips: f1 and f2. The yellow line is S11 return loss. In theory, you can measure the Q using the contactless probe but in practice the S11 dip is too small to allow accurate 3db readings. Last edited by regenfreak; 6th Dec 2020 at 4:04 pm. |
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6th Dec 2020, 6:35 pm | #28 |
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Re: Stagger-tuning of FM stereo valve IF stages
Q is difficult to measure, the very act of getting some signal out to measure the growth of peak voltage damps it, the very act of injecting current etc, ditto.
What you do is make a measurement, then try again with lighter coupling? Did the result change? If so lighten the coupling yet more until you start getting pairs giving the same result. David
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6th Dec 2020, 8:05 pm | #29 | |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
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Re: Stagger-tuning of FM stereo valve IF stages
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The VNA works like a weak transmitter. The amount of reflected RF power becomes less when the VNA sweep frequency coincides with the resonant frequency of the LC resonator that it "draws" the energy. |
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6th Dec 2020, 8:39 pm | #30 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
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Re: Stagger-tuning of FM stereo valve IF stages
I could not resist it in the name of science. It is difficult to get a steady dip reading...very fiddly.
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