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Replacement for the PM9610 Prescaler for Philips PM6654 High precision Counter

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SaabFAN:
I have looked at the difference between the connection of the first and second prescaler:
The only difference is that the first one has a trace running under it that is connected to the +5V supply.
Could that be enough to cause oscillation if there's some noise on this particular trace?

TheSteve:

--- Quote from: SaabFAN on October 20, 2016, 03:31:22 pm ---I have looked at the difference between the connection of the first and second prescaler:
The only difference is that the first one has a trace running under it that is connected to the +5V supply.
Could that be enough to cause oscillation if there's some noise on this particular trace?

--- End quote ---

Hard to say for sure but having a solid ground plane under the IC is important and standard practice for the frequencies involved.

Performa01:
I’ve had a brief look at your circuit and there are a few issues striking me immediately.

D2 and D3 are specified as BAT54? These are totally inadequate, with a capacitance of 10pF @ 1V, compared to the original BAT17, which is 1pF @ 0V.

This means that the input of the MMIC is loaded with ~20pF and the corner frequency is 318MHz, thus making the circuit very insensitive at high frequencies.
Furthermore, at higher frequencies like 1GHz, the input impedance seen by the MMIC is very low and almost purely capacitive, which might lead to stability issues.

Apart from the capacitive loading, these diodes appear to be part of an RF detector which produces the control voltage for the pin diodes. Therefore these diodes need to work up to several GHz, whereas a slow BAT54 will not be good for more than maybe 100MHz.

Then the output of the MMIC is loaded with 25 ohms, which means a major mismatch (C13, R11 in parallel with C14, R20).
BTW. What is that shorted C18? Even if it’s not shorted, it will still act like a short at 1GHz or above anyway. This again will not aid stability and clean signals.

So I recommend solving these issues first before looking any further.
BAT14 is still available, but of course any other UHF mixer diode will do the job, as long as the capacitance is <1pF and the max. forward current can be at least 30mA.

SaabFAN:

--- Quote from: Performa01 on October 20, 2016, 06:13:50 pm ---I’ve had a brief look at your circuit and there are a few issues striking me immediately.

D2 and D3 are specified as BAT54? These are totally inadequate, with a capacitance of 10pF @ 1V, compared to the original BAT17, which is 1pF @ 0V.

This means that the input of the MMIC is loaded with ~20pF and the corner frequency is 318MHz, thus making the circuit very insensitive at high frequencies.
Furthermore, at higher frequencies like 1GHz, the input impedance seen by the MMIC is very low and almost purely capacitive, which might lead to stability issues.

Apart from the capacitive loading, these diodes appear to be part of an RF detector which produces the control voltage for the pin diodes. Therefore these diodes need to work up to several GHz, whereas a slow BAT54 will not be good for more than maybe 100MHz.

Then the output of the MMIC is loaded with 25 ohms, which means a major mismatch (C13, R11 in parallel with C14, R20).
BTW. What is that shorted C18? Even if it’s not shorted, it will still act like a short at 1GHz or above anyway. This again will not aid stability and clean signals.

So I recommend solving these issues first before looking any further.
BAT14 is still available, but of course any other UHF mixer diode will do the job, as long as the capacitance is <1pF and the max. forward current can be at least 30mA.

--- End quote ---

Thanks for pointing out the problems.
I completely missed the parallel connection of the termination resistors :)
I just replaced the resistors with 100Ohms each, which improved the sensitivity a bit.

The Diodes will have to stay the same for the time being, as I don't have any better ones, but I will try to get better ones on ebay (digikey requires 65€ order and I just ordered something about a week ago^^).

C18 is connected in parallel with an tiny microstrip-inductor on the board, forming a notch-filter at about 1,55GHz. My best guess is that it is supposed to stop RF-Power from reaching the detector if the input-frequency exceeds 1,5GHz and therefore the Prescaler is disabled - I haven't paid too much attention to that part though and just copied it from the original schematic :)

The biggest success I just had by inserting some copper foil underneath the first prescaler that I connected to ground. At least when connected on the bench, I now have the correct division ratio at the output. It seems to require some "warm-up" time, but after about 30 to 60seconds it shows the correct frequency (this might be a problem with the Si5351 though).

Performa01:
So you’re making progress …  :-+

Don’t you have a ground plane under the prescaler? If so, the copper foil shouldn’t make a difference. Sorry if this sounds daft, but I haven’t really studied your PCB design.

As for the diodes – for the time being, even a humble “computer diode” like 1N914 or 1N4148 would surpass the BAT54 by quite a margin. Even better would be 1N4151 or at least 1N4448.

Yes, these are no Schottky, but that doesn’t really matter for now.

Try to replace the BAT54 with the fastest Si diode (lowest capacitance) you have, and then compare the sensitivity at 1GHz or above – you should see a substantial improvement, even though quite obviously no Si diode can ever match a dedicated RF detector/mixer Schottky diode.

EDIT: oh, forgot to mention the notch filter at 1.55GHz - isn't your design meant to go higher than that?

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