A couple of years ago, my oh my doesn't time rush by when you're having fun?:-), I posted a recommendation for 3GHz prescalers from Pawel Witkowski, an Ebay seller in Poland, for the HP 53131 and 53132 counters. I'm now very happy to report that Pawel has also produced a similar prescaler for the Pendulum range of counters, his Ebay ID is pa-fko and a typical auction is 111794690250. Pawel actually lists these as being for the Philips/Fluke PM6685, this being the counter he designed them for, and with a comment that they also work with the PM6680, but those are rebadged Pendulum counters and a look at the service manuals suggested that the original Pendulum prescalers were likely to be a one module fits all solution, and this has turned out to be the case. For the past few weeks I've been using a pair of these modules, one fitted to a CNT90 and the other to a Tekktronix FCA3100, which is a rebadged CNT91, and I'm very happy with the results. Once installed the counter automatically recognises that it now has an input "C", and that's all there is to it. The built in statistical plotting options work for input C just as they do for inputs A and B, but note that the level measuring functions of inputs A and B are hardware dependent and aren't available for input C, this also applies to the original Pendulum prescalers. There's a graph of measured sensitivity and "application area" included with the auction photos and I'm seeing results that agree closely with that. Up to 1GHz I've run tests using a Marconi 2022E signal generator and above that I've been playing with a Chinese clone of the Anolog ADF4351 evaluation kit. I don't have any independent means of checking the levels from the ADF4351 kit above 1.5GHz but I've no reason to believe it's not performing as it should, certainly the frequency accuracy is as expected. The service manuals indicate that later models have extra pins on the prescaler connector but the functions of the rear sixteen pins have remained unchanged, and ensuring that the prescaler is mounted to the rear of the connector, regardless of the number of pins that leaves spare, is all that's required. A couple of points to note, the mounting clips shown in the auction photos are already fitted to the PM6685 but were not fitted to my CNT90 or FCA3100, so these had to be supplied with the prescaler. Anyone ordering these should make it clear what counter they will fitted to. There are two predrilled 2.5mm diameter holes in the front panel for the N connector that will accept M2.5 screws but the shape of the casting does not allow any nuts to be fitted. The prescaler board is provided with self tapping screws, although I chose to run an M3 tap through the holes and use standard screws, but I suspect the best solution would probably be to use M3 Taptite screws.
For anyone who might be interested I've uploaded a file containing a set of similar photos for each counter showing stages of the installation, and also some photos of the counters under test and the ADF4351 module...... https://mega.nz/#!qNYiDCzC!HHWEpI8TUDW6VO_fuyk34uU6Tr4V-pXRLru-DMtaq2 Apologies for the messy looking link, it's nothing sinister, just that the site encrypts all files and most of that link is the decryption key. For these tests, both counters and both frequency sources were referenced to the same Trimble Thunderbolt. At 1GHz, using the Marconi 2022 signal generator, I checked sensitivity down to -40dBm, possibly could have gone lower but didn't see too much need for that:-) The ADF4351 evaluation kit only has selectable output levels of +5, +2, -1, and -4 dBm, so for higher frequencies I took a different approach and tested for the maximum frequency that would display properly on the counter at levels of -4dBm, as indicated by the ADF4351 software, and also at -7dBm and -10dBm by adding attenuators. The module was mounted a very short distance from the counter using semi-rigid coax to an SMA to N adapter, again this can be seen in the photos, and 23GHz rated Aeroflex 3dB SMA attenuators were added directly between the synthesiser module and the counter input, although this is not shown. Both counters worked fine at 3.00 GHz with the -4dBm signal and whilst there was a difference between the two counters as the frequency increased it's obvious that both would have worked fine at 3.00GHz with the -7dBm signal also and very close at -10dBm. It might be interesting to swap the modules between counters, to see if that has any effect on the overall sensitivity at higher frequencies but it's not something I've tried so far, and I don't see it as a problem either way. With the FCA3100 (CNT91), the limits I measured were..... at -4dBm ---- 3.500GHz at -7dBm ---- 3.400GHz at -10dBm --- 3.325 GHz With the CNT90, the limits I measured were...... at -4dBm ---- 3.200GHz at -7dBm ---- 3.025GHz at -10dBm --- 2.925GHz The usual disclaimers apply, I paid full price for both my units and I'm just a very happy customer with no vested commercial interest in this item whatsoever, but believing it to be a great product and excellent value I felt that others here might be interested too. I've mentioned the Pendulum service manuals, and will comment further on that in a follow up post. Regards Nigel GM8PZR _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.