Chapter 3, TECHNICAL DESCRIPTION
3-11
When the reference frequency is below 10 Hz the synchronous filter option is
available. When selected, the actual time constant of the filters is not generally the
selected value TC, but the closest smaller value equal to an integer number of
reference cycles. If TC is greater than 1 reference cycle, then the time constant is
between TC/2 and TC. Where random noise is relatively small, synchronous filter
operation gives a major advantage in low-frequency measurements by enabling the
system to give a constant output even when the output time constant is equal to only
1 reference cycle.
3.3.16 Fast Curve Buffer
The fast curve buffer is a feature common to the models 7124, 7270, and 7230 lock-
in amplifiers. It allows up to 100,000 sets of eight signals to be recorded at rates of
up to 1 MSa/s (1 µs per point), and supports a variety of trigger modes. The buffer
can also be used in a circular fashion, with new data overwriting the oldest data, to
allow capture of events up to the time of an applied trigger.
Although eight signals can be recorded, these include the X2 and Y2 outputs that are
only generated in the dual reference and dual harmonic modes, and the input to the
second demodulator. Hence unless one of these modes is selected, only five signals
are stored.
The signals that are stored are therefore as follows:
Single Reference/Virtual Reference Mode
SIG ADC This is the raw digitized data out of the main ADC
X(1) The X channel output from the first demodulator
Y(1) The Y channel output from the first demodulator
ADC 1 The digitized representation of the input to the auxiliary ADC1
ADC 2 The digitized representation of the input to the auxiliary ADC2
Dual Reference/Dual Harmonic Mode
DEMOD 2 The digital signal input to the second demodulator
X2 The X channel output from the second demodulator
Y2 The Y channel output from the second demodulator
Data that has been stored to the buffer can be downloaded to a computer.
3.3.17 Main Output Processor - General
The same eight signals that pass to the fast curve buffer also pass to the main output
processor. This carries out further output filtering if required, generates derived
outputs, such as signal magnitude and phase, drives four DACs that in turn generate
analog representations of the instrument outputs, and implements the standard curve
buffer. These features are described in more detail in the following sections.
3.3.18 Main Output Processor - Output Offset and Expand
Following the output filter, an output offset facility enables ±300% full-scale offset
to be applied to any or all of the X(1), Y(1), X2, Y2 output signals. The output
expand facility allows a ×10 expansion, performed by simple internal digital
multiplication, to be applied to the same output signals.
3.3.19 Main Output Processor - Vector Magnitude and Phase
The processor also implements the magnitude and signal phase calculation, which is
useful in many situations. If the input signal V
s
(t) is a reference frequency sinusoid of
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