BNC to XLR cables

[warpie]

I'm thinking of making some BNC-to-XLR cables so I can plug my signal generator straight into my audio gear.

a) Is better to use a coaxial cable or an audio cable? FWIW, a coaxial cable would be easier to make as I already have some BNC-BNC leads. So, it's a matter of replacing only one side of the lead with an XLR. Saying that, I'm obviously looking for best performance so using an audio wire isn't a problem at all.

b) Should I connect Pin3 of the XLR to gnd (Pin1) or leave it unconnected.

Thanks!

 

[Newmarket]

You have a single ended source (SigGen BNC) going into a balanced input (XLR) ?
There type of cable isn't really the issue. Rather the method of interconnection is.
I'm assuming that the audio kit gets 'Earthed' somewhere. Is the signal generator 'Earthed' ie a 3 wire mains connection. And is the BNC screen connected to that ?

I just saw that you previously asked in a different post:

"Feb 23, 2022
Wouldn't a 1:1 (600:600?) transformer at the output of the generator solve the issue? Probably the level would be slightly different but this can be measured."

and this might be you're preferred solution (although the 600:600 isn't relevant).

 

[warpie]

You have a single ended source (SigGen BNC) going into a balanced input (XLR) ?
There type of cable isn't really the issue. Rather the method of interconnection is.
I'm assuming that the audio kit gets 'Earthed' somewhere. Is the signal generator 'Earthed' ie a 3 wire mains connection. And is the BNC screen connected to that ?

I just saw that you previously asked in a different post:

"Feb 23, 2022
Wouldn't a 1:1 (600:600?) transformer at the output of the generator solve the issue? Probably the level would be slightly different but this can be measured."

and this might be you're preferred solution (although the 600:600 isn't relevant).

Thanks Newmarket.

The signal generator is earthed indeed and the probing the BNC's outer part shows continuity with the IEC earth pin. So, yes and yes.

I could use a 1:1 Tx but I want to use the generator together with a scope to do bode plots and a transformer would limit the bandwidth considerably.

 

[Newmarket]

I could use a 1:1 Tx but I want to use the generator together with a scope to do bode plots and a transformer would limit the bandwidth considerably.

What bandwidth are you aiming to measure.

 

[warpie]

What bandwidth are you aiming to measure.

Probably a couple of MHz. Maybe a bit more...

 

[Bo Deadly]

I want to use the generator together with a scope to do bode plots

Bode plot? How do you use an oscilloscope to plot time domain data?

The best arrangement would probably be to use an XLR cable and connect BNC signal to hot and shield to cold and connect the XLR shield to pin 1 only at the XLR end.

But when you connect a bunch of clunky unbalanced earth grounded lab devices to studio gear, you run the risk of introducing ground loop currents. So any kind of measurement that requires a good noise floor is going to be limited. You could use cheater plugs to un-earth the lab gear but you're sort of risking shock. A better solution would be to use a USB audio device and a laptop running some kind of FFT software. The USB is floating and designed to be quiet so you'll get much better results. With the proper cables you can get a very good noise floor lower than the device you're measuring which is all that matters.

 

[Newmarket]

Probably a couple of MHz. Maybe a bit more...

+1 to Bo's suggestion to use an unearthed source (eg laptop audio setup) for the signal generation.
But I don't think I'm getting what you are trying to setup / measure.

"Probably a couple of MHz. Maybe a bit more..."
Well that's' not going to 'get through' your audio kit, never mind being bandwidth limited by a transformer.
Or maybe you want to look at what effect it has on in band audio ?
In any case Good Luck.

 

[Bo Deadly]

Probably a couple of MHz. Maybe a bit more...

RF is a completely different animal. You will need to explain in detail exactly what you are trying to do.

 

[warpie]

How is the noisefloor going to affect a frequency response measurement? For anything else other than FRA I can (and I do) use my audio interface.

A couple of MHz won't get through indeed but it\s good to have the option to test some opamps for example.

Bo Deadly, quite a few modern scopes can do bode plots. Keysight and Siglent comes to my mind. Probably other too.

All I'm trying to do is to be able to measure the frequency response well beyond the 50-60-70kHz that most Audio interfaces offer.

 

[ccaudle]

a) Is better to use a coaxial cable or an audio cable? FWIW, a coaxial cable would be easier to make as I already have some BNC-BNC leads.

Coax is fine. I assume you are talking about something in the 1'-6' range (0.3-1 m range). You aren't really worried about noise induction over those distances for what you are trying to do.

b) Should I connect Pin3 of the XLR to gnd (Pin1) or leave it unconnected.

Pin 3 should connect to the coax shield, since in this case the shield is also the signal return.
Pin 1 may be left unconnected in many cases, discussed below.

The signal generator is earthed indeed and the probing the BNC's outer part shows continuity with the IEC earth pin.

You will need to be aware of how the audio equipment handles safety earth connection. If the audio equipment also has a three wire power cable, then just plug both the signal generator and the audio equipment into the same outlet or next to each other on a power strip and it should be fine.
If the audio equipment uses double insulated power supply and so only has a two wire power cable, you will need to connect the audio equipment ground either to safety earth at the power strip or outlet, or to a chassis or ground connection on the signal generator. Without some connection to restrict the common mode voltage to earth the equipment can float to an arbitrary voltage, although with high impedance.
You could make the connection at the cable, by also connecting the XLR pin 1 to the coax shield, but that would put power return currents on the signal return, which you do not want (it's the reason unbalanced connections are a pain to begin with). Make the connection elsewhere chassis-to-chassis (which is done for you already if both chassis have safety earth connections). Make the connection as short and direct as possible (which you get with earthed chassis by powering from adjacent outlets).

use an XLR cable and connect BNC signal to hot and shield to cold and connect the XLR shield to pin 1 only at the XLR end.

That will also work, but will likely to a little more work to wire up, since BNC connectors are typically made for crimping onto coax cable. Getting the second wire of the balanced pair connected securely to the BNC shield could be a pain because of that.
Note also that using balanced cable doesn't affect the above discussion about making sure the common mode voltage difference is handled. All using the balanced cable does is add a shield around the connection, but for connections just a couple of feet long noise induced into the signal conductors isn't usually a problem (the cable is too short to be an effective antenna at the frequencies which cause the most problems).

 

[Bo Deadly]

Bo Deadly, quite a few modern scopes can do bode plots. Keysight and Siglent comes to my mind. Probably other too.

Not in any useful way for audio. Digital scopes use 8-10 bit AD converters that are spec'd for speed, not resolution. Even at 10 bits the plot is going to be very crude. There are 12 bit scopes that use software trickery to make a nice looking plot of audio. But for even a vaguely accurate audio measurement, you need a USB audio interface which has 24 bit AD converters that are spec'd for resolution, not speed.

 

[pucho812]

why not just buy a bnc to xlr plug this way you just make a BNC cable..

https://www.bhphotovideo.com/c/product/1026925-REG/neutrik_na2mbnc_3_pole_xlr_male_to.html

 

[warpie]

Not in any useful way for audio. Digital scopes use 8-10 bit AD converters that are spec'd for speed, not resolution. Even at 10 bits the plot is going to be very crude. There are 12 bit scopes that use software trickery to make a nice looking plot of audio. But for even a vaguely accurate audio measurement, you need a USB audio interface which has 24 bit AD converters that are spec'd for resolution, not speed.

So, you mean that it's not useful for measuring the bandwidth of an amplifier or the ringing frequency of a transformer, etc...?

Obviously it won't be ideal for EQ curves but this can be done with an interface anyway.

why not just buy a bnc to xlr plug this way you just make a BNC cable..

https://www.bhphotovideo.com/c/product/1026925-REG/neutrik_na2mbnc_3_pole_xlr_male_to.html

Because I already have the wires and the connectors I need. Also, these Neutrik connect Pin1&3 together and maybe there're better configurations (as already suggested)

 

[ccaudle]

Not in any useful way for audio. Digital scopes use 8-10 bit AD converters that are spec'd for speed, not resolution. Even at 10 bits the plot is going to be very crude. There are 12 bit scopes that use software trickery to make a nice looking plot of audio. But for even a vaguely accurate audio measurement, you need a USB audio interface which has 24 bit AD converters that are spec'd for resolution, not speed.

He said he was trying to make a Bode plot, not measure noise and distortion. Exactly how much resolution do you suggest is needed? What if averaging is used? How much resolution can you accurately view by eye looking at a graph?

I looked at the Siglent app note for creating Bode plots (not a very expensive 'scope at all), and it displays phase to the tenth of a degree and fractional dB values for each data point. That's more than good enough for checking phase margin, or even trying to match circuitry between channels of a stereo pair. Suggesting that such a measurement is useless without a 24 bit converter is not reasonable.

 

[moamps]

I have recently been using one R&S digital 10 bit oscilloscope with generator and bode plot function and I must say that I am very satisfied even though I have never been a fan of digital scopes. And certainly that scope with that function can be used for all measurements and settings that a DIYer needs.

 

[Bo Deadly]

He said he was trying to make a Bode plot, not measure noise and distortion. Exactly how much resolution do you suggest is needed? What if averaging is used? How much resolution can you accurately view by eye looking at a graph?

For quick and dirty measurements I don't doubt that it can work. But averaging is not going to help much if you only have 1024 possible values. At 10 bits SNR is 60 dB. That might be fine for RF but not so much for audio.

I looked at the Siglent app note for creating Bode plots (not a very expensive 'scope at all), and it displays phase to the tenth of a degree and fractional dB values for each data point. That's more than good enough for checking phase margin, or even trying to match circuitry between channels of a stereo pair. Suggesting that such a measurement is useless without a 24 bit converter is not reasonable.

What app note? Is it about audio or RF? Is the frequency axis log scale? For audio the low end could be really chunky.

Maybe the newer scopes do a better job with certain measurements but a USB audio interface is cheap and works much better in just about every scenario.

 

[ccaudle]

At 10 bits SNR is 60 dB. That might be fine for RF but not so much for audio.

And yet it does work just fine, as attested by moamps in the previous post. I'm not hearing any specific complaints from you indicating that you have tried it and found it lacking for specific reasons. Is this a case of moamps saying "I use this, works great" and you saying "I've never done this before so it can't work?"
I think there are a few things you are missing. One is that the quantization noise is spread over the entire Nyquist bandwidth, so that -48dB or -60dB noise floor is spread over a couple hundred MHz, not over 20 or 30 kHz. That makes the noise density much lower, which has a direct impact on the accuracy of narrow band measurements like you would use for frequency response or Bode plot.
The other is that this isn't an audio recording, if there are discontinuities it doesn't really matter, i.e. you can change the gain range to get the best resolution at the amplitudes you need, you don't have to pick a single gain and then stick with it forever.

What app note? Is it about audio or RF?

Siglent Bode plot app note
Includes a link near the beginning to a video showing this in action, but there are plenty of screen shots in the web page, I don't know that the video really adds much.

I think your whole "audio or RF" question is a red herring (with apologies to the non-native English speakers for that rather odd idiom).
I'm not seeing any specific assertions about exactly what makes RF measurements so different from audio measurements. The same principles apply, admittedly with different concerns emphasized, but it still comes down to appropriate bandwidth, phase response, and SNR for the measurements you need to make. Controlled impedance becomes important as the wavelengths get shorter, but that isn't really detrimental to low frequency performance.

a USB audio interface is cheap and works much better in just about every scenario.

I think maybe you don't really understand the use case. How are you going to generate a Bode plot of an audio bandwidth device with another audio bandwidth device? I guess it might work if you have a 96kHz audio interface measuring an intentionally bandlimited device like a guitar amplifier, but the OP already stated he was looking for measurements up to at least a couple of MHz. You aren't going to get that in a USB audio interface. Even if you got a 192kHz audio interface, and even if that interface did have flat response up to 90kHz (which is very rare), that isn't nearly high enough for most devices. Take the OPA1642, which has been discussed recently in the mic forums (it is used in the alice design). That has pretty flat open loop phase response out to 400kHz, and you can't see the knee where the upper poles kick in until 10MHz or 20MHz. That is 3 orders of magnitude above audio bandwidth, so any type of audio interface is completely useless for seeing that, no matter how good the SNR. That few hundred dollar Siglent scope will however show it very clearly.

 

[john12ax7]

From a practical standpoint the easiest solution is to get BNC cables, then cut in half and solder XLRs. This avoids needing to crimp the BNC.

 

[Bo Deadly]

I think your whole "audio or RF" question is a red herring (with apologies to the non-native English speakers for that rather odd idiom).
I'm not seeing any specific assertions about exactly what makes RF measurements so different from audio measurements. The same principles apply, admittedly with different concerns emphasized, but it still comes down to appropriate bandwidth, phase response, and SNR for the measurements you need to make. Controlled impedance becomes important as the wavelengths get shorter, but that isn't really detrimental to low frequency performance.

The rationale behind the audio or RF question is that FFT resolution improves with frequency so looking at FFT of RF with a digi scope might be markedly better compared to LF audio. FFT is basically comparing the signal to sin + cos at the specific frequencies so the generated sin + cos period is going to be more limited at low frequencies. That's why the resolution gets very chunky in the LF audio range (based on deductive reasoning anyway - I could be wrong about that). Combined with already limited resolution of a digi scope (the SDS1000 series looks like only 8 bit incidentally) I have to believe that it would not be usable for FFT of LF audio at all. Even at 50 MHz like in that video it's super slow and grainy. But if you're doing RF, then yes, you're right, you don't have any choice. I didn't think people did a lot of RF work around here. I can't imagine scenario where anyone would need to test an audio device doing anything at greater than 100 kHz (actually more like 20 kHz but I suppose one might want to sanity check for oscillation or strange peaking or digital noise or some such).

 

[warpie]

From a practical standpoint the easiest solution is to get BNC cables, then cut in half and solder XLRs. This avoids needing to crimp the BNC.

This was my initial thought but althought I have BNC cables I also have BNC connectors with a screw instead of a crimp so it shouldn't be too difficult to do the connection. I'm gonna try one of these days.

I think I'm gonna stick to what it was suggested here and on the RANE notes.

The DUT-to-Scope wiring (i.e XLR balanced-to-BNC) might be more complicated though. RANE suggest two possible solutions (4b or 6). Most of my gear is either transformer balanced of IC balanced (that12xx) so I believe that fig.6 would be the right solution. Excuse my ignorance but what is "cross-coupled? Is it a (+) & (-) signal provided by two opamps? If so, is the THAT chip considered cross-coupled?

Many thanks for all you help and comments!