SET UP OF AMPLIFIER:
There is no single best way to set up the lock-in amplifier. It usually depends on the properties of the sample being measured and the nature of the measurement. If the sample has never been measured, setting up and trouble shooting the lock-in amplifier can be a tedious exercise.
The first measurement in any pulsed field experiment should be of the raw background noise without pulsing the magnet. The following steps and suggestions should be helpful with the set up:
1. RC time constant of sample and cable capacitance will dictate maximum frequency.
2. Desired time response affects the time constant. use the longest one consistent with excitation frequency and desired time response.
3. Gain should be set so that lock-in overloading is positively prevented.
4. Use transformers for low-impedance sources or a differential pre-amp for high impedance sources.
5. 150 kHz is often a good choice but it may be desirable to use other frequencies.
MINIMIZING NOISE ENTERING MEASUREMENT INSTRUMENTS:
There are different input options on lock-in amplifiers, pre-amplifiers and other instruments that if set properly can minimize interference and noise. In general there are two coaxial inputs: inverting and non-inverting.
One can connect a single coaxial cable into either the inverting input or the non-inverting input. The other input is grounded. In this case, the instrument is measuring the voltage difference between the inner and outer conductors of the coaxial cable. This is a good choice if the open-loop area is injecting excessive pickup because there is only one cable. However, because the outer conductor of the coax is part of the voltage measurement, it is not shielded from interference. This can allow noise to enter the instruments on the outer conductor of the coaxial cable.
The second choice is to measure the voltage difference between two coaxial center conductors. This is done by connecting one voltage lead to input non-inverting, via the inner conductor of a coaxial cable, and the other voltage lead to input inverting, via the inner conductor of another coaxial cable, and setting the pre-amplifier to the differential mode. Because both the V+ and V- conductors are shielded from outside noise by the outer conductor of the coaxial cable, differential mode and common mode noise caused by external sources may be lowered. However, a larger loop area can couple to the magnet and other interference sources.
Noise that appears equally on both the inner and outer conductor of a coaxial cable plugged into a single input, or noise appearing on both terminals of a differential measurement is called COMMON MODE noise. Common mode noise is much less of a nuisance than differential mode noise, because both inputs see the same interference, and ideally it should cancel when subtracted. The accuracy of this subtraction in differential mode is expressed as common mode rejection ratio (CMRR), usually measured in dB. 100 dB of CMRR means rejection of common-mode voltages by . Most pre-amps cannot do better than 60-80 dB at 150 kHz.
Once the wiring configuration
has been selected, the lock-in amplifier time constant should be adjusted
to obtain a high signal to noise ratio, consistent with the desired time