noise in analog input signals

[imperial-aero]

Hi,

I’ve been having problems with noise in my analog inputs. Here is a description of my problem. I hope I have explained the problem clearly.

I have created a VI that outputs signals to 8 loudspeakers (after the signals were amplified from an 8-channel power amplifier). I checked the individual output signals (after amplification), both by reading them back into the computer using a BNC-2090 terminal block and also using an oscilloscope. They have the correct frequency and amplitude as expected.

However, I observe noise in the signals when I connect all 8 signals from the amplifier into the BNC terminal (I input the signals back into the computer, because I need to know the exact signals that goes into the loudspeakers, i.e. after they are amplified). I observe this noise (pic shows signal with and without noise), both on the input waveform graphs and also I can hear a high-pitched noise from the loudspeakers.

To find out whether the noise was caused by all 8 channels being in use; the first thing I did was to use just 1 channel. Once I found that 1 channel gave the correct input signal then I checked 2 channels and so on. I found that if I connect a third coaxial cable to my BNC terminal, this is when the noise starts to kick in. I got the same observation from different coaxial cables, i.e. there’s nothing wrong with the cables. Basically, a third connection to the terminal leads to noise in the input signals.

I tried using bundled up coaxial cables, 5 coaxial cables in a bundle with common ground (I think). However, now I find that a second connection on the BNC terminal lead to noise, as opposed to the third connection with the single coaxial cables.

I suspect that the noise might be something to do with the grounding in the cables or the instruments I’m using (since, the only difference between the bundled coaxial and single coaxial cables is the grounding).

I’m wondering whether anyone could give me some suggestions on what the problem might be, or any further checks I could do. Any help will be much appreciated.

Message Edited by imperial-aero on 03-18-2008 12:29 PM

[Brooks_C]

Hello Imperial-Aero,

Sounds like you're doing something cool.  I have a couple questions that may help us figure out what's going on.

1.  Are you generating a single analog output and splitting it 8 times for your amplifier or are you generating 8 analog outputs and sending one to each channel?  If you're splitting the signal I'd be inclined to think that it is a cabling issue--something having to do with reflections and impedance issues, but if you're using 8 analog outputs there may be something strange going on with the card.

2.  What hardware (DAQ card) are you using to generate the analog output/s?

3.  Can you perform a Fourier transform of the waveform you read back into the BNC-2090 with you hear the high pitched noise?  It would be interesting to see if the frequency you hear is a harmonic of the frequency you intend to generate.

If you're currently splitting a single analog output 8 times I would recommend trying to generate more than one so you split the signal fewer times.  I.e. if your board has 2 analog outputs see if you can split each twice without problems.  If this works then it is likely a cabling issue that may be difficult to solve, but you can always get more analog outputs.  If you're already using multiple analog outputs then please post back with what hardware you're using and I'll see if I can reproduce the issue on my end.

Cheers,

[imperial-aero]

Hi Brooks,

Thank you for your response. Actually just yesterday I was able to solve the problem. I saw a KnowledgeBase article on NI that had the solution. All I did was change the RSE/NRSE switch on my BNC-2090A to RSE and then modified my VI accordingly. This got rid of all the noise and I could not see any peaks (except my input signal) on the FFT waveform. So I think that has solved my problem. Also, I'm using a PCI-6229 card which has 4 analog outputs (2 from BNC-2090A and 2 from SCC-68) and I am using all of these and splitting each into 2 to get 8.

I've got another question on a completely different topic. From my BNC-2090A connector block I can only obtain 16 analog inputs, whereas ideally I'd like to have 24 analog inputs. I know that I can connect wires into my SCC-68 and obtain these additional inputs. I'm wondering whether the software setup would have already been made for my SCC-68 since it's already producing 2 analog outputs (I wasn't the person who initially set the DAQ system). Also, considering the fact that I have limited knowledge on electronics, would you say that it'd be okay for me to do it myself as I really don't want to destroy any equipment. I guess one good point to start will be to look at the user manuals.

[Tom O]

Hi Imperial-aero,

Your M-series device (6229) has two connectors. It sounds like one of them is connected to your BNC connector block, the other to your SCB-68. You have 32 single-ended analogue inputs (or 16 differential). These are split evenly across the two connectors. This means that, as you have stated, you have access to the first 16 RSE channels on your BNC (connector 0) and access to the other 16 on your SCB-68 (connector 1).

For most applications, you won't need to configure anything in software to use your connector blocks. They give you direct access to certain pinouts on the card for you to connect the signals to. If you have a look at the M-series user manual, it will give you more information on this. Specifically, page A-46 gives you the pin mapping for your device, which ties in with the pin numbers on the SCB-68. You can then, using the manual, identify the appropriate pins and make connections. In the case of your microphones, everything should be pretty safe even if you make a mistake! As an example, to connect an RSE input to AI 16 of your device, you would attach the signal line to pin 68 of your SCB-68, and you ground to any AI GND, for example pin 67.

Something that may be of further use to you is this KB, which contains SCB-68 reference sheets for your device. These reference sheets will show you the pinouts and pin numbers in a table for the SCB-68, which is easier than going between the manual and the connector block! For your convenience, the reference sheet for the SCB-68, when connected to connector 1 of your 6229, can be downloaded directly here. Please confirm before using that reference sheet that the SCB-68 is indeed attached to connector 1 (the second connector!).

I hope this is all useful for you, please post back if you have more questions about connecting your signals!

Best regards,