What is the 'best' way to connect these items? - Page 1
Forum section: electronics / beginners

author: Poe
author_rank: Frequent Contributor
timestamp: September 12, 2019, 08:10:27 pm
content: We have problems and our customer feels it's due to the order of the connected components. We disagree, but maybe I can learn something here.So we have the following 4 components all connected via 12" BNC cables and/or BNC "T" connections.1. A signal generator creating a 10KHz signal.2. A device under test (DUT) receiving that signal.3. A very accurate voltmeter checking the amplitude (1Vrms) of the signal.4. A single 50 ohm terminator.How should these devices be connected?My setup:The signal generator and 50 ohm terminator positioned on either end of the signal chain with the DUT and voltmeter between, in any order.Their setup:The signal generator and 50 ohm terminator on one side, the voltmeter in the middle, the DUT on the end.They have found that when switching from one setup to the other, they see a ~1% change on the voltmeter. Note the signal frequency is low at 10KHz.Any thoughts?

author: bob91343
author_rank: Super Contributor
timestamp: September 12, 2019, 08:43:06 pm
content: The terminator, as its name may imply, should be at the end of the run. The T connector disturbs the integrity of the coaxial setup.However if you want to measure the signal reaching the DUT, you need to connect the voltmeter there, perhaps with a T. That is where the terminator belongs. The run from that point to the voltmeter should be short and direct and the voltmeter needs to be high impedance. Best to use a probe so as not to add any complexity.Measuring to within 1% is not simple, as even the voltmeter may not be flat over the frequency range.If the DUT has high input impedance, disconnecting it and leaving only the terminator and voltmeter should not change the voltmeter reading.You didn't mention the output of the DUT so I assume that's not part of the question.

author: fourfathom
author_rank: Super Contributor
timestamp: September 12, 2019, 08:54:16 pm
content: Quote from: bob91343 on September 12, 2019, 08:43:06 pmWhat are the input characteristics of your DUT? If the DUT does not have a high impedance input then you may need to change your terminator. You don't want to double-terminate your signal source.

author: Poe
author_rank: Frequent Contributor
timestamp: September 13, 2019, 01:39:03 pm
content: Thank you for the responses.The DUT's input impedance appears to be around 2Kohm at 10Khz. So not that high. Although the signal is fairly low, so I wouldn't have thought it'd matter much. We're slightly adjusting the signal generator's "1Vrms" output so that the (very accurate) voltmeter is perfectly at 1Vrms each time.The DUT is just being calibrated, so there isn't really a DUT output to monitor.This isn't my design so, unfortunately, no schematic.If we replace the voltmeter's T connection for probes, in my setup, we get the same result. There is an additional error doing this with the customer's setup.Correct me if I'm wrong, but doesn't the customer's setup have multiple signal paths? One from the signal generator to the 50ohm terminator, then another from that point to the 2k DUT load. If the voltmeter is placed after the DUT, then there is a third path with essentially no terminator.

author: magic
author_rank: Super Contributor
timestamp: September 13, 2019, 01:59:02 pm
content: Wavelength at 10kHz is 20km. Whatever you see probably is not a matter of impedance matching but ohmic effects. Perhaps the terminator at the DUT end draws current which causes resistive loss in the cable and connectors?Generally, for calibration you should probably reproduce the same setup that will be used in production. And put the voltmeter wherever you want 1V RMS to appear in production.

author: macboy
author_rank: Super Contributor
timestamp: September 13, 2019, 02:38:40 pm
content: You do not need to terminate a 10 kHz signal to maintain signal integrity over those small distances. You could simply remove the terminator entirely, and adjust the signal generator output accordingly (the generator assumes a terminated output). This will substantially reduce the current flowing through your cables and connections, which reduces voltage drop.Using your setup (not theirs) you should be able to observe that swapping the DUT and voltmeter results in a different measured voltage. When the voltmeter is closer to the source, it measures higher. When the DUT is closer to the source, the reading is lower. Draw the circuit on paper, including current flow from source to termination, and think about why this happens. It will be obvious once you see it.Neither their setup nor yours is ideal. I would go Signal generator to DUT to VM. This way the VM is sampling the signal at the DUT, not at the middle of the signal path. The terminator can optionally be used to load the signal generator output down to the expected voltage, by connecting it as in their setup (near the source). This prevents the relatively large current flowing into the terminator from affecting the measurement.