I’m setting up a Moku:Go lock-in to measure an optical detector output. The laser input to the detector is mechanically chopped at ~5.2 kHz to modulate it.
The detector signal is ~2 mVp-p, and AC coupled into Input 1. There is a 50 ohm terminator on Input 1, as required by the detector.
The signal into Input 2 is the sync signal from the optical chopper, a clean ~2.2 Vp-p square wave, 50% duty cycle, ~5.2 kHz.
I set the gain to +72.6 dB to get about +4.5 volts DC at Output 1. When I blind the detector, I still get about +2.3 volts out. The leakage current from the detector is blocked by AC coupling, so why don’t I see ~0 volts?
I’m not sure what the operating environment limits are for the Go, but the ambient temperature in the room is about 90F, 55% RH.
Setup:
% Saved 2024-07-25 T 10:08:23 -0700
Moku:Go Lock-In Amplifier
Input 1, AC coupling, 0 dB attenuation
Input 2, AC coupling, 0 dB attenuation
External PLL demodulation: AC coupling, 1 MOhm impedance, 0 dB attenuation, phase 180.000 000 deg
Lowpass filter corner frequency 15.9 Hz (time constant 9.99 ms), slope 6 dB / octave
Output 1 - R signal, +72.6 dB gain, invert off, offset 0.000 0 V, output enabled
Output 2 - Theta signal, +0.0 dB gain, invert off, offset -150.0 mV, output enabled
Hey John, it looks like your images aren’t loading properly, so I’ll just tell you what I am thinking could be happening.
You mentioned that the signal is 2 mVpp, so the amplitude will be 1 mV. After the amplification, I would expect the output voltage to be around 4.265 V, which sounds like what you are seeing before blocking the detector.
The 2.3 V you’re seeing when the detector is blind sounds in the region of expected to me, given that the gain is very large and that the noise power within the demodulation frequency range is amplified. This can cause the non-zero output you are seeing. So, if the 2.3 V signal you are seeing behaves like noise then I think that is okay.
If the 2.3 V is not behaving like noise, then can you try upload your screenshots again?
I’ve been recreating this on my end and I have a couple of suggestions for you.
Firstly, I would suggest that you change your Polar range setting from 2 Vpp to 7.5 mVpp, as the two Input signals multiply together to produce a 4.8 mVpp signal through the LP filter, so the 7.5 mVpp range is more appropriate.
Secondly, what are your PLL settings? When I set the PLL bandwidth to 10 Hz I get a signal with less visible quantization, with a lower signal mean in the tens of uV (with a gain of 0 dB). I’d love to know if you get similar results when you have these settings and remove the gain from your system, as that will give me an understanding of the underlying signal without the gain.
The only other thing I came across in my testing was some potential crosstalk between the cables, as after I moved the cables around, the LIA Output 1 mean value had reduced. So, if you have the chopper signal and the photodiode signal running next to each other for any distance, then even blocking the detector might still be getting some signal at the target frequency due to crosstalk. You can check if this is affecting your measurement by disconnecting the signal cable at the Moku and observing any changes, rather than just blocking the detector.
Summarizing the setup - the purpose of this setup is to measure a low signal coming from an optical detector. The beam impinging on the detector is a CW laser beam chopped at about 5 kHz with an optical chopper. The chopper also provides a synchronous square wave output of about 4.7 Vpp. I’m looking at the magnitude, R, at Out 1 which will be proportional to In 1. Since the two inputs are the same frequency, I should be able to adjust the phase shift for a difference of zero degrees, producing a DC only signal at Out 1 and zero out of Theta. With that in mind, I’m setting a minimum cutoff frequency and the steepest roll off (24 dB / octave). Am I thinking correctly?
Addressing your questions:
Firstly, I would suggest that you change your Polar range setting from 2 Vpp to 7.5 mVpp, as the two Input signals multiply together to produce a 4.8 mVpp signal through the LP filter, so the 7.5 mVpp range is more appropriate.
I understand that the magnitudes of the input signals multiply, but where did you get 4.8 mV? Should I probe the two mixer inputs to see the levels? Selecting the Polar range setting to accommodate the product of the signal magnitudes makes sense and answers a question I had about that setting.
