Using The Sr844 As A Double Lock-In - Stanford Research Systems SR844 User Manual

2-22 SR844 Basics

Using the SR844 as a Double Lock-In

The ratio feature of the SR844 can be used to provide a second stage of demodulation.
Consider the following application: you have an experiment providing a signal at
100 MHz. Because the environment is noisy and RF interference is everywhere, you
arrange the experiment so that the signal of interest is modulated at a low frequency, say
on/off at 100 Hz. This could be a laser beam with a 100 MHz pulse rate going through a
light chopper spinning at 100 Hz. The problem is to measure the modulated component of
the 100 MHz signal.
One solution is to use a 100 MHz power meter to measure the signal, and put its 100 Hz
output into a traditional low-frequency lock-in such as an SR830 and measure the 100 Hz
component. Or you could put the signal into an SR844 and make a narrowband
measurement at 100 MHz, using a 1 ms time constant, and take the 100 Hz analog output
and look at it with either an oscilloscope or a low-frequency lock-in. Or you could have
the SR844 detect both the 100 MHz and 100 Hz signals as follows: put the 100 Hz
reference signal into the SR844's AUX IN 1 input, turn on ratio mode, and have the
SR844 make the measurement for you directly, say with a 1 s time constant.
The key to this technique, sometimes referred to as a double lock-in, is putting in a
bipolar square wave into AUX IN 1. Remember that the DSP is dividing the incoming
data by AUX IN 1, and then low-pass filtering the result. If AUX IN 1 is
effectively demodulating the 100 Hz output and averaging it.
In order to get accurate measurements of the 100 Hz modulated component of the
100 MHz signal, it is important that any unmodulated 100 MHz signal be rejected. You
can do this by turning off the 100 MHz modulation and adjusting the DC offset of the
AUX IN 1 square wave until the SR844 reading is nulled. In the above example, simply
turn off the light chopper and pass the beam 100% of the time. If the AUX IN 1 signal
comes from a source that allows independent phase adjustment without disturbing the
experiment, you can also perform the following test: change the phase of the AUX IN 1
source by 180
but stay at the same value.
While the use of AUX IN 1 for demodulation can be a handy technique, it does suffer
from two limitations. First, the AUX IN 1 input is bandwidth limited to about 3 kHz
(minimum sampling rate is 12 kHz), so the modulation signal into AUX IN 1 should be
considerably slower than this, say up to a few hundred Hz. Second, there is no phase
adjustment on the ratio input. Remember, the X and Y outputs are both modulated at the
same phase of the 100 Hz modulation. In the above example, they both turn on and off
together with the light chopper. If the ratio input is 90° out of phase with the this
modulation, the result is zero on both X and Y.
In general, using a low-frequency lock-in amplifier is preferred. However, in many
instances, the SR844 provides a convenient solution for both modulation frequencies.
SR844 RF Lock-In Amplifier
°
— if the signal is properly nulled, the instrument reading will change sign
±
1 V, the DSP is