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Chapter 3
Make sure that the SR860 External Trig is set to Pos TTL and the Input is 1 MΩ. Adjust
the SR860 internal frequency to a value that the SR540 can reach with the selected blade.
The Phase setting in this screen is the 'blade' phase. This is the phase of the blade
opening relative to the optical detector at the base of the chopper head. When a single
chopper is being used, this is not important since the demodulator reference phase of the
SR860 can be adjusted to maximize the signal. When using multiple choppers, this
'blade' phase can be used to align multiple chopped beams to arrive in phase together at
the experiment. This would otherwise be done by moving the position of the chopper or
beam.
The <Cursor> knob will also adjust the phase value in this screen.
External [Input]
This key sets the termination of the Ref Input BNC to either 50 Ω or 1 MΩ. Be careful
that the termination does excessively attenuate the signal or cause ringing on its edges.
External [Trig]
This key selects the external reference input trigger mode.
TTL
When either Pos TTL or Neg TTL is selected, the SR860 locks to the selected edge of a
TTL square wave or pulse train. For reliable operation, the TTL signal should exceed
+1.5 V when high and be less than +0.5 V when low. This trigger mode is dc coupled.
This input mode should be used whenever possible since it is less noise prone than the
sine wave discriminator.
For very low frequencies (<1 Hz), a TTL reference MUST be used.
Sine
Sine input trigger locks the SR860 to the rising zero crossings of an analog signal at the
Ref In BNC. This signal should be a clean sine wave at least 200 mVpp in amplitude. In
this input mode, the Ref In is ac coupled (above 1 Hz).
Sine reference mode cannot be used at frequencies far below 1 Hz. At very low
frequencies, the TTL input modes must be used.
Unlock
In External or Dual reference source, the
lock to the external reference. This can be because the external reference amplitude is too
low or the frequency is out of range.
In Chop reference,
because the switches on the SR540 are incorrectly configured or the blade is not turning
freely.
Unlock
Unlock
indicates that the SR540 chopper is unlocked. This can be
Operation
indicator turns on if the SR860 cannot
SR860 DSP Lock-in Amplifier
77
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