Specifications

Instrument Overview
11
INSTRUMENT OVERVIEW
50Ω
IN 1
SELECT
A
DISC
50Ω
IN 2
SELECT
B
DISC
T
DISC
T
DISC
10.0
MHz
DUAL GATE
GENERATOR
S
E
L
E
C
T
DWELL
TIME
300 MHz
AMPLIFIERS
DISCRIMINATORS
FAST LOGIC
TRIG
DISC LEVELS GATE CONTROL COUNTER INPUTS
MICROPROCESSOR CONTROL
GPIB
A
DISC
A
GATE
B
DISC
B
GATE
T
DISC
DWELL
OUTPUT
D/A
OUTPUTS
RS-232
A - COUNTER
CLR
B - COUNTER
CLR
PRESET OR T
COUNTER
LOAD
BLOCK DIAGRAM
SR400 OVERVIEW
The SR400 architecture is diagrammed above.
There are three fast counters identified as counters
A, B, and T. All three counters operate at rates up
to 200 MHz. The input to each counter is selected
from a number of sources, including the two
analog signal inputs, the crystal timebase, and the
external trigger input. Counter T is presettable and
determines the measurement period. For pulsed
experiments, counters A and B may also be
synchronized to external events via the two
independent gate generators. The gate generator
provides gates from 5 ns to 1 s in duration with a
delay from external trigger ranging from 25 ns
to 1 s.
SIGNAL INPUTS AND DISCRIMINATORS
There are two independent analog signal inputs
labelled INPUT 1 and INPUT 2. They are
internally terminated into 50 Ω. The inputs can
accept signals of either polarity up to ±300 mV
and are protected to ±5V dc. Each input is
followed by a dc to 300 MHz amplifier. This
allows detection of pulses as small as 10 mV. If
greater sensitivity is required, the SR445A remote
amplifier is available. The SR445A can provide
gains from 5 to 625 at bandwidths up to 350 MHz.
There are three discriminators, one for each
counter. Each discriminator has selectable slope
and a threshold programmable from -300 mV to
+300 mV in 0.2 mV steps (referenced to the
inputs). Pulse-pair resolution is 5 ns and pulses of
either polarity may be detected. Each threshold