Products > Test Equipment

Replacement for the PM9610 Prescaler for Philips PM6654 High precision Counter

<< < (6/18) > >>

SaabFAN:
I actually thought about putting in some LL4148 but didn't do that because I though it was more important to use schottkys there.
Although at the moment it probably doesn't really make a difference because the highest frequency I can generate here is about 140MHz.

I have a solid ground plane underneath the second prescaler, but I have routed the +5V supply to the Standby-Pin on the top layer.

Interestingly, I have discovered that there seems to be a mechanical problem as well - If I put pressure on the board from the right side and at the correct spot, it suddenly puts out a perfectly clean ECL-Signal at 6,25MHz (Input = 100MHz). I have checked the signal at basically every place and it stays constant until the first prescaler. That one is the pressure-sensitive one...

I am now soldering another board with the copper-foil in place from the beginning and putting in 3,3nF coupling capacitors (easier to solder), as well as all the other hacks I have already employed to get it to work. Once I have it up and running I'll incorporate all the changes into the design  and order another batch in China.

It is true that my design could go higher than 1,5GHz because I use 2,5GHz prescaler-chips, but the counter is only able to count up to 120MHz, which translates to 1950MHz at the prescaler-input. Changing the capacitor will pull the notch higher in frequency - I just assumed 1nH inductance of the short 6mil trace parallel to the capacitor btw.

Performa01:
Regarding D1 and D4, at low frequencies the only difference between normal and Schottky diodes is the control voltage offset for the PIN diode attenuator/limiter. At 100MHz the difference is negligible in this application.

Much more important is that these diodes need to be fast enough to provide a proper detector function up to at least 2GHz. I’m not aware of any normal Si-diode that could do that, but an average Schottky diode won’t be any better either. You really need an ultrafast diode specifically designed for RF detector/mixer applications.

Just to illustrate the difference, I’ve simulated your input circuit with three different diodes for the RF detector: BAT15 (which is a proper RF mixer diode and roughly equivalent to BAT17, but appears to be more robust) BAT54 (which is not intended for HF) and 1N4148. See the results below:



As can be seen, with BAT 15 the input circuit works up to 5GHz within less than 3dB, no problem. Of course, the parasitic elements in a real world circuit would make the results worse, but we are just looking at the diodes right now.

BAT54 on the other hand causes 10dB attenuation at 1GHz due to its large capacitance already, and the automatic level adjust for input attenuation/limiting will not work at that high frequencies either. Ironically, the latter will most probably not even be a problem, as it merely causes additional attenuation at higher input levels.

1N4148 does a significantly better job than the BAT54 and causes little more than 3dB attenuation at 1GHz.

One more remark with regard to the input circuit. R10, L2 and C12 can be omitted, as this quite obviously is nothing more than a low frequency gain correction for the discrete amplifier in the original Philips design, not needed for the MMIC. This will further improve the overall frequency response, because less components in the signal path also means less parasitics.

Regarding the notch filter with C18, I don’t quite get it.
What’s the benefit of the prescaler turning off at higher frequencies?
And if there’s a benefit, wouldn’t be a lowpass filter be more suitable?

Finally for the ground plane. So I understand you only have a 2-layer PCB and there is no solid ground plane, i.e. other traces running on the same layer, thus disrupting it. Well, in this case, it’s not a ground plane any more. For a high frequency design like this, a 4-layer PCB with true ground (and supply) plane(s) would be clearly indicated.

SaabFAN:
I don't know what the Philips-Engineers were thinking either when they put the notch-filter in there. Maybe it was smaller than using a low-pass on the crammed board... We'll probably never know because I doubt one of the designers of this board will come here and tell us :)

I have now ordered a few BAT17 from Conrad and a VCO to generate higher frequencies. But for the time the LL4148 will have to do. They are the ones with the lowest capacitance and specified reverse recovery time - some manufacturers specify it in the datasheet some unfortunately don't...

The ground planes on the top and bottom layer around the RF-Path is basically uninterrupted except for the supply of the MMIC and next to the Prescalers. The one mistake I probably made was that I routed the pullup for the first prescaler on the top layer underneath the chip instead of the bottom layer.
The updated PCB has the pullup-line for the standby-pin of both prescalers routed in a way that the RF-Path up to the second prescaler has an uninterrupted ground-plane beneath it.
A 4-Layer board seems like overkill to me and would also be prohibitively expensive (27€ vs. 84€ for 10 boards).

Performa01:

--- Quote from: SaabFAN on October 21, 2016, 04:08:01 pm ---I don't know what the Philips-Engineers were thinking either when they put the notch-filter in there. Maybe it was smaller than using a low-pass The updated PCB has the pullup-line for the standby-pin of both prescalers routed in a way that the RF-Path up to the second prescaler has an uninterrupted ground-plane beneath it.

--- End quote ---

That sounds good.

Basically, you're right, the ground plane is most important for the high frequency section of the PCB.

But behind the 2nd prescaler, frequencies up to 120MHz aren't exactly low either. More importantly, this is the output of a digital divider, hence a digital signal with steep edges, hence lots of strong harmonics. So you should try hard to get a true (uninterrupted) ground plane for the entire signal path from input to output of the entire circuit - where "ground plane" refers to a reasonable low inductance (hence as wide as possible, but never less than say 3mm) copper trace going straight from input to output. This is the only thing that really matters. All the other ground connected copper areas will give some shielding, but do not contribute to signal integritiy in any way.

SaabFAN:
I'm pretty sure it is working now. The little piece of copper foil underneath the first Prescaler seems to have been the solution. The counter is now able to count 100MHz sent to the prescaler. Above 100MHz the output of the Si5351 gets really jittery and the frequency count jumps up and down several MHz at the prescaler-input and even worse at the normal Input (terminated with 50 Ohms of course).
I haven't checked or adjusted the Standby-Circuit yet because I'm currently using a LL4148 as the detector-diode and BAS70 Schottky-Diodes (those were the ones with the lowest capacity I could find in my part-box - Unfortunately without any information about reverse recovery in the datasheet) for the automatic Attenuator.

Now I have to wait for the diodes and build something that can generate higher frequencies. I have 2 GHZ VCOs in my part-box, as well as an ADF4351 Module intended for my Spectrum Analyzer. I'm still at least a little bit tempted to buy the 8922H that is offered on ebay for about a year now for 350€ - It has an RF-Generator in it that goes up to 1GHz. :)

Well, if someone want's one of these prototype-boards, you can have them for 5€ per board plus shipping. They require cutting one trace and soldering a 1nF cap manhattan-style to the first pin of the first prescaler, as well as the copper-foil underneath the first prescaler (see Screenshot of the 3D-Rendering of the board).
I'm going to order the new design in 2 or 3 weeks I think. Still trying to get a HP Spectrum Analyzer and I need to save money^^

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod