Reference and Phase Menu 
Reference Source
Ref. Source
External
Ref Slope
Sine
Ref. Source
Internal
Ref Frequency
50.000 Hz
The Reference Source key selects the source of the
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 MΩ. 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.
