Stanford Research Systems SR865A, Operation Manual

Page: 59 / 221

Chapter 2 Basics 41
SR865A DSP Lock-in Amplifier
Thus, in the previous example with a 2 V pk
–
pk square wave input, the SR865A would
detect the first sine component, 1.273 sin( ω t). The measured and displayed magnitude
would be 0.90 V (rms), e.g. 1.273
/√2
.
Degrees or Radians?
In this discussion, frequencies have been referred to as f (Hz) and ω (2πf radians/sec).
This is because people measure frequencies in cycles per second and math works best in
radians. For purposes of measurement, frequencies as measured in a lock-in amplifier are
in Hz. The equations used to explain the actual calculations are sometimes written using
ω to simplify the expressions.
Phase is always reported in degrees. Once again, this is by custom. Equations written as
sin(ωt + θ) are written as if θ is in
radians mostly for simplicity. Lock-in amplifiers
always manipulate and measure phase in degrees.
Block diagram
A simplified block diagram of the SR865A ’s lock -in circuit is shown below and
explained in the following sections.