Beginner question relating to HP3456A - Page 1
Forum section: electronics / general

author: Atomillo
author_rank: Regular Contributor
timestamp: March 02, 2022, 01:48:41 pm
content: Hello:I've recently started learning about electronic metrology. For christmas, I got myself an HP34401A and an HP3455A. Both came from eBay and had no calibration stickers. However, after a bit of warm up, the both agreed pretty much EXACTLY (difference of around1-2uV10-20uV). I recently got and repaired an HP3456A. The reparation didn't touch any of the analog boards so I would expect any calibration to still be in effect. HOWEVER, the unit has the crystal used for 60Hz.When connecting the three meters to a 5V regulator and configuring all three for 6-1/2 digits however, the 01A and the 55A still agree, but the 56A is consistently5-7uV50-70uV higher than the other meters. Both have been on for about 5 hours and this test has been repeated two times, with identical results.My question is: could this be caused due to the poorer mains line rejection due to not having the correct crystal, or could it be that the 56A is out of cal?Many thanks for helping this newbie!

author: daqq
author_rank: Super Contributor
timestamp: March 02, 2022, 02:21:00 pm
content: I'd say that it's not out of cal, but well within spec on the +/-0.0008% accuracy (0.0008% of 5V is 40uV). If there was an issue with the 50/60Hz rejection, I'd expect it to jump around more if nothing else.

author: Atomillo
author_rank: Regular Contributor
timestamp: March 02, 2022, 02:49:42 pm
content: Daqq thanks for your reply!Yes, based on other posts, I was expecting the last two digits to jump around. But the meter is remarkably stable. Only the last digit fluctuates, and always no more than by 1-2 counts in 6 measurements at the native measuring speed.If the 56A is ok, then I must wonder: why do the other two meters agree so closely? One of the reasons I originally bought two voltmeters was to be able to check one agaisnt the other, the idea being that if the read something close one could judge if they were ok without having to use a reference voltage. Of course I was happy when I saw such close agreement.But is it really expected that such an old instrument with no known calibration date agrees so well with a much more modern one?

author: bdunham7
author_rank: Super Contributor
timestamp: March 02, 2022, 02:49:56 pm
content: If I'm reading it correctly, your 3456A is showing 50uV (5 counts) lower than the other two. Even if the other two are deemed to be exactly correct, the 5-count error is within tolerances for even the 24-hour specifications. IOW, within specs. As for the PLC issue, it is hard to say for sure. I would expect the wrong PLC frequency to give you a bit of random variation that would decrease as you go from 1 PLC to 10 and then 100, but it might also cause a small offset. Careful use of twisted wire pairs for your connections can help here.

author: Atomillo
author_rank: Regular Contributor
timestamp: March 02, 2022, 02:56:09 pm
content: You are correct, it's 50uV not 5uVMy badI will try to see if the reading changes when using more PLCs and I will also twist the cables connecting the 55A to the 56A. The new crystal has been order and should arrive within three weeks.Even then, wow, more than 30 year old equipment within specs?

author: TimFox
author_rank: Super Contributor
timestamp: March 02, 2022, 02:57:36 pm
content: You can adjust the overall full-scale voltage by a trimpot in the 3456A, but obviously you need to tread carefully with such an adjustment.

author: Atomillo
author_rank: Regular Contributor
timestamp: March 02, 2022, 03:02:40 pm
content: I think that until I have a trustworthy voltage reference (and more experience and knoledge on the topic) I would prefer to mess with the instrument.If right now it is on specs, then I would prefer to let the beast sleep. Guess I will have to more slowly on the lab from now on...

author: Atomillo
author_rank: Regular Contributor
timestamp: March 02, 2022, 04:15:15 pm
content: Hi:I've left the meter at 100PLCs and also twisted the wires. The difference esentially is the same, but the readings are very stable. Once the crystal arrives I will post more results.Still very very impressed with this meters.Thanks for the help as always, still trying to fully grasp the meaning and specification of accuracy, stability, etc...

author: Kleinstein
author_rank: Super Contributor
timestamp: March 02, 2022, 04:23:58 pm
content: The clock frequency ( 60/50 Hz mode) is mainly a thing with signals that have large hum. The 5 V or 10 V ref. signals are usually rather clean and thus little effected from the wrong clock.The main effect of hum would be more like noise or a very low frequency beat frequency.There is a chance that one may have to redo the calibration with the 3456 if the crystal is changed. So it would be good to have a good comparitson with the other meters and maybe ref. circuits before the crystal swap.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 02, 2022, 05:50:00 pm
content: The 3455 and 3456 meters tend to be inexpensive, probably because of their large size, but are remarkably good. My old 3455 has been stable and within a dozen ppm for decades.

author: daqq
author_rank: Super Contributor
timestamp: March 02, 2022, 09:28:11 pm
content: QuoteAgreed. Had a 3456A (https://www.daqq.eu/?p=577), beast of a machine, made a sound like it was going to take off, but when I bought it I compared it to our regularly calibrated DMM at work. Spot on.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 02, 2022, 11:55:37 pm
content: Quote from: daqq on March 02, 2022, 09:28:11 pmThat's then a first generation 3456A. Later ones were fan-less and hence dead-silent.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 03, 2022, 12:00:58 am
content: Quote from: Conrad Hoffman on March 02, 2022, 05:50:00 pmI can't help thinking that they're also comparatively cheap because, although they do have GPIB ports and can be read-out remotely, can't be automatically calibrated (adjusted).

author: Atomillo
author_rank: Regular Contributor
timestamp: March 03, 2022, 06:05:33 am
content: Quote from: daqq on March 02, 2022, 09:28:11 pmIn my case I bought one specifically without the fan. Not only for the noise but also because I suspect better thermal stability I'd achieved this way.However the HP3455A is much more noticeable. Maninly the clicking of the auto cal relays. If I'm tired or sleepy I have to be careful because the ryrhmic sound could just made fall over the desk.

author: Atomillo
author_rank: Regular Contributor
timestamp: March 03, 2022, 06:07:32 am
content: Quote from: guenthert on March 03, 2022, 12:00:58 amI personally see the trimpot calibration as a pro: no need to worry about some non volatile memory becoming volatile.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2022, 08:27:12 am
content: Quote from: Atomillo on March 03, 2022, 06:07:32 amTrimpots can also become volatile: trimmers are common point of failure and they tend to drift more than fixed reistors. The trim range is limited and trimmers may have to be combined with selectable resistors/jumpers for the initial adjustment to compensate for resistor / reference tolerances.If done right, an adjustment via numerical cal constants can be fully transparent and reversible. So one can do an adjustment and still keep a cal history without extra errors. This is at least the theoretical side - practical implementations may fall short and often lack documentaion.The fan helps to get faster thermal stability. So the warm up time can be shorter with the fan. A lower internal temperature also tends to reduce the long term drift and may result in longer life of some parts (e.g. capacitors). A downside of the fan is collecting more dust and dirt, which can be a problem.

author: Atomillo
author_rank: Regular Contributor
timestamp: March 25, 2022, 08:35:35 am
content: Small update: yesterday I received the custom ordered crystals from Krystaly. Very happy customer: reasonable prices and great support.Unfortunately, this change doesn't seem to alter much. After a couple hours of warm up, the offset of around 70uV is still there, which indicates me that it's just a cal difference.

author: dietert1
author_rank: Super Contributor
timestamp: March 25, 2022, 08:53:13 am
content: Unless somebody did it for you, the 3456A may need some maintainance of the inguard power supply caps. There are two voltage doublers and their caps don't last as long. Another weakness of the first series with fans is the big "bone" relay they used for range switching. When measuring 5 V that relay is closed, but how well? It may need exercise. Also the front-back switch may need exercise. If you want precision, you may have to exercise those contacts regularly, e.g. once per day or per week.Instrument checks usually start with getting a stable and precise null reading with a short over the inputs. And there is a jumper inside the 3456A to test the ADC null independent of the front-end.Regards, DIeter

author: Atomillo
author_rank: Regular Contributor
timestamp: March 25, 2022, 01:58:39 pm
content: Many thanks for your kind input.After reading it, I decided to measure 15V, that is, to use the 10V scale and bypass the relay. The offset still was there and was pretty much equal in magnitude.After that, I just shorted the inputs together, and in the .1V range I obtained a reading of 0.0040 E -3, when the maximum permissible according to the manual is 0.0024 E -3. Unfortunately, there doesn't seem to be an offset trimmer.Do you know anything I could do to try and solve this? I also used several the times the "front-rear" switch but this had no change.EDIT: THIS IS A MISTAKE IN MY PART. The offset with nulled inputs is higher when the "Filter" function is turned on (this is NOT indicated in the service manual). Configured as instructed, i get an offset of 0.0020 E -3, within specs. My badEDIT: also applying 15 V will also, in auto mode, result in using 10V range. Really should think before writing eh

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2022, 07:06:46 pm
content: Enabeling the filter can add a little leakage currentfrom the switches - though this should not be very much. Chances are actuall that the off mode may have more leakage from the negative side and this may compensate some of the offset. A shift of some 2 µV with 100 K of resistance from the protection would be some 20 pA of leakage current, which is a bit on the high side.To see if the observed offset is more of a true offset voltage or more of excessive bias current, one could do a test with a higher resistor (e.g. 10 M) across the input. This could also work with the meter in a mode with 10 M input resistance mode an otherwise open input.