Stanford Research Systems SR844 User Manual page 37

SR844 Basics 2-7
The SR844 contains a phase-locked loop that locks to the external reference and
generates reference signals with the correct amplitude, frequency and phase for both the
in-phase and quadrature mixers. Since the SR844 tracks the external reference, changes in
the external reference do not affect the measurement. Furthermore, the measurements
made by the SR844 are independent of the amplitude of the external reference, with one
exception. The phase relationship between the external reference and the internally
generated signals depends slightly on the amplitude of the external reference.
It is not necessary to provide an external reference to the SR844. The SR844 contains a
digital frequency synthesizer that may be used as an internal reference source. This is a
convenient feature in those cases where an external generator is not available. To use the
internal source, the front panel REF OUT must be used to excite the experiment
appropriately. REF IN is left unconnected. The mixer reference signals are generated from
the synthesizer.
It should be noted that high-frequency mixers operate by using the reference (or local
oscillator) signal to switch pairs of diodes or transistors on and off. Consequently it is
more accurate to view the mixer operation as multiplication by a square wave rather than
multiplication by a sine wave. In fact, the reference signal provided to the mixers in the
SR844 is a square wave.
SR844 RF Lock-In Amplifier