SRS Labs SR530 manual Reference Mode, Reference Display, Phase Controls, Time Constant

going transitions of the reference input. This mode triggers on a negative pulse train or on the falling edges of a TTL type pulse train (remembering that the input is ac coupled). The pulse width must be greater than 1 ∝ S.

Reference Mode

The REFERENCE MODE indicator toggles between f and 2f whenever the MODE key is pressed. When the MODE is f, the lock-in will detect signals at the reference input frequency. When the MODE is 2f, the lock-in detects signals at twice the reference input frequency. In either case, the reference oscillator has a maximum frequency of 100 KHz, thus, when in the 2f mode, the reference input frequency may not exceed 50 KHz.

Reference Display

The REFERENCE DIGITAL DISPLAY shows either the reference oscillator frequency or phase shift. The displayed parameter toggles between the two whenever the SELECT key is pressed. The appropriate scale indicator below the display will be on. It is useful to check the frequency display to verify that the lock-in has correctly locked to your reference. The reference frequency is measured to 1 part in 256 resolution at all frequencies. The display reads .000 if there is no reference input and 199.9 kHz if the input frequency exceeds 105 kHz.

Phase Controls

The phase shift between the reference oscillator of the lock-in and the reference input signal is set using the four keys in the PHASE section. The two keys below the FINE label increment the phase setting in small amounts. A single key press will change the phase by 0.025 degrees in the desired direction. Holding the key down will continue to change the phase with larger and larger steps with the largest step being 10 degrees. The two 90° keys are used to change the phase by 90 degree increments. The upper key will add 90 degrees and the lower key will subtract 90 degrees. Holding both keys down at once sets the phase shift back to zero. The REFERENCE DIGITAL DISPLAY automatically displays the phase whenever any of the PHASE keys are pressed. The phase ranges from -180 degrees to +180 degrees and is the phase delay from the reference input signal.

Time Constant

There are two post demodulator low pass filters, labeled PRE and POST. The PRE filter precedes the POST filter in the output amplifier. Each filter provides 6 dB/oct attenuation.

The PRE filter time constant ranges from 1 mS to 100 S and is selected by the two keys below the PRE filter indicator LED's. Holding down either key will advance the time constant four times a second in the desired direction.

In many servo applications, no time constant is needed. The SR530 may be modified to reduce the output time constant to about 20 ∝ S. Contact the factory for details.

The POST filter time constant can be set to 1 S or

0.1S, or can be removed altogether, NONE, using the two keys below the ENBW indicators. When set to NONE, the total attenuation is that of the PRE filter, or 6 dB/oct. When the POST filter is 1 S or 0.1S, the total attenuation is 12 dB/oct for frequency components beyond the larger of the POST and PRE filter bandwidths (reciprocal time constant).

Noise Measurements

When the DISPLAY is set to X NOISE Y NOISE, none of the PRE and POST indicator LED's are on. Instead, one of the two ENBW indicators will be on, showing the Equivalent Noise Bandwidth of the rms noise calculation. The ENBW is set using the keys below the ENBW indicator LED's (same keys as used to set the POST filter). The PRE filter keys do nothing in this case. Pressing the upper key when the bandwidth is already 1 Hz will reset the rms noise average (output) to zero, restarting the calculation. Likewise with pressing the lower key when 10 Hz is already selected.

The noise is the rms deviation of the output within a 1 or 10 Hz equivalent noise bandwidth about the reference frequency. A dc output does not contribute to the noise, the noise is determined only by the ac 'wiggles' at the output. By measuring the noise at different frequencies, the frequency dependence of the noise density can be found. This usually has the form of vnoise ~ 1/f. The noise computation assumes that the noise has a Gaussian distribution (such as Johnson noise). Since the computation takes many time constants (reciprocal ENBW), the noise output

15