Cannot lock reference signal on lock-in amplifier

[ash1204]

Hello everyone,

I am using a lock-in amplifier SR860 with the aim of reading an optical pulse from a laser break-out board operating at 30 kHz as the reference.
The reference signal is not being picked up by the lock-in amplifier. However, it is being read by the oscilloscope.

I have used a function generator, which I have set to operate at 30 KHz, and at a similar amplitude and pulse width of that of the laser optical pulse. Witht the function generator, the lock-in amplifier is able to read this signal just fine. The ‘Unlock’ light turns off with the function generator, and it reads 30 KHz on the screen.

I am unsure why it is not doing the same with the optical pulse.
Please see the oscilloscope readings for both the optical pulse from the laser and that from the function generator

The amplitude of the optical pulse does not seem to be a problem, because when I drop the amplitude of that of function generator (even lower than that of the optical pulse), it is still picked up by the lock-in amplifier.

When connected to the laser break out board, set at 30 KHz, this is what it reads on the lock-in amplifier:

The ‘unlock’ sign remains on, and it reads out a frequency of 90 KHz.

On another note, one thing that I have noticed is that when I change from the currently used External TTL setting to the External Sine setting on the lock-in amplifier, the lock-in amplifier reads the laser optical signal briefly ( the ‘Unlock’ light turns off for maybe about a quarter of a second, and it reads 30 Khz, before turning on again).

When connected to the function generator, this is what I see:
I have tried the different settings for the reference, i.e., Sine Positve TTL, negative TTL, at 50 ohm and 1 Mohm. At any of these settings, the lock-in amplifier is able to pick up the signal from the function generator.

I am very new to using lock-in amplifiers, and my background is not in electrical engineering, so I am not sure what else can i try to make it work.

Any help will be very much appreciated.
Best regards,
Ash

 

[marcusl]

What is the input impedance of your lock-in and are you sure your optical system can drive full voltage into that impedance?

 

[DaveE]

What is the input impedance of your lock-in and are you sure your optical system can drive full voltage into that impedance?

Yes, good idea. You can put a coax tee on the amplifier input and look at the signal with your o'scope with the amplifier load. Use the high impedance setting on the scope. At audio frequencies you don't have to worry about impedance matching, etc. for a crude test like this.

It sounds like it might be trying to lock onto the third harmonic. You could try a LPF. IDK if the phase shift from a filter would be OK with your experiment, you'd have to account for that.

It looks like it should work. I suspect something isn't the way you think it is. Recheck all of the basic stuff.

 

[Tom.G]

A good place to start is signal levels.

The signal generator amplitude is 6V.
The Laser optics amplitude is 4V.

See if you get lock-in for the signal generator when its output is 4V.

Cheers,
Tom

 

[DaveE]

See if you get lock-in for the signal generator when its output is 4V.

But in the OP:

The amplitude of the optical pulse does not seem to be a problem, because when I drop the amplitude of that of function generator (even lower than that of the optical pulse), it is still picked up by the lock-in amplifier.

Thanks for playing... I doubt that experiment needs to be repeated.

 

[Baluncore]

Welcome to PF.

A narrow pulse signal will have a baseline close to zero. If zero is also the threshold for the reference input, then there will be noise with harmonics of the reference signal entering the reference PLL.
I would look first at the reference input DC-Level setting. For a TTL input signal, that threshold should be set to about 1.2 volts, which is halfway between TTL low and high voltages. For an AC coupled sinewave reference, the threshold should be zero volts.