Can someone tell me what this does...(lock-in amplifier/voltmeter)

[PStevenson]

can someone shed light on what this is/does maybe even how to use it. I've tried the net but any information regarding it usually assumes some prior knowledge. it was given to me to salvage parts from along with a load of other stuff however I was told this piece of equipment worked so I kept it + it looks cool in my "lab"

I have always been curious about it and I thought this might be the place to get some answers

[tekfan]

Nice piece of test equipment you've got there. That and the Telequipment scope.
Are the inputs on the back? Can you take a shot of the back?

Here's what it does:
http://en.wikipedia.org/wiki/Lock-in_amplifier

Here's a pdf to describe it's use in practice (look at page 4 and onward):
http://www.physics.rutgers.edu/ugrad/387/nmr.pdf

And here's how the setup described in the pdf actually looks like:
http://www.physics.rutgers.edu/ugrad/387/388s06/photos/F05387/nmr.JPG

[ejeffrey]

Lockin amplifiers are the coolest instruments in the world   I have a few of that model sitting around, although most of them don't work any more.

A lock-in amplifier measures the amplitude and phase of a signal at a specific known frequency.  It multiplies the input signal by a sine wave at the known reference frequency.  Through the normal non-linear conversion process, this generates sum and difference frequency components.  For the special case of the component of the signal that is at the same frequency as the reference oscillator, the difference frequency is at 0 Hz or DC.  That signal is filtered with a low-pass filter, and produced as a slowly varying voltage or displayed on the meter.

The magic is that the analog multiplier followed by a low-pass filter is equivalent to a bandpass filter with the same bandwidth.  This is nice because it is easy to make a 0.01 Hz lowpass filter, but quite difficult to make a bandpass filter centered at 5 kHz, but with 0.01 Hz bandwidth.

The second important point is that the lock-in measurement is phase sensitive.  For any given frequency you can have a sine wave or a cosine wave, and they are distinct.  A single-phase lock-in measures only one, the component in phase with the reference signal (they will have an adjustment to change the reference phase to match the signal).  A dual phase lock-in measures both the sine and cosine terms independently.  This is how most LCR meters work: they inject an oscillating voltage and measure the conducted current with a dual-phase lockin.  The in-phase term gives the resistance, while the out-of-phase term gives the reactance.

Lockin measurements come in many guises, but in many ways they are a much more sophisticated cousin of a 'relative mode' measurement.  If you are trying to measure a small voltage with a DMM, you can compensate for the DC offset by in turn shorting the probes together and connecting them to the source you want to measure.  Subtracting the background reading from the signal, and you eliminate the offset error in your DMM -- as long as the offset doesn't drift in the time it takes you to switch measurements.   By hand you can only switch the signal at 0.25 Hz or so, but a lockin amplifier can work at 10 kHz.

A classic lockin use is to measure an optical signal in the presence of a huge background noise.  If you are trying to use a photodiode to measure the amount of light from a laser pointer scattered off a wall in broad daylight, you might expect it is impossible.  The sun is orders of magnitude brighter than the little laser pointer.  However, if you use an optical chopper to block and uncover the laser at a few kHz, a lock-in amplifier can recover the signal in the presence of much larger noise.

[PStevenson]

that is pretty cool, I think I understand what it does - pin pointing a specific signal in a sea of noise to measure it basically.

the insides of it are brilliant though, it's got a great smell to it aswell so if you have some of this model and you haven't yet, take it apart !
thanks so much for the reply and explanation.

@Tekfan I can't really get it down off the shelf cause there's tons of stuff on it, that scope is a really nice one but unfortunately doesn't work properly, the trace has become really thick and is pretty much unusable, I keep getting it down and try to fix it but I have so far always failed.

[ejeffrey]

Yeah, a lot of that old equipment is really beautiful inside.  I should see if I can take apart one of the PAR 124 lock-ins we have around and take some photos...

[PStevenson]

you definitely should do, I'd love to see those pictures.
I usually take pictures of inside all the equipment I have for future viewing pleasure.
one of the most beautiful things I have for internals is a SE Labs EM102 scope, it's all discrete aswell
unfortunately that's broke too, it turns on and I can even get a good trace but it just drifts up the screen till it's gone

[carlitos49]

For those of you who have come across the term “Lock-In Amplifier” but are not yet sure what do they really do and how do they really work. Well some of the people in this forum have given excellent and intuitive explanations but sometimes it is hard to put it all together in your head until you can see the whole picture in a step by step basis.   Lock-In Amplifiers basically take a very tiny signal buried in noise and by a process of signal multiplication we can eliminate out of phase and out of  frequency noise. When we say frequency multiplication we are referring to two different signals where one is derived from the other, lets say we want to measure a change in an AC signal voltage that is 1mVpp  and its buried in out of frequency noise that is just as high or higher than what we are are looking at. As a simple example, let’s say we have a resistive sensor that is varies it’s value in milli-ohms  and let’s say we want to be able to measure each change in milli-volts so we see a 1V/mV or an amplifier that has a gain 1000.  Obviously if this signal was the only signal present and there was no other noise component all you would need is an amplifier with a gain of 1000 and you be done but unfortunately in the real world this may not be so easy to do.   Lock-In Amplifiers come in various topologies, you can use a integrated circuit such as the  AD630 from Analog Devices which is a balanced modulator / demodulator or you can simply use a +/- 1 gain amplifier as a synchronous detector whereby the */- basically converts the input signal Fs with the same frequency Fr with a gain of -1 or +1 which basically multiplies the frequency by 2 as long as (Fs = Fr) and depending on the phase difference between these two the final DC value after a low pass filter is either positive or negative.

I have put together here a simple circuit example of the above described Lock-In Amplifier using LTspice (see attached LTspice simulation file, LockIngAmp.asc) In this example I am injecting a bipolar 1 KHz sine wave ( Fs) wit a 1 Vp-p amplitude this goes through a voltage divider trough a 10K resistor and a 10 ohm resistor so basically we have a 1 mVp-p across the 10 ohm resistor which I call Rsensor. At the same time I also inject an interfering noise signal at a frequency of 250 Hz with the same amplitude as the signal I’m trying to amplify. So the goal here is to get a DC signal that represents the amplitude of the 1 mV signal with a 1V/mV scaling factor.  The final DC voltage is Vout which is the output of the 10Hz LPF stage, notice that the LPF stage has a gain of  1.6 rather than 1 and this is because the LPF will average the peak to peak value which in this case is ~ 0.62V for a 1Vp-p signal.  The +/- synchronous detector is driven by a switch that is toggling at the same frequency as Fs actually this switch is driven by a square wave that is generated via a comparator, I call the Square wave Fr so if the phase angle between Fs and Fr is = 0, the output at Vout will be -1.0 Vdc for a 1mV input signal. However, if the phase angle between Fs and Fr is = 180, the output at Vout will be +1.0 Vdc for a 1mV input signal.

Hopefully you have access LTspice which can be downloaded from here: https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html
           
If you have used LTspice before and you can try this simulation, the nice thing about it running the simulation, is that it allows you to see and understand every step of the functionality of the Lock-In Amplifier by probing the various inputs and outputs of the circuit it also gives you a more intuitive feel of what’s goin on when you look at the FFT plot of the various intermediate circuit stages.

Enjoy!