Products > Test Equipment
7265 Lock In Amplifier
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_Wim_:

--- Quote from: percypeng on December 26, 2020, 07:25:15 am ---Now I get it running OK with the program, all I do is remove 232 cable and reboot 7265 as well as PC, then open the program, it seems the unit was locked to 232 connection if it was used at first time, to using GPIB, you have to reboot the system.

--- End quote ---

It indeed seems the firmware of the 726x is sensitive to lock-up the communication. I have this in the past also even using the official acquire software.
i3v:

--- Quote from: jhenderson0107 on December 25, 2020, 11:38:36 pm ---I just uploaded a new version which tests successfully here.  When invoked for the first time, it is necessary to click the Discover button and click the GPIB checkbox in the Instrument Discovery Options dialog shown so that the GPIB will be scanned.  Subsequently, the device should be discovered and begin scanning immediately. 

Here's the link:  https://www.dropbox.com/s/xcr7ioz8s3ugvgx/LockIn.zip?dl=0

--- End quote ---
It's a pity that this link is dead again...
jhenderson0107:
I've updated the link in the original post. 
rubidium:
Just noticed this post. I became a big fan of lock-in amplifiers a few years ago when designing a sensor for measuring the electrical conductivity (EC) of aqueous liquids that can sustain continuous immersion for long periods. Typical EC sensors use 2 immersed electrodes and measure the resistance between them to infer EC. Unfortunately, these electrodes are subject to degradation by corrosion or other fouling if left immersed for too long a period. My approach was to immerse 2 small back-to-back toroids, drive one with a square wave and measure the magnitude of the induced signal in the other. Without a closed conducting path linking the cores of the 2 toroids, the induced signal should be zero, if the toroids are assembled properly (i.e. with bifilar windings) and thus exhibit no external magnetic fields. With a closed conducting path passing through both toroids, the signal will be linearly proportional to the conductivity of that path. The issue I encountered with this approach was a very low signal to noise ratio. For sufficiently low EC levels, the SNR was less than 0 - sometimes a lot less. That's when I thought of a lock-in amplifier. Since I had a square wave driving one of the toroids, I used that same signal to switch the gain of an opamp between +1 and -1 synchronously, and then integrate. With a couple of pre-amplification stages, this worked like a charm, even though a square wave might not be the optimum waveform in light of its harmonics for taking the most advantage of that simple trig identity that underlies the concept. In any event, I became forever sold on the lock-in amplifier.

I suppose one can do what I did in digital form by appropriately sampling a signal with a microcontroller's ADC and then adding up the measurements with appropriate sign flips that are synchronous with the drive signal (probably by having the drive signal also trigger an MCU pin change interrupt.
Kleinstein:
One can do the lock in amplifier digital with a µC. However the dynamic range is limited by the ADC. So when there is a strong interfering signal (e.g. mains hum) considerably larger than the signal of interest one may run into the limits of the µC internal ADCs with usually 10 or 12 bit resolution. The modern lock-in amplifiers generally use higher resolution ADCs (like 16 to 24 bits) to allow for a lower dynamic range.

The square wave demodulation is not that bad a choice. It is relatively simple and can be done low dift in the analog form. Analog multiplication with a sine wave is tricky. Depending on the actual signal the square wave can also use the additional harmonics part, provided there is not extra phase shift and the modulation is also more square wave. In those cases with a square wave signal the square wave demodulation is actually the better choice.  Even some of the analog LIs use square wave demodulation and filter out the harmoics of the signal up-front, because the square wave demodulation is easier and lower drift than analog multiplication.
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