Stanford Research Systems SR810 Manual page 65

[Source]
[Trig]
that the SR810 is detecting signals at a multiple of the reference
frequency.
Always check the harmonic detect number before making any
measurements.
If the harmonic number is set to N, then the internal reference
frequency is limited to 102 kHz/N.
If an external reference is used and the reference frequency
exceeds 102 kHz/N, then N is reset to 1. The SR810 will always track
the external reference.
Pressing this key displays the harmonic number in the Reference
display. The harmonic number may be adjusted using the knob.
Harmonics up to 19999 times the reference can be detected as long as
the harmonic frequency does not exceed 102 kHz. An attempt to
increase the harmonic frequency above 102 kHz will display the
message 'hAr ovEr' indicating harmonic number over range.
This key selects the reference mode. The normal mode is External
reference (no indicator). The Internal mode is indicated by the
INTERNAL led.
When the reference source is External, the SR810 will phase lock to the
external reference provided at the Reference Input BNC. The SR810 will
lock to frequencies between 0.001 Hz and 102.0 kHz. Use the [Freq]
key to display the external frequency.
When the reference source is Internal, the SR810's synthesized internal
reference is used as the reference. The Reference Input BNC is ignored
in this case. In this mode, the Sine Out or TTL Sync Out provides the
excitation for the measurement. Use the [Freq] key to display and adjust
the frequency.
This key selects the external reference input trigger mode.
When either POS EDGE or NEG EDGE is selected, the SR810 locks to
the selected edge of a TTL square wave or pulse train. For reliable
operation, the TTL signal should exceed 3.5 V when high and be less
then 0.5 V when low. The input is directed past the analog discriminator
and is DC coupled into a TTL input gate. 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 input mode locks the SR810 to the rising zero crossings of an
analog signal at the Reference Input BNC. This signal should be a clean
sine wave at least 200 mVpk in amplitude. In this input mode, the
Reference Input is AC coupled (above 1 Hz) with an input impedance of
1 MΩ.
Sine reference mode can not be used at frequencies far below 1 Hz.
At very low frequencies, the TTL input modes must be used.
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Front Panel