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| Replacement for the PM9610 Prescaler for Philips PM6654 High precision Counter |
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| mos6502:
If you're looking for a different prescaler, check out the Fujitsu MB506. I've had very good results with it. On perfboard even. |
| SaabFAN:
Well, I got it mostly to work now. It took some cutting and component-acrobatics, but I finally managed to get a decent waveform at the output that the counter can use. I also discovered that the biggest problem wasn't the board itself, but the Si5351 Adafruit board I used as a signal generator. Using it at 120MHz caused the output to contain considerable jitter which made the output of the prescaler-board look a lot worse than it actually was. Self-Oscillation and a lot of harmonics is riding on top of the output waveform, but at as there is a circuit on the board that shuts off the output if there is no input-signal, this is no problem as long as the input-signal is handled correctly. Which brings me to the biggest problem I'm having at the moment: It does not handle the signal correctly! The second Prescaler performs the correct /4 division, but the first one doesn't. I already changed the chip because I suspected a faulty one from the 5 piece lot I bought on ebay, but the problem remains: The first prescaler only does division by 2. Both ICs have basically the same connection with both SW-Pins grounded. Btw. the datasheet specifies the SW-Pin to be open for a low-condition, but if I leave the pin unconnected the voltage at the pin rises to about 3,5V and it still keeps dividing just by 2. The picture with the counter displaying 100MHz was taken with the math function active which added a multiplier of 0.5 to the counter-result. When searching for other prescaler-chips I also found some interesting replacements (pin-compatible). A suitable set of chips would be the MC12026 (both control Pins set to high) together with the MC12083 (/2 divider with standby). The MC12026 could also be used standalone with some board-hacking, but that would defeat the standby-control as this chip has no standby-input. This chip is compatible with the boards I have here now. About building the board: I've specified BAT54JFilm and BAP50-03 diodes in several places, but with a cut off leg regular BAT54C or schottky diodes in a similar package will also fit. I also used 0402 components in several places so components ranging from 0805 down to 0402-package fit on the footprints. With board revision D some cutting of traces and mounting resistors and capacitors on top of each other is necessary. I don't know yet if I'm going to order another batch, as this board seems to work except for the problem with the first prescaler mentioned earlier (anyone have an idea about that?). EDIT: Was a bit late and forgot to add the picture of the counter with the prescaler in it :) |
| TheSteve:
Is there any chance your input signal isn't clean enough and the first prescaler is triggering on an undesired harmonic? Are you able to scope the input and output to see it really only dividing by 2? My only other thought is that the input signal is so noisy going into the first prescaler that it sees voltage on pin 6 even when it is grounded. |
| SaabFAN:
The sine-wave at the input is pretty clean. But as you can see in the attached pictures, there's excessive jitter on it at the output of the first prescaler. When I look at the output signal of the first prescaler more closely, it seems as if there's some frequency-mixing going on... |
| SaabFAN:
By the way, this is the signal from the Si5351. It is not exactly clean, but I don't have any better signal source upwards of 10MHz here at the moment. |
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