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dB calculation using Signal Generator and Oscilloscope

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oliveandme

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Hello,

Here is an experiment I'd like to do. I would like some advice on anything I might be doing wrong.

Aim of experiment:

To determine the dB level of a signal being generated on the Signal Generator. This dB level signal will be played through a calibrated speaker to a microphone on a CSR8675 Dev board to determine the energy level of different dB level signals. Based on a code I have written on CSR, I'll collect the FFT data and calculate the signal magnitude to determine the energy.

Hardware I have access to:

A PE/IEPE Signal conditioner, Signal Generator (SG), An Oscilloscope (OSc)

I'm going to explain this experiment in 2 steps for easy understanding.

First half of experiment:

I'll connect the output of the Signal Generator (SG) to the input of the Oscilloscope (Osc). I'm debating whether a Signal conditioner should be used as an intermediate between the SG and OSc. Once the SG is connected to the OSc, I'll generate a Sinusoidal Signal (1 kHz) of Peak-Peak Voltage of 1.0 V on the Signal generator. I'll observe the aforementioned signal on the Osc. The Signal on the Osc should have a Peak-Peak Voltage.

Using the above data that I have, I want to determine what is the dB level of the signal being generated on Signal generator . Does the 1.0 Vpp correspond to a 40 dB or a 60 dB .

This is the part where I am confused. I read this post which covers a similar topic.

https://www.edaboard.com/showthread.php?t=235708

based on which I decided a couple of options would be an appropriate way to calculate the dB level.

I'm not sure if either of my assumptions are wrong or both are wrong.


1. dB = 20 log10 (Voltage 1 / Voltage 2)

Voltage 1 = Vpp on the Oscilloscope ; Voltage 2 = Vpp on the Signal Generator.


2. dB = 20*log_10(Voltage 1/ V ref)

Voltage 1 = Vpp on the Signal Generator. Vref = Reference voltage of the Signal generator (I'm not sure what this reference voltage might be)

So are both my assumptions wrong ? How can I calculate the dB level ?

The final goal is to have this signal from the SG resemble a real world 40/60/80/100 dB signal and determine the energy level.

The questions that I have are

a. Is my experiment setup correct ?
b. Is the signal conditioner necessary ?
c. Which of my calculations (1 or 2 ) are correct to calculate the dB level ? Or are both wrong ?

Any help would be appreciated.
 

What I'd like to do is calculate the dBV level for sinusoidal signals with varying Vpp level.
Then just use your 1st formula using Voltage2=1 V peak-to-peak.
 
Hi,

Then just use your 1st formula using Voltage2=1 V peak-to-peak.
Are you sure with "Vpp"?

I´d rather use V_RMS.

Klaus
 

Are you sure with "Vpp"?

I´d rather use V_RMS.
It does not matter. The user must know what the voltage in dBv actually means. I do not even use peak-to-peak quantities, I use amplitudes, but as I said, the one using that must clearly state what does the quantity represent.
 

Hi,

Your formula with Vpp works for 1GHz, ...1MHz, 1kHz, 1Hz, but not for 0Hz

If you use the formula with RMS, then it works down to 0Hz.

Klaus
 
Thank you all for your replies. I'll be using the range from 100 Hz- 10 kHz. So I guess I don't need Vrms and I'll stick to Vpp.
 

I do not quite get it; are you measuring a signal against itself? Your experiment setup means to me that you are using the scope to measure the signal from your generator. What's the reference here?

Are we considering gain? Would it not be 1 (0dB) here?
 

I do not quite get it; are you measuring a signal against itself? Your experiment setup means to me that you are using the scope to measure the signal from your generator. What's the reference here?

Are we considering gain? Would it not be 1 (0dB) here?


I'm later using a CSR8675 Dev Board to generate the signal from the SIGnal generator. I'll capture the signal using a calibrated microphone in a sound proof chamber. I'll use the sensitivity of the calibrated microphone as a reference.

- - - Updated - - -

I do not quite get it; are you measuring a signal against itself? Your experiment setup means to me that you are using the scope to measure the signal from your generator. What's the reference here?

Are we considering gain? Would it not be 1 (0dB) here?

Since you seem to know about this. I have another question. I'll be using connecting the output of the signal generator directly to input line of a cSR8675 Dev board. I'll pass the output of the CSR8675 to a Knowles receiver (speaker).

Then in a sound proof chamber, I'll use a B&K microphone to capture the sound and pass this sound onto the oscilloscope.

The sensitivity of the B&K microphone I'm using is

Open-circuit Sensitivity (250 Hz)a:
50 mV/Pa, ñ26 dB ±1.5 dB re 1 V/Pa.

The reference voltage is 250 Hz. All my calculations are later done with 1 kHz as the reference . They do not provide a sensitivity curve. So I'm not sure if the sensitivity at 1kHz is the same as the sensitivity at 250 Hz

The product sheet for the B&K microphone.
https://www.bksv.com/-/media/literature/Product-Data/bp2210.ashx

They do have a free field response curve though. Any idea on how I can calculate the sensitivity at 1 kHz ?

I also have another question. Since I am using a Knowles receiver (speaker) to output the sound, does it have to factored in during the dB SPL calculation ?

Any ideas ? Thank you.
 

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