Download RF Lock-In Amplifier - Frederick Seitz Materials Research Laboratory

SR844 Basics
2-13
Offsets
User entered offsets can be added to X and Y. These offsets are added before taking
ratios, filtering and computing R and θ.
Offsets are useful for making relative measurements or to cancel the contribution from an
unwanted phase coherent signal. In analog lock-ins, offsets were generally used to remove
DC output errors from the mixer outputs. The SR844 demodulator is digital and has no
DC output errors, however, it does have coherent pickup at high frequencies, which can
be canceled using offsets.
Important!
Xoffset and Yoffset are applied to the X and Y before other processing occurs. R and θ
are computed from the offset values of X and Y. Adding offsets to X or Y changes the
value of R and θ.
In addition, changing the Reference Phase will modify the values of Xoffset and Yoffset.
Think of (Xoffset, Yoffset) as a signal vector relative to the Reference (internal or
external) which cancels an actual signal at the input. This cancellation is preserved even
when the detection phase (Reference Phase) is changed. This is done by circularly rotating
the values of Xoffset and Yoffset by minus the Reference Phase. This preserves the phase
relationship between (Xoffset, Yoffset) and the signal input.
Since the vector (Xoffset, Yoffset) is used to cancel a real signal at the input, Xoffset and
Yoffset are always turned on and off together. Turning either offset on (or off) turns on
(or off) both offsets. Auto offsetting either X or Y performs auto offset on both quantities.
These statements are true even if only one of the quantities X or Y is currently being
displayed.
Ratio
If ratio mode has been selected, the reciprocal of the appropriate input (1.0 V/AUX IN 1
or 1.0 V/AUX IN 2) is computed, and both X and Y are multiplied by this quantity. Since
the value of R is computed after the ratio, R is also scaled by the ratio.
Another application of the Aux Input voltages is to provide a second demodulation,
sometimes known as the Double Lock-In Technique. This is described in the next section.
Time Constant Filters
The signals are filtered by a chain of simple low-pass filter/amplifiers. Using 1, 2, 3 or 4
stages provides the selected rolloff of 6, 12, 18 or 24 dB/octave. Distributing the gain
among the filters allows near-optimum signal recovery without causing internal overloads
or losing bits of resolution. The appropriate filtered X and Y are used for all subsequent
computations.
The individual filters are the digital equivalent of an RC low-pass filter, although being
digital they can easily incorporate gain. The numerical coefficients of the filter are chosen
to provide the selected time constant and a gain appropriate to the sensitivity. Since the
filters are digital, very long time constants (up to 30 ks) are easily achievable.
Selecting No Filter removes the filtering operations while leaving just gain. This mode is
useful when the highest possible analog output bandwidth is required from the X and Y
SR844 RF Lock-In Amplifier