WTB: Mixer or Prescaler for Pendulum CNT-90(XL) (minimum 6GHz) - Page 1
Forum section: electronics / general

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 22, 2025, 08:26:29 pm
content: I'm looking for a prescaler for the Pendulum CNT-90 (minimum 6GHz). I got my CNT-90XL on eBay, and the previous owner took the 40GHz prescaler out of it.Anyway, I'd love the Cojotech PMO-080, or if somebody has an official Pendulum mixer or prescaler available (for a reasonable price), please let me know.Thanks,Josh

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 22, 2025, 10:48:00 pm
content: I'd be interested in compatible prescalers even if they need repair. I'm open to reverse-engineering something if needed.AFAIK PM6690, PM6680, PM6681, PM6685, CNT-81, CNT-90, and CNT-91 all use the same prescaler design.Thanks,Josh

author: testpoint1
author_rank: Frequent Contributor
timestamp: August 03, 2025, 05:45:34 am
content: Quote from: KungFuJosh on July 22, 2025, 10:48:00 pmPM6690 add on interface is different as PM6680, 30 pins for PM6690.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 03, 2025, 05:50:11 am
content: Quote from: testpoint1 on August 03, 2025, 05:45:34 amI dunno how they did it, but people have this cojotech prescaler working on those:https://www.eevblog.com/forum/buysellwanted/fs-8-ghz-input-c-option-for-pm6680-pm6681-and-pm6685/But it's okay, you know which card I really want.Thanks,Josh

author: Cyclotron
author_rank: Unknown
timestamp: August 04, 2025, 03:14:33 pm
content: Quote from: testpoint1 on August 03, 2025, 05:45:34 amThe prescaler in my. PM6690 does not use all 30 pins. See photo.

author: testpoint1
author_rank: Frequent Contributor
timestamp: August 06, 2025, 05:29:27 am
content: Quote from: Cyclotron on August 04, 2025, 03:14:33 pmoh, sorry, my counter is CNT-90XL. seems hardware is different.

author: DaJMasta
author_rank: Super Contributor
timestamp: August 06, 2025, 05:52:32 am
content: FWIW, it probably shouldn't be. There are two major board revisions (different main processors) for the PM6690 which are very well documented in Fluke's Service Manual, and while I've found a few more board revisions with component placement differences, they all seem to be pretty equivalent and use the same connectors. I've worked in a handful of PM6690s and Tek FCA and MCAs, both CNT-90 and CNT-91 based, and they've all been effectively the same for the C channel riser. That said, the riser cards aren't. The pictured riser doesn't use all the pins but some use more - the microwave prescalers (10s of GHz) tend to use more for digital communication and maybe signal level whereas the 2.7GHz board pictured has somewhat less flexibility. I believe the Cojotech prescaler uses an even smaller set of pins than the Fluke one here, there are very few required to make a C channel function for counting.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 06, 2025, 03:15:07 pm
content: The hardware is the same. Allegedly, the only difference between the CNT-90 and CNT-90XL is that they cut the trace on J5 (for the XL version), and they code something on the board differently. They don't seem to want to share specifically what though.There are two types of C channel cards; prescalers and mixers. The non-Pendulum branded units, and the regular CNT-90s all use prescalers, which are available up to 20GHz. Above 20GHz they use higher quality mixers for 27GHz, 40GHz, 46GHz, and 60GHz. How much of the pin header is used is dependent on the card, not the counter.Anyway, I need one of the mixers, not prescalers, unless I can figure out how the models are coded differently. I don't want to send it to Pendulum for them to cripple my counter in case I do eventually get a mixer. If I can switch back and forth, that would be cool.Thanks,Josh

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 07, 2025, 04:02:58 pm
content: Hey, I think I saw your mom telling you not to try and scam people.