FAQ - Applications

You can use a lock-in amplifier to do this. The idea is to use its oscillator to generate a current through the coil, by connecting the coil in series with a known value of resistor. You then use the lock-in to measure the amplitude and phase of the voltage across the coil, from which you can derive the coil's impedance. By changing the oscillator frequency you can also investigate frequency-dependent effects in this impedance. Typically you also need several interchangeable test resistors to give a wide impedance measurement range. Our model 7225 or 7265 lock-in amplifiers are probably best suited to this application. However, you should be aware that they are not certified for medical use and as such we cannot recommend their use where there is any prospect that a patient could come into contact with exposed conductors that are in turn connected to the lock-in amplifier. Without further information it is difficult to see if this is the case with your experiment.
It rather depends on how you will measure the resonant frequency...
If you will apply an excitation frequency to the crystal and measure the response from it then our lock-in amplifiers can in general do this. The 7265 can generate frequencies up to 250 kHz with a 1 mHz (0.001 Hz) setting resolution, so if your device resonates at say 240 kHz you could perform a frequency scan over a range of say 100 Hz about this and look for the peak. Our software package, Acquire, allows such scans to be easily set up and run. 

However, one problem is the frequency of 250 kHz. If you need to go above this then you will need the model 7280 2 MHz lock-in instead. Other than its increased frequency range it can make exactly the same measurements as the 7265. 
I also read about the Harmonic measuring capabilities, and was wondering if you could explain the capabilities to in more detail. I will be using the lock-in to measure a detector current at the output of a monochromator, wavelengths 700nm to 1800nm - I haven't worked out what the signal strength will be, but I'm certain it is something the 7265 could handle.

The 7265 can be referenced to an external frequency in the range 1 mHz to 250 kHz so it will certainly cover your requirements. 

The harmonic measuring capability means that, for example, if your chopper signal is at 1 kHz but the chopped waveform is nominally a squarewave then the lock-in can be set to measure the third harmonic of this signal at 3 kHz (i.e. 3 x F). This mode is most commonly used for distortion measurements where the signals measured sequentially at several harmonics are summed. 
If you simply want to measure the ratio of the signal being displayed by the 7265 to another signal then I suggest you use the User Equations menu to define an appropriate equation, typically something link ((MAG + 0) * 1)/ADC1, and apply the signal representing the denominator to the ADC1 input. If you want the result to appear on the main display then one way of doing it is to configure the CH1 or CH2 analog outputs to represent the equation and then connect this output to the ADC2 input, and set the display to show ADC2 as required. 

Alternatively you may be able to use the dual reference mode of the 7265 in conjunction with a suitable light chopper to make ratio measurements on two chopped beams of different frequencies. This is discussed in our application notes AN1000 "Dual-Channel Absorption Measurement with Source Intensity Compensation (Models 7260 & 7265)" and AN1003 "Low Level Optical Detection using Lock-in Amplifier Techniques" which you can download from this website. 
It depends on what you want to do with the signals from the detectors. 

If you need to independently measure the amplitude of the signals at each of the detectors then you will need four measurement channels. This generally requires four lock-ins, although it may be possible to use the dual reference mode of the 7265 if you can arrange that the signals you are measuring on one pair of detectors is at a different frequency to those on the other pair. Then you will only need two lock-ins.
If, when the laser is not pulsed, the light through the sample is a fixed level (i.e. there is no external modulation on it by means of a chopper) then it appears that there will be a signal that is repetitive and triggered by the laser when it is pulsed. Given this, you can use a signal averager running at the laser rep rate of 10Hz to record many waveforms and average them. 

For this you can use the model 9846 transient recorder (as the repetition rate is low) or, if you might want to move to higher repetition rates in the future, the model 9826 or Eclipse (the latter only if your 10 Hz laser can be externally triggered). Your existing scope may even be able to do the averaging. 

If the "normal" light intensity is drifting with time during the experiment then the above technique will not work well. Assuming that your laser gives a (stable) 10 Hz trigger output then I would use the following system: 

Use the laser to trigger a model 9650A delay generator, with the delay set to 99.8 ms. Take the trigger out of this and use it trigger a 9846, set to 32 ns per point and 65 k points and a sweep time of 2 ms. Hence on each 100 ms trigger of the laser, the 9846 will acquire a sweep of data "bracketing" the laser pulse. Each sweep is then read by the PC, which using your own software based on our free LabVIEW driver can then be programmed to read the data at a few early channels (corresponding to the time before the trigger) and then ratio rest of the data curve to the average of these values (to correct for source fluctuations), with the resulting curve being averaged again. Given the above and data transfer times it is possible that only every other laser pulse would be used, but nonetheless it still represents a viable solution to your problem. 
The simplest option is a model 5105, which includes a LabVIEW driver. It can take several readings/second, so over a 10 second time frame you could acquire a reasonable curve of data. It doesn't have an oscillator - if you need this then you can use the cheapest sort of unit from any T&M supplier - you could probably buy this for at most a few hundred dollars.