Good morning all!
Quick question for you. I've done plenty of 3325A/B function generators in my time but had one question that has always eluded me for some time. In the Air Force, I was told that the 50Ω termination needed to be placed on the 3325A/B despite the rule of being placed on the input device (meter). However, the Army and even the manufacturer manual, shows that the 50Ω term is on the meter. Which is the correct way, or really better way? I can get two different results from these tests, depending on the cable and term.
If I do end up doing the AF way, I usually measure the resistance on the 3458A and match the closest to 50Ω as possible.
Thanks guys.
I had the same question in the past and was told that you always terminate at the meter because that's where the actual impedance mismatch is. You are correct though about getting a different reading. I just tried it on a 3325B and 8508A.
I read the AF K procedure and it had a note that wasn't there when I left years ago. It said that the manufacturer specifications state the DC Offset accuracy at the output connector only. But I looked up a manual and I don't see that reference. I'm just afraid that I'm calibrating things the wrong way and getting false negatives and positives. I would just like a reference point is all.
I agree with the termination at the input to the meter. The only time I differ is on the larger Tektronix oscilloscope mainframes 80000/82000 series, that one calls for the term on a tee at the output, it's the difference between pass & fail.
Cables are part of your calibration process and bad cables can make a huge difference in the measurements.
I will always terminate at the DMM input.. Reason.. The voltage is higher before the termination and lower after it. Higher voltage can drive current a greater distance in a cable.
If you place the 50 Ohm term at the 3325 output, you will have less voltage going through the cable to the meter.. The cables resistance will cause a greater drop on the voltage measured by the meter..
*** Bottom line.. Make sure your lab has good cables.. If you find a bad one.. Cut the cable up in the 2 inch chunks and toss it in the garbage!
Mike..
If that's the case then why didn't HP provide a cable and termination too be mated to the generator?
I agree with putting the termination at the meter and the reasons given for that make sense. All the AF procedures I can recall put the termination at the meter end (but they don't explain why)
I have seen transmission Line Pulsers that will not pass for risetime if the attenuator is placed at the end of the cable. The cables used had plenty of bandpass to see the pulse but I believe they were breaking down under the high voltage being applied. I wasn't willing to risk my Gore cables to see if I was right, I put the attenuator on the TLP output and measured the risetime there.
The spec for those units was at the instrument output and I wasn't calibrating the end user's cables.
I found in the performance section and a note in the 3325B manual that they want you to calibrate it that way. I didn't see it in the 3325A. It's a weird check but HP has done a lot of weird things through the years.
You say weird things.. But at the end of the day it is not the company.. It is the engineer writing the test. He is just an engineer, they are not always right.
Good metrology is good metrology! I don't have a problem arguing with an engineer if I think something is wrong.. And just because it is in print doesn't make it right.
It the 3325 passes with the load connected to the 3325 and fails with the load connected to the DMM. Then there is something wrong! (maybe 5 whys?). I could be that the engineer who wrote the manual didn't take the time to figure out why this was an issue. Because industry practice is to terminate the signal in at the end of the cable.. Hence the word terminate!
Mike!
Quote from: CalLabSolutions on 12-05-2016 -- 12:08:14
Because industry practice is to terminate the signal in at the end of the cable.. Hence the word terminate!
Industry uses weird terms like feed through termination, when in fact it isn't terminating anything. We have a tech who's favorite always-repeating complaint is just that. A feed through termination is an oxymoron.
An explanation for the original question of the 50 Ω DC Output of the 3325A/B.
When a known 50 Ω load is attached directly to the output and the DCV measured with a DMM (normally 10 MΩ or greater) the source is correctly loaded since the parallel DMM resistance is insignificant to this measurement at DC.
We know this is not how the 3325 A/B will normally be use.
When the 50 Ω load is placed at the DMM we normally use a BNC cable of some length between the 3325A/B and the load.
This BNC cable has some resistance which is unknown unless we measure it before use (I have seen many older BNC cables with close to 1 Ω due to the outer shield crimp connection oxidizing).
A good 24 Inch BNC cable will be less than 0.1 Ω so if we set the 3325A/B to output 5 V we will lose some voltage through the cable, say the cable is actually 0.1 Ω. We would also need to know the design of the 3325 A/B at 5 VDC, in this case it is a constant 10 VDC source that goes through an approximate 50 Ω resistance.
For the 50 Ω load we get 5.00 VDC at the output connector but for a 50.1 Ω load (total cable and load) we get about 5.005VDC at the output connector but only 4.995 VDC at the load/DMM.
So if a cable is used, as in the Army procedure, then it must be known to be good enough to meet the requirements of the test. Some procedures require the cable and load be measured before use and some procedures require matching a cable with the source but the 3325 A/B does not.
End of line termination becomes essential when dealing with higher frequency AC and pulse measurements where VSWR becomes an important measurement consideration.
There is always more but hopefully this helps.
Thanks Mr. M. I was pretty sure I figured it out but what you said makes a lot of sense. I was thinking of going into engineering but decided on a business degree instead. I still am thinking of getting two degrees because I love metrology and I would love to run a cal lab one day! Many calibrators don't ask why things are done and I think it needs to be a prerequisite to be curious. If you don't understand the science, then you are just being a trained monkey.
Thanks all for the helpful responses!