Analog signal acquisition

[santo_13]

I don't have any literature that I can refer you to, but here is a summary,

• Any measuring device which is not adjusted to a known value is un-calibrated. This known value has to be somehow traceable back to the SI units or any national standard. 
• Uncalibrated measurements are unreliable, if a uncalibrated DAQ says it measures 1V, there is no guarantee that it was really 1V, it could as well be 100V.
• On the other hand, calibrated instruments are adjusted to adhere to a known tolerance level against national standards in defined intervals.
• If a DAQ has a specification that states, the accuracy is +/-1mV and it measures 1V, the actual could be anywhere between 0.999V and 1.001V

[Henrik_Volkers]

If you have a carrier with 30 MHz .. what is the bandwidth of the signal?

I work a lot with 40-80 MHz laser doppler vibrometer signals .. in some applications we use a simple analog mixer to bring down the carrier to 1 MHz...

another technique is to use the sampler as the downmixer

AD and TI make nice RF frontend ICs and have demoboards , ok some migth be above 2k$  .. but usually they can make special offer to students 😉

Where does the carrier come from? an chance to get that signal ?

A nice switching demodulator works with IQ carrier signal on a 1:4 switch , followed by caps and two OP... simple cheap IQ mixer ... 

however design need to follow the desired bandwidth.

If you look for a traceability for your interferrometer signal : If you tell a little more about your application, .. usually it's your timebase and ligth-wavelength .. salted by phasenoise 😉

[Soumya_MIT]

Hi Henrick,

Thank you for your response!

I am building an interferometer to detect ultrasound vibration. We have a doppler vibrometer from polytech.

The interferometer I am developing is based on the phase-generated carrier-homodyne (PGC-homodyne) method. In the PGC-homodyne approach, there is a phase modulator instead of a frequency shifter as in the case of a heterodyne doppler vibrometer.

The interferometer is operated at a 1550 nm laser diode procured from ThorLab, which is a single wavelength laser diode as mentioned in the datasheet. However, it is not a single wavelength laser diode as inspected by me using a spectrum analyzer.

The phase modulation happens at a rate of 20-50 MHz, which carries the phase changes happening due to the surface vibration (1-10 MHz). Later I am trying to apply a differentiate-cross multiply algorithm and phase lock loop to separate the carrier signal from the signal that carries the information about phase changes that occurred due to the ultrasound vibration.

Now I can operate oscilloscope and analog discovery 2 with LabVIEW. But it will not give a resolution of less than a few millivolts during ADC. However, I have a good transimpedance amplifier with the photodiode which should amplify the signal that represents the phase changes due to vibration. Hopefully, It will work. Otherwise, I have to go for a more sophisticated ADC.

I can't use an IQ demodulation scheme as the quadrature signal I will get will have different magnitudes and will give an extra signal which contains Sine of the twice of the phase change in addition to the phase change.

Looking forward to hearing back!

Best,

Soumya

[Henrik_Volkers]

Now even 12bit scopes (100MHz bdwdth) are getting close to 2k$  ok, the siglent is 3k ... 10bit should be able to find.

12 bit SDR with 60MHz samplerate (you have to use the included downmixer) are in the <1k range (lime SDR, ..)

a cheap source for 1550nm LD (9mW)  are SFP modules (80km fiber length in 25 buck range sfp1g-zx-55-80km from fs.com )  but I never checked the spectrum

and you get a fast PD incl log amplifier with LVDS output (good for some GHz) for free 😄  (OK thats rectangular output) 

add another 5 bucks and you get a duo fiber cable with APC endconnectors ..add a lens and you have your collimator 🙂

(here I 'played' with them)

[Soumya_MIT]

Thank you so much for sharing all this information.

I will try to include your suggested modules and equipment in my experiments.

I will try to update my progress here to get your help. Thank you in advance.

Best,

Soumya