2461Type C User Manual
Theory Of Operation 3-2
Racal Instruments
1998
Trigger levels for the two channels are derived independently in
the DAC (Digital to Analog Converter) System and applied
independently to each input channel. These trigger levels are
also made available at the front panel (buffered by 10k
Ω
nom.
resistance). The DAC System incorporates separate trimming
DACs which are used to calibrate the trigger DACs. The DAC
System also incorporates DACs that allow channel 1 and
channel 2 offset voltage correction, and DACs that allow control
of channel 1 and channel 2 sensitivity. Control of all DACs is
via the microprocessor system. DAC calibration values are
stored in non-volatile memory.
Control signals for the relays are supplied by the
microprocessor system.
Input Channel 3
System
The Input Channel 3 System processes the signal applied to
Input 3 (if Option 41 is installed) and feeds it to the
measurement system. A block diagram of Channel 3 is shown
below.
To
Measurement
System
Ch. 3 Signal
Detected
Signal
Gate
To
Microprocessor
System
To
Measurement
System
Low
Signal
Latch
÷
64
dB
Signal
Limiter
Set Low
Signal
Threshold
Low Signal
Detector
and
Comparator
Continuous
Signal
Latch
Ch. 1 Inhibit
Ch. 3 Selected
Reset
Gate
Fuse
Input 3
From
Measurement
System
From
Microproscessor
System
{
Ch. 3 Inhibit
Ch. 3 Signal Detected
Throughout Gate Period
Channel 3’s input is protected by a fuse mounted in the input
connector and by a signal-limiting circuit. Next is an automatic
level control circuit which reduces the range of the signal level
applied to the amplifier.
After amplification, the signal is pre-scaled by 64 before being
passed via a buffer and a signal gate to the measurement
system.
The amplitude of the signal at the amplifier output is monitored
by a detector and a comparator. The comparator output
controls the low signal latch. If the detector output is below the
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