currently displayed parameter. If the output is greater than 1.024 times full scale, the REL function will not be able to zero the output and the ON LED will blink. The offset value will then be set to its max value. If NOISE is being displayed when the REL function is turned on, the noise ouptut will require a sew seconds to settle again.

If the manual OFFSET in ON when the REL function is turned on, the manual OFFSET will be turned OFF before the auto zero is done.

The REL function and the manual OFFSET are both ways to enter the offset value. When the REL function is turned off using the REL key the offset

is turned off but the value is not lost. If the manual OFFSET is now turned ON, the offset will be that set by the REL function.

Offset

The OFFSET section controls the manual OFFSET. The offset is turned ON and OFF using the upper key in the OFFSET section. When the offset is ON, the lower two keys are used to set the amount of offset. A single key press will advance the offset by 0.025% of full scale. If the key is held down, the offset advances in larger and larger increments, the largest increment being

10% of full scale. When the offset is turned OFF the applied offset returns to zero but the offset value is not lost. The next press of the upper offset key (returns to ON) sets the offset to the previously entered value.

If an attempt is made to advance the offset value beyond full scale, the OFFSET ON LED will blink. An offset up to 1.024 times the full scale sensitivity may be entered. When the expand is on, this is 10X the full scale output.

If the REL function is ON when the manual OFFSET is turned ON, the REL function is turned OFF but the offset value remains the same. The OFFSET keys may now be used to adjust this offset value.

Note that the offsets (either manual offset or those generated by the REL function) represent a fraction of the full scale reading, and so their absolute value will change when the sensitivity scale is changed. A signal which has been nulled by an offset will not be nulled when the sensitivity scale is changed. The analog meter and the output BNC indicate the same value given by the equation:

Vout = 10Ae(AvVicosØ+Vos)

where...

Ae

= 1 or 10 per the Expand setting

Av

=

1/Sensitivity

Vi

=

magnitude of the signal

Ø= phase between signal & reference Vos = offset (fraction of FS < 1.024)

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 twice a second in the desired direction.

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

When the DISPLAY is set to 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

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