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| Fast precision rectifier/peak detector needed (5 MHz, 100 mV) |
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| ebastler:
--- Quote from: BrianHG on January 01, 2019, 10:01:32 pm ---Careful, TV color delay lines not only delay 1 line, they delay 2 or 3 while super-imposing the results on top of each other with inverted amplitudes creating a color-comb filter effect. This effect is used to remove vertical cross color artifacts in color decoding and makes the glass delay line filter out and amplify a narrow frequency, adding gain, or canceling out stray what you might call 'single bits' in your type of application. --- End quote --- Hmm, I can't confirm any multiple reflections. If I send in a single burst, I see the output after 64µs as shown in the OP. At 128µs and further out, the output signal is within the noise. That's true for the 5 MHz signal shown in the OP, but also at the 4.4 MHz resonance. I thought that in PAL TVs, the Chroma processing between successive lines was done in analog electronics, and the "delay line" is really only providing the delayed information? I use 390 Ohm termination, as seen in an old datasheet, but the termination value does not seem too critical. I did not provide any LC filters at the input and output however, in contrast to the datasheet circuit. --- Quote ---You may get lucky and find a really weird odd old glass delay line filter which doesn't do this, however, my experience is that they were designed to improve color separation beyond a R-L-C filter by creating a delay with internal reflections to specifically create the color combing effect I described above. --- End quote --- So far I have tried three different types/makes of delay lines. All of them seem to be PAL chroma types, with approx. 64µs delay and a resonance around 4.4 MHz. They all behave as discussed above, and all seem to provide a single-reflection (64 µs delayed) output. --- Quote ---Also, at the tuned frequency, I got 1vp-p going through a few of the NTSC units I had. --- End quote --- Yes, that part has me wondering. I don't get anywhere near the output amplitudes Mike showed in his video. (At resonance, he shows output amplitudes in excess of the input.) That's true for all three delay lines I played with. I'm using 390 Ohm in series at the input, 390 Ohm in parallel at the output. Is there any additional trick to this? |
| ebastler:
I found a nice article from Philips about the first generation of glass delay lines for PAL (1968), http://www.extra.research.philips.com/hera/people/aarts/_Philips%20Bound%20Archive/PTechReview/PTechReview-29-1968-243.pdf, and a datasheet for various delay lines which are apparently still available from China: www.eectech.info/DocumentDownloas.aspx?id=YNjvLwD6n5BtaNw4QXUZ3LiNOytfWQ3ink6quaWab1xkcjliV44b6g== (see pages 6 and 7 for the 64µs glass delay lines). Both sources describe a typical insertion loss of 9 to 10 dB. So the factor of approx. 10 between input and output amplitudes, which I observe somewhat above the resonance frequency, seems reasonable. Not sure what Mike did to achieve his very high outut amplitudes -- maybe the scope was still set to assume a 1:10 probe which wasn't there? Also, the old paper and the new datasheet specify an attenuation of higher-order reflections by at least 21 db and 26 dB, respectively. Hence, I am pretty sure that the delay line itself is not meant to create and mix multiple delayed versions of the input signal, but is designed to produce a single, delayed copy of the input. |
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