Differential cables

I’ve had a QA400 for a while, and of course it’s single ended. I’ve used it primarily to test component upgrades to 1970’s Phase Linear preamps. I make my own BNC to RCA cables using Amphenol 69475 BNC connectors, Beldon 83284 50 ohm RG316 coax, and Switchcraft 3502A RCA plugs.
So, I’ve had the new QA403 for a few weeks now, and want to make use of the differential inputs and outputs for preamp testing.
Think I’ve read every post on the forum and whatever Google offers up. I see STP cable seems to be preferred, and cable grounds configuration may be determined by DUT grounding. I’m using a laptop running on battery most of the time.
I sure would welcome some thoughts about best cable to use and cable construction for a differential to single ended configuration.

Hi @George_S,

I’ve had really good luck with PI Manufacturing cables mentioned at the Getting-Started link below. They are very flexible, hold up well, they have lots of lengths to choose from, and are a great price. And in case you haven’t seen it, the Getting-Started page also has info on wiring up TRS for balanced measurements, making single ended measurements (and shorting the unused input, etc).

I was just in a EMC lab earlier this week doing RF susceptibility tests on a QA404 and the PI cables were used. For this test, you put the QA40x into a large walk-in RF chamber, turn on a RF power amplifier into a directional antenna to immerse the hardware in a 3V/meter field (the RF is modulated with a 1 kHz tone), step the frequency from 80 MHz to 1 GHz and look for disturbances that creep in to measurements while the QA40x is running in fully differential loopback. The PI cables seemed to do fine–you want to test with a cable with reasonable RF shielding to make sure an externally applied field isn’t creeping into the measurements through the cables.

Thanks Matt, hope I’m understanding the differential inputs and outputs correctly, but been through the wiki and found no reference to differential cable construction.
No problem. Think I’m going to solder two BNC’s with short coax pigtails to a length of STP (Shielded Twisted Pair) with a RCA on the other end.
I’ll join the coax shields of the STP and pigtails on either the input or output cables and leave the other set with a break in the shield at the junction so there’s no ground loop.
General construction will follow examples of differential to single ended cables found elsewhere on the web.
I’ll post a photo if this works out.
Thanks to QuantAsylum for making such awesome products available!

Here’s a link to the guide I’m going to use for cable construction. Figure #2 would represent a RCA connection on one end, and two BNC on the other.
I’ll experiment with and without a break in the shield concerning ground loops, but the document says no shield breaks are preferred.

Got the differential loop back and four differential to single ended cables completed.
The cable is Monoprice balanced STP I cut out of some balanced interconnects I wasn’t using.
Now it’s time to do some testing on loop back and see what kind of noise floor I get.
My goal is to greatly reduce the 60 Hz and harmonics noise I see with typical single ended cables.

Success. Loop back using the differential cable. Have to go to maximum FFT, dropping the noise floor to see the 60 Hz spike.

If you used just 2 regular bnc cables, one from say L+ out to L+ in and another cable from L- out to L- in, what results did you get?
Just trying to understand the need for a special bnc to bnc cable.
A properly constructed rca to bnc is another matter and really worthwhile.

I’ll post a photo using two BNC cables and exact same settings this coming weekend as I travel for work all week. Note I’m a hobbyist and have no background in electronics. I’m just trying to learn more.
I welcome and encourage others to comment. Help us learn and get the most out these great products please.

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Here are the results of testing loop back using two BNC coax cables to utilize the differential inputs and outputs and the STP cable. The bitmaps are labeled. If I can find my RCA to RCA connector, I’ll post loopback of the differential to single ended cables.

I see. I have a QA403 and I hadn’t been bothered with that. Here are 2 cheap 1.5’ bnc cables in loopback on my
QA403. This is on laptop battery. I may try building a single twp cable to test for improvement.

Thanks Moto, nice to see your results, very helpful. I have a issue with 60 Hz polluting my work space.
Next weekend I think I’ll try taking it outside for a measurement.
Spent a couple hours today measuring a couple preamps using the single ended to differential cables and BNC coax cables and just general experimenting with settings.
Strong 60 Hz spikes and harmonics.
I’m about to drive a dedicated ground rod into the soil outside my work room. I do use a laptop with a good battery.
Thanks. More experimenting is needed here (and a Faraday cage). :grinning:

Been doing some reading about differential to single ended cables this morning.
Presently I have the differential low wire tied to the shield at the RCA.
Coming weekend I’m going to disconnect the shield from the RCA and rely on the shield to ground connection at the BNC.
Think I’m upsetting the pseudo-balance of the cable and creating ground loops.
Those diff/single ended cables are too damn noisy as I have them wired.

Did some loop back testing with the differential/single ended cables. Used a adapter to join the two RCA ends.
Bitmaps are titled. Tested one pair with shield soldered to the BNC’s and RCA’s, and tested the other pair with the shield soldered to the BNC’s only.
Note the additional noise and the amplitude of the 1 kHz signal at approximately 4 dBV. Weird.
So shield connections at each end of the cables are needed as the article I linked to in my third post states.
Going to do some further testing using different USB cables, using a desktop computer with ECC memory, and taking the laptop on battery outside away from 60 Hz.

I’m beginning to think that the Lenovo T410 laptop is the source of my 60 Hz issue. Walked out away from the house about 200 feet and ran loop back with the differential single ended cables and battery. Is it normal to see a 60 Hz spike generated by the laptop on battery?

Results using a Lenovo ThinkStation S30 series 2 with ECC memory and a Xeon processor.
This is a workstation configured as a PC. Not bad results.
Going to run with this and start testing actual preamps.
I did update to v1.176 and run the DC offset utility with good results.
I really like this little red faced box. It’s so cool to have this kind of diagnostic power with repeatable results at a reasonable price point.

Hi @George, when you are able to measure nV, it’s very possible for power line energy to creep in.

As a first step, short the 4 inputs, and look for power line in the spectrum. Try to move it closer to power cords, further away, change the orientation, etc. The goal here is to understand how much power line is creeping in when the system is mostly buttoned up (no external sneak paths). And once you have a feeling for that, then you can move to loopback and replicate the experiment. That will give you a clue how much is leaking in through shielding in the BNCs.

Thanks Matt, will do. Late last night I did short all four inputs and did the noise self test as shown in the getting started manual from the Wiki. It looks very, very good with no 60 Hz.
You guys did a great job on EMI proofing the unit. No 60 Hz anywhere showing. So it’s got to be entering on the cables and it’s not the QA or the laptop.
I also tried various external ground schemes with flat ground strap including running one out to the ground rod for my long wire short wave antenna.
Going to do some reading about BNC cables and 60 Hz.

Hi @George_S, you can build a pretty good “sniffer” for 60 Hz if you have a marginal BNC cable connected to the QA403. The cable will have a fair bit of directivity to it, meaning you can spin the unit around, move it closer/further to various sources, and get a good feel for your environment as you watch the 60 Hz level on the display. From time to time, you’ll probably encounter various USB devices on your desktop creeping in to measurements too. USB tends to have some recurring activity around 1 kHz, so if you ever see some very low level “sparkles” around 1 kHz on your desktop setup, you might see if moving USB cables around can help too. But just keep asking “Is this better or worse” as you explore the environment and monitor on the display, and you’ll find the offender. Good luck!

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Matt, thank you! Now that is a great idea. Will do.

Well, found several offenders. I’m back on the laptop on battery. The desktop monitor and cables were radiating badly.
Unplugged everything upstairs where my work room is, and took a reading. My laptop on battery is generating a 60 Hz spike. I have a longer appx 6’ Monoprice Monolith USB cable, so used it to gain distance and this lowered the spike.
Starting plugging things in and watching results. General noise floor increased, which I can deal with.
Found the source of my 180 Hz harmonic spikes, a early 70’s homebuilt class A stereo amp on the other side of the room from my bench.
Swapped it out for a White Oak Audio Designs Phase Linear 700 Pro. Much better. I really enjoy listening to Classic Rock digital files while I work.
Also going to try a different laptop to see if the 60 Hz changes. I did walk far away from the house with the QA and laptop on battery. The spike was definitely there.
Thanks y’all!

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