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 bandwidth), the noise output should be allowed to approach a steady value before a reading is taken. For the 1 Hz ENBW, this time is on the order of 15 to 30 seconds; for the 10 Hz ENBW, the output stabilizes much faster. The noise output will vary slightly since there will always be noise variations that are slow compared to the bandwidth. Any DC component in the output will not contribute to the noise. However, a large DC output will cause the noise computation to initially rise to a large value before approaching the final answer. As a result, the computation will take longer to settle.

To obtain a value for the noise density, the noise reading should be divided by the square root of the ENBW. Thus, when the ENBW is 1 Hz, the noise output is the noise density, and when the ENBW is 10 Hz, the noise density is the noise output divided by √ 10. For example, if the input noise is measured to be 7 nV with the ENBW set to 1 Hz, the noise density is 7 nV/√ Hz. Switching the ENBW to 10 Hz results in a faster measurement and a reading of 22 nV on the output. The noise density is 22 nV/√ 10 Hz or 7

nV/√ Hz. At frequencies » 10 Hz, the noise density should be independent of the ENBW.

Reference and Trigger Level

The REFERENCE INPUT BNC is located in this section. The input is ac coupled and the impedance is 1 MΩ . The dc voltage at this input should not exceed 100 V and the largest ac signal should be less than 10 V peak. The three indicators above the input BNC display the TRIGGER MODE. The single key above the input BNC is used to select the TRIGGER MODE.

If the center TRIGGER MODE LED is on, the mode is SYMMETRIC and the reference oscillator will lock to the positive zero crossings of the ac reference input. The ac signal must be symmetric (e.g. sine wave, square wave, etc.) and have a peak to peak amplitude greater than 100 mV. A signal with 1 Vrms amplitude is recommended. The phase accuracy of the reference channel is specified for a 1Vrms sinewave in the symmetric trigger mode.

If the upper TRIGGER MODE LED is on, the mode is POSITIVE. The trigger threshold is +1V and the reference oscillator will lock to the positive going transitions of the reference input. This mode triggers on the rising edges of a TTL type pulse train. The pulse width must be greater than 1 ∝ S.

If the lower TRIGGER MODE LED is on, the mode is NEGATIVE. The trigger threshold is -1V and the reference oscillator will lock to the negative 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.

Above the TRIGGER MODE indicators are the REFERENCE MODE LED's. The key below the REFERENCE MODE indicators toggles between f and 2f. 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.

The REFERENCE DIGITAL DISPLAY shows either the reference oscillator frequency or phaseshift. 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.

Phase Controls

The phase shift between the reference oscillator and the reference input 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

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