Signal Recovery

FAQ - Applications

Q1. I want to measure the inductance of a coil by incorporating it into a parallel RLC circuit and either (1) tuning the exciting signal frequency until resonance is achieved or (2) measuring the frequency response and fitting it to the theoretical expression for that configuration. The coil I am interested in has a resistance of about 3 ohms, and by applying the formula given in the AIP handbook I estimate the inductance to be of the order of 10 microhenries. I propose to connect the coil in series with a 1000 ohm resistor, and to connect this combination in parallel with a 10 picofarad capacitor, and apply a sinusoidal voltage. The output signal would be the potential across the 1000 ohm resistor, and this would be in phase with the exciting potential at an angular frequency, omega, of about 1000. Is this within the nominal capabilities of your instruments?

Lock-in amplifiers are commonly used to accurately measure the network response of systems, so what you propose is feasible. However I calculate that the resonant frequency of your network will be in the order of 50 MHz, so there will be little phase shift at typical measurement frequencies of up to a few megahertz with the capacitor values given. In any case the input capacitance of the amplifier as a shunt across the 1000 ohm resistor will change the response. I suspect that you would be better using a capacitor value in the order of 10 nF.

Q2. I need to measure the AC impedance from say 2 kHz to about 4 kHz of some very thin wire coils. These coils are 92% platinum and 8% tungsten and between 0.001 inch to 0.002 inch thick wound into about 2 cm coils. These coils are used in a medical application and placed inside the brain of patients. I need to measure the AC resistance (using a lock-in amplifier) and am thinking of a Wheatstone bridge approach. What do you recommend?

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.

Q3. We are interested in purchasing a Lock-in Amplifier for our research. But, before we do, we want to make sure it is capable of doing the kind of things we need, hence, this message to you. We are hoping to measure resonant frequency from a sensor at an approximate value of 250,000 Hz. We are further attempting to accurately measure and detect shifts in resonant frequency of about 5 Hz increments. Can you tell us if SIGNAL RECOVERY lock-in amplifiers are capable of this level of sensitivity? In other words, could I expect to detect the differences between a resonant frequency of 250,000 and 250,005? Which model would you recommend?

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.

Q4. I am looking for a lock-in amplifier that will be able to synchronize with a chopper frequency in the range 25 Hz to 5 kHz. I have used an EG&G model 7260 in the past, and was quite impressed with it's ability and interface, but I understand that it has been replaced by the 7265 model. Is the 7265 able to measure at the frequency range I'm interested in chopping at?

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.

Q5. We would like to receive technical information about the possibility to acquire a device to add to our lock-in preamplifier 7265 in order to perform ratiometric spectroscopy. Do you have any tip for us? Does SIGNAL RECOVERY produce any device of this kind?

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.

Q6. We are building a system that uses 4 detectors at the same time in one package, which we would like to connect to lock-in amplifier. Do we need a 4 channel lock-in or 4 separate instruments?

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.

Q7. I am performing an experiment where I use a PMT to measure the light transmitted through a sample. The sample is excited by a 10 Hz pulsed laser that causes the transmission to change. I need to see the changes in detected light over a time frame from about 100 ns to a few milliseconds (with lower rep-rates if measuring millisecond response times) compared to the "normal light intensity" which is taken as that about 500 ns before the laser fires. Can I use my existing oscilloscope? What do you suggest?

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.

Q8. I am interested in a simple LIA measurement of a battery at a fixed frequency of 1 kHz via a potentiostat in as short a time frame as possible. The preferred interval is in the order of a few seconds. I would like a PC LIA card with GPIB or with LabVIEW drivers. What would you recommend?

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.