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3-10
Chapter 3, TECHNICAL DESCRIPTION
reserve is often expressed in decibels, for which
DR( in dB) = 20 log(DR( as a ratio))
Applying this formula to the model 7220 we may put in the maximum value of
INPUT LIMIT (3 V) and the smallest available value of FULL-SCALE
SENSITIVITY (20 nV) to reach a value of about 1E8 or 160 dB for the maximum
available dynamic reserve. Figures of this magnitude are available from any DSP
lock-in amplifier but are based only on arithmetical identities and do not give any
indication of how the instrument actually performs. In fact, all current DSP lock-in
amplifiers become too noisy and inaccurate for most purposes at reserves of greater
than about 100 dB.
For the benefit of users who prefer to have the AC Gain value expressed in this form,
the model 7220 displays the current value of Dynamic Reserve in decibels on the
input full-scale sensitivity control.
3.6 System Updates and Reference Frequency Changes
Both the signal channel and the reference channel contain calibration parameters
which are dependent on the reference frequency. These include corrections to the
anti-alias filter and to the analog circuits in the reference channel.
In external reference operation the processor uses the reference frequency meter to
monitor the reference frequency and updates these parameters when a change of
about 2 percent has been detected.
All the parameters are also updated when the SET key is pressed or the LOCK
command is executed. Therefore if the most accurate and reproducible settings are
required, the SET key should be pressed or the LOCK command executed after
every intentional change in reference frequency, when in the external reference
modes. Note that sufficient time must be allowed for the frequency meter to give a
fully accurate value.
With internal reference operation, regardless of the frequency mode, the frequency-
dependent parameters are updated on any change of reference frequency, without the
need to press the front panel SET key or to issue the LOCK command.
3.7 Reference Phase and Phase Shifter
If the reference input is a sinusoid applied to the REF IN socket, the reference phase
is defined as the phase of the X demodulation function with respect to the reference
input.
This means that when the reference phase is zero and the signal input to the
demodulator is a full-scale sinusoid in phase with the reference input sinusoid, the X
output of the demodulator is a full-scale positive value and the Y output is zero.