Stanford Research Systems SR810 Manual page 64

Front Panel
[+90°] and [-90°]
[Freq]
[Ampl]
[Harm #]
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.
If θ is not stable, Auto Phase will abort.
The [+90°] and [-90°] keys add or subtract 90.000° from the reference
phase shift. The phase does not need to be displayed to use these keys.
Zero Phase
Pressing the [+90°] and [-90°] keys together will set the reference phase
shift to 0.00°.
Pressing this key displays the reference frequency in the Reference
display.
If the reference mode is external, then the measured reference
frequency is displayed. The knob does nothing in this case. If the
harmonic number is greater than 1 and the external reference goes
above 102 kHz/N where N is the harmonic number, then the harmonic
number is reset to 1. The reference will always track the external
reference signal.
If the reference mode is internal, then the internal oscillator frequency is
displayed. The oscillator frequency may adjusted with the knob. The
frequency has 41/2 digits or 0.1 mHz resolution, whichever is larger. The
frequency can range from 0.001 Hz to 102.00 kHz. The upper limit is
decreased if the harmonic number is greater than 1. In this case,
the upper limit is 102 kHz/N where N is the harmonic number.
Pressing this key displays the Sine Output Amplitude in the Reference
display. Use the knob to adjust the amplitude from 4 mVrms to 5 Vrms
with 2 mV resolution. The output impedance of the Sine Out is 50 Ω. If
the signal is terminated in 50 Ω, the amplitude will be half of the
programmed value.
When the reference mode is internal, this is the excitation source
provided by the SR810. When an external reference is used, this sine
output provides a sine wave phase locked to the external reference.
The rear panel TTL Output provides a TTL square wave at the reference
frequency. This square wave is generated by discriminating the zero
crossings of the sine output. This signal can provide a trigger or sync
signal to the experiment when the internal reference source is used. This
signal is also available when the reference is externally provided. In this
case, the TTL Output is phase locked to the external reference.
The SR810 can detect signals at harmonics of the reference frequency.
The SR810 multiplies the input signal with digital sine waves at a multiple
of the reference. Only signals at this harmonic will be detected. Signals
at the original reference frequency are not detected and are attenuated
as if they were noise.
Whenever the harmonic detect number is greater than 1, the
HARM# indicator in the Reference display will flash to remind you
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