18
Nov
14

ESR Meter Repair and Investigating Jitters

This weekend I decided I’d tackle a repair that has been waiting quite some time. A bit more than a year ago I built an ESR meter from a kit. The version I have is the 2007 Bob Parker meter that was featured in Silicon Chip. I built the kit and was quite pleased with the result except during calibration of the battery voltage warning I manage to break the trimpot. Fortunately the unit still worked quite well, so I wasn’t all that concerned, the only downside being that it warned of a low battery all the time.

This weekend I happened by Jaycar to buy some unrelated stuff when I remembered the damaged trimpot and looked through the parts bin. I didn’t remember the value of it exactly, I thought it was around 200k Ohms. So I ended up getting a 250k part as it was the closest value, thinking that would be close enough. However I had remembered the value incorrectly as it is actually 10k Ohms.

trimpot circuit diagramRather than going back and looking again for another part I looked at the circuit diagram. It seemed that the resistance value of the trimpot shouldn’t matter too much. Here’s a diagram of where it fits into the circuit.

It looked like the trimpot is basically providing a voltage for the IC to detect a low battery, it also has a resistor either side to limit the current flowing through it. I figured the higher value should only reduce the amount of current flowing through the trimpot and that I’d still be able to adjust it such that the IC would get the correct sense voltage. So I went ahead and replaced the broken one with the 250k new one.

After fitting the new part I tested the meter, it powered up and didn’t report a low battery, success! I needed to adjust the trimpot so that the low battery alarm would work correctly. I didn’t have a variable power supply so I created the low voltage by putting in some older cells that had about the right low voltage I needed. Adjusted the trimpot and checked that good batteries continued to not set of the low battery alarm.

As an extra measure I checked the meter against the calibration resistors to make sure the meter is still accurate.

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Something else I heard about recently was people finding trigger jitter in particular models of Rigol Oscilloscopes. I first heard about it from watching a recent EEVBlog episode where Dave does some tests on his own equipment. I decided to test out my Rigol DS2072.

First let me explain trigger jitter a little. An oscilloscopes trigger mechanism is what starts the scope both capturing and displaying at a particular point in a waveform. Trigger Jitter is basically a situation where the scope will not start at the same part of a waveform each time, causing the waveform to literally jitter backwards and forwards on the display. This can be a problem if timing is sensitive in the signal and for performing some measurements.

Unfortunately I don’t have a signal generator (something I should rectify) so I had to use the 1Khz square-wave output on the scope normally used for compensating the probes. At first glance it wasn’t apparent that there was any jitter at all.

I also checked the AC coupled waveform and didn’t see a difference, then I realised I should probably zoom in on the waveforms to see if the jitter was just too small to see on the scale I was using.

I checked the DC coupling again like Dave did, checking intervals to see if there was any periodic jitter. I found none just like he did.

I then checked the AC coupled version and you can see here that there is indeed some jitter. I didn’t realise it when I did the test, but this is actually a different instance of jitter than what Dave on the EEVBlog found. He was testing AC coupling for the trigger input, here I’ve tested AC coupling on the input itself. I’ve run out of time this weekend so I’ll test the trigger coupling soon, but this illustrates that AC coupling on the input can also cause jitter in these scopes.

So what does this mean for me and using my scope going forward? Well as long as I use DC coupling I don’t have to worry about it, as that doesn’t seem to have the issue. I need to be aware of it when using AC coupling, and use appropriate means to reduce the problem. Although given what I’m interested in, I’m fairly unlikely to really need AC coupling most of the time as digital circuits that I’m interested in can be probed with DC coupling mostly. I’m still happy with my scope, although I haven’t had much call to use it for a while.

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