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Chapter 6, COMPUTER OPERATION
FRQ[.]
LOCK
VRLOCK [n]
6.7.03 Signal Channel Output Filters
NOISEMODE [n] Noise Measurement Mode control
NNBUF [n]
TC [n]
TC.
6-14
Reference frequency meter
If the lock-in amplifier is in the external reference source modes, the FRQ command
causes the lock-in amplifier to respond with 0 if the reference channel is unlocked, or
with the reference input frequency if it is locked.
If the lock-in amplifier is in the internal reference source mode, it responds with the
frequency of the internal oscillator.
In fixed point mode the frequency is in mHz, and in floating point mode the
frequency is in Hz.
System lock control
Updates all frequency-dependent gain and phase correction parameters.
Virtual reference mode lock
The Seek option of the frequency sweep mode must be used before issuing this
command, for which the value of n has the following significance:
n
Mode
0
Disables virtual reference mode
1
Enters virtual reference mode by enabling tracking of the signal frequency
The value of n sets the noise measurement mode according to the following table:
n
Function
0
Noise measurement mode off
1
Noise measurement mode on.
When the noise measurement mode is turned on, the output filter time constant is
automatically adjusted until it lies in the range 500 µs to 10 ms inclusive, the
synchronous time constant control is turned off, the fast analog output mode is
turned on (FASTMODE 1), and the output filter slope is set to 12 dB/octave if it had
previously been 18 or 24 dB/octave.
Noise Buffer Length control
The value of n sets the noise buffer length according to the following table:
n
Function
0
Noise buffer off
1
1 second noise buffer
2
2 second noise buffer
3
3 second noise buffer
4
4 second noise buffer
Filter time constant control
The value of n sets the output filter time constant in accordance with the following
table:
NOISEMODE
= 0
n
time constant
0
10 µs
1
20 µs
2
50 µs
NOISEMODE
= 1
time constant
n
N/A
16
N/A
17
N/A
18
NOISEMODE
NOISEMODE
= 0
= 1
time constant
time constant
2 s
N/A
5 s
N/A
10 s
N/A
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