SRS Labs SR850 - Manual (Page 114)

5-4
Reference and Phase Menu
Reference Source The Reference Source key selects the source of the lock-in reference.
Turn the knob to select the desired source, either Internal , Internal
Sweep, or External. Keypad entry is not allowed for this entry field.
External When the reference source is External, the SR850 will
phase lock to the external reference provided at the Refer-
ence Input BNC. The SR850 will lock to frequencies
between 0.001 Hz and 102.0 kHz. The upper limit is
reduced to 102 kHz/N where N is the harmonic number if
N is greater than 1.
Ref Slope This key selects the reference input discriminator mode.
This selection is only available when the reference source
is external.
When either TTL Rising Edge or TTL Falling Edge is
selected, the SR850 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 discrimi-
nator 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 SR850 to the rising zero cross-
ings 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 M
Ω
.
This mode can not be used at frequencies far below 1 Hz.
At very low frequencies, the TTL input modes must be
used.
Internal When the reference source is Internal, the SR850's syn-
thesized 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.
Reference Frequency Pressing this key selects the internal oscillator frequency
as the active entry field. This field is only available when
the reference source is internal. The frequency may adjust-
ed with the knob or directly entered from the keypad. The
frequency has 5 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.
Internal
Ref Frequency
50.000 Hz
Ref. Source
Sine
External
Ref Slope
Ref. Source