Stanford Research Systems SR865 Operation Manual page 96

78 Operation
Timebase
The SR865 has a 10 MHz TIMEBASE input and output on the rear panel. Apply a
10 MHz sine signal (1 Vrms) to the TIMEBASE input to lock the SR865's timebase to an
external 10 MHz timebase. The internal reference frequency of the SR865 is derived
from this timebase. When multiple SR865's are locked to the same 10 MHz timebase,
then harmonically related internal frequencies on different units will stay in phase
indefinitely.
The Ext 10 MHz
The SR865 can also output its own 10 MHz timebase to another unit.
Note that external function generators, even DDS locked to the same 10 MHz timebase,
will not stay in perfect phase with the SR865. This is because the resolution of the
frequency tuning word in the external synthesizer differs from the SR865.
<Phase>
This knob may be used to adjust the phase. The phase shift ranges from −180° to +180°
with 0.001° resolution. Press and hold this knob to set the phase to zero.
The phase value is shown in the info bar at the top of the screen. Touch this {Phase} tile
to show the numeric entry screen.
When using an external reference, the reference phase shift is the phase between the
external reference and the digital sine wave which is multiplying the signal in the PSD.
This is also the phase between the sine output and the digital sine wave used by the PSD
in either internal or external reference mode. Changing this phase shift only shifts internal
sine waves. The effect of this phase shift can only be seen at the lock-in outputs X, Y and
θ. R is phase independent.
Auto Phase
Touching {Auto} in the numeric entry screen or pressing the [Auto Phase] key will adjust
the reference phase shift so that the measured signal phase is 0°. This is done by
subtracting the present measured value of θ from the reference phase shift. It will take
several time constants for the outputs to reach their new values. Auto Phase may not
result in a zero phase if the measurement is noisy or changing.
Phase [+90°] and [−90°]
The [+90°] and [−90°] keys add or subtract 90.000° from the reference phase shift.
Touching {+90°} and {−90°} in the phase numeric entry screen will do the same.
<Frequency>
If the reference source is Internal, Dual or Chop, this knob adjusts the internal reference
frequency.
The internal frequency is shown in the info bar at the top of the screen. Touch the {F
tile to show the numeric entry screen.
SR865 DSP Lock-in Amplifier
LED is on when the unit is locked to an external timebase.
Chapter 3
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