[time-nuts] 3GHz prescaler for Pendulum counters
GandalfG8 at aol.com
GandalfG8 at aol.com
Wed Oct 14 17:36:01 EDT 2015
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
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
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......
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
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
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.
More information about the time-nuts