Stanford Research Systems SR830 Manual page 73

Front Panel
[Source]
[Trig]
UNLOCK
quency 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 SR830 will always track
the external reference.
Pressing this key displays the harmonic number in the Reference dis-
play. 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 harmon-
ic frequency above 102 kHz will display the message 'hAr ovEr' indicat-
ing harmonic number over range.
This key selects the reference mode. The normal mode is External refer-
ence (no indicator). The Internal mode is indicated by the INTERNAL led.
When the reference source is External, the SR830 will phase lock to the
external reference provided at the Reference Input BNC. The SR830 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 SR830'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 SR830 locks to
the selected edge of a TTL square wave or pulse train. For reliable oper-
ation, 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 when-
ever possible since it is less noise prone than the sine wave discrimina-
tor.
For very low frequencies (<1 Hz), a TTL reference MUST be used.
SINE input mode locks the SR830 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.
The UNLOCK indicator turns on if the SR830 can not lock to the external
reference.
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