Delay in Reading Data from SR44

[Benvalet]

Hi Guys,

I am trying to write a simple code that I can use to perform a frequency sweep of a tuning fork with a resonance freqquency of 32768 Hz.

Well my code works but its a bit creepy. I can get the what I want but there is a DELAY in reading the signal from the lock in amplifier. It takes a good number of seconds before the reading begins. It therefore means that the sweep using a function generator ends before the data is completely read from the SR844 Lock in amplifier

I think I am missing out on something very small, but unfortunately I could not figure it out yet.

I am sure my code is not good but its my best at the moment haha.

Any suggestions or clues on how I can solve the problem will be highly appreciated.

I have attached my code and a front panel screen shot of the output.

Thanks

Opuk

[mcduff]

Look at the manual for the SR844, in particular the SANP command. This is from the manual.

The values of X and Y are recorded at a single instant, as are the values of R[V] and θ. Thus reading (X,Y) or (R,θ) yields a snapshot of the input signal. A SNAP? measurement of X, Y, R and θ may give R,θ measured at a time up to 84 μs before the X,Y measurement.

R[dBm] and the Aux Inputs are computed with a maximum interval of 84 μs. A SNAP? measurement of X, Y, and AUX IN 1 will give AUX IN 1 measured at a time up to 84 μs before the X,Y measurement. The reference frequency is computed 6-12 times a second.

Thus there are delays in your measurements. My advice, make your sweep rate slower.

Cheers,

mcduff

[Benvalet]

Thank you very much for your response mcduff. But I not sweeping at a very high speed. In fact that is not my aim at the moment. I noted that even if I use a wait time of 450 ms, I face a similar problem.

Maybe let me explain again. The oscillator in this case a tuning fork, resonance occurs at 32,768 Hz, If you look at the picture I attached, this shows that resonace occured at a very low freuency which is not true.

My main problem is, (1) for every frequency step, how can be assured of a rsponse from the lock in amplifier, (2) When I start the sweep, the first response from the lock in begins after several seconds. this is my main problem.

Regards,

Opuk

[mcduff · Accepted solution]

This willbe a short reply due to the holidays and the fact I do not have LabVIEW on this computer.

The lockin gives a "valid" response after 5 time constants, in practice a minimum of 3 time constants can be used. So what is your time constant? If you are sweeping faster than that the response will be slow. The other option is to sweep point by point, that is, set the frequency, wait 5 time constants, get your measurements, then repeat as needed.

Cheers,

mcduff

[Benvalet]

Hi mcduff,

I implemented the point by point sweep and it solved the problem, together with other few modifications. Thank you so much for your help.

But incase I want to do a slightly faster sweep, what are some of the approcahes I can use with the same instruments i.e. Finction Generator and the lock-in amplifier SR844?

Kind Regards,

Opuk