Adjusting HP 5315A Reference Oscillator? - Page 1
Forum section: electronics / general

author: E63S4Me
author_rank: Contributor
timestamp: April 12, 2021, 12:19:00 am
content: I have an HP 5315A frequency counter which is showing between 45 and 50 Hz off when measuring the 10 MHz from my 10 MHz Rubidium frequency reference.I've tried adjusting the two capacitors referenced in the manual, these are the course tune and fine tune, but can't get the frequency to read any closer to 10 MHz than the +45 Hz value.Any suggestions? I'm thinking I'm going to have to swap out the fixed capacitor that is part of the crystal alignment circuit to compensate for the aging to bring the frequency back down 50-100 Hz or so to get it within the adjustment range of the variable caps, but would welcome suggestions.

author: tkamiya
author_rank: Super Contributor
timestamp: April 12, 2021, 03:48:43 pm
content: It's very possible that in 30+ years, the internal reference oscillator has drifted out of range. If you have a rubidium reference, why not use that as a reference? Typically, 10MHz crystals can be moved up to around 10Hz before it starts to lose stability. If you insist on having an internal reference, I'd place OCXO instead. You can buy them on eBay for less than 50 dollars. Adopting one is usually straight forward.Typical of HP and few others I know, standard oscillators are rather poor even when they were new. They were placed there just to show the device works. Buyers are expected to use house reference or buy expensive optional OCXO.

author: E63S4Me
author_rank: Contributor
timestamp: April 12, 2021, 04:22:40 pm
content: Thanks,The unit that I have does not have the optional input jack for an external reference. That said, I do already have plans to add that capability to the frequency counter so I can use the Rubidium while the unit is in the shop.But mainly, I would like be able to use this counter in the field or to perhaps loan it to a friend. So it won't always have a Rubidium handy.I did actually order an OCXO off eBay last week thinking I could add that since there is a connector and switch inside the unit that facilitates using an internally mounted TCXO or OCXO. Should be straightforward to fab a matching interface to connect the OCXO to the unit.In the meanwhile, I would like to get the unit on frequency for fun. More of an intellectual exercise than anything else. Currently the unit does tune from ~10,000,045 Hz to 10,000,250 Hz, but I didn't do any testing of how stable the frequency was when set.So, I guess I'll swap out the fixed capacitor and see if I can move the frequency down a bit. Straightforward and easy to revert if it doesn't work.Thanks again

author: bdunham7
author_rank: Super Contributor
timestamp: April 12, 2021, 04:39:38 pm
content: IIRC, there is another trim capacitor on the main board of my 5316B that will also affect the frequency of the base option oscillator board, but not the TXCO or OXCO versions. I'm not sure if yours has the same.User amc184 designed a very nice OXCO board for these counters using a used Morion 5V 10MHz OXCO. I built one and I think it would be the optimal solution if you really want a stable and accurate reference. I believe the info required is in this thread:https://www.eevblog.com/forum/testgear/hp-5316a-teardown-and-frequency-standard/https://www.ebay.com/itm/Morion-MV85-5V-10-MHz-OCXO-SC-Sine-TESTED-DATE-2008-2009-50-7dBm-2/312303291797?hash=item48b6ba0d95:g:ijgAAOSwjg1b4sn~

author: E63S4Me
author_rank: Contributor
timestamp: April 12, 2021, 04:52:53 pm
content: Thanks for the link to the 5316a teardown,I will have a look on the board for other adjustments.That 5 V OCXO looks very similar to the one I purchased.

author: E63S4Me
author_rank: Contributor
timestamp: June 06, 2021, 06:36:34 pm
content: Just a quick follow-up to report that I finally got around to replacing the fixed capacitor in the base crystal reference oscillator circuit. The unit now tunes a bit above and below 10 MHz and I was able to set it spot on at 10 MHz +/- the basic stability of the oscillator circuit, which seems to be about +/- 5 Hz at a relatively stable temperature.Next stop is to build up the adaptor board for the OCXO I purchased and add the capability to use an external reference like my Rubidium or GPSDO when using the counter at home.