Stanford Research Systems SR844, User Manual

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3-6 Overview
SR844 RF Lock-In Amplifier
REFERENCE
OUT The reference out signal is phase coherent with the reference signal
internal to the SR844. It is a square wave, nominally 1 Vpp into 50
Ω
.
In external reference mode, this signal is phase-locked to the external
reference input, while in internal mode it is derived from a frequency
synthesizer and locked to an internal 20 MHz crystal oscillator.
Important!
The shields of all the connectors are connected to the instrument ground and thereby to
chassis ground.
Do not under any circumstance attempt to apply voltage to the
connector shields.
Rear Panel Connectors
The rear panel has six BNC connectors, the power entry module and connectors for the
GPIB and RS-232 remote interfaces.
Power Entry
Module The power entry module is used to fuse the AC line voltage input, select
the line voltage, and block high-frequency noise from entering or exiting
the instrument. Refer to the beginning of the manual under
Safety and
Preparation for Use for instructions on selecting the correct line voltage
and fuse.
RS-232 The RS-232 connector is configured as a DCE (transmit on pin 3, receive
on pin 2). The baud rate and parity are set with the
Setup
key. To connect
the SR844 to a standard PC/compatible serial port, which is a DTE, use a
straight-through serial cable.
IEEE-488
(GPIB) The 24-pin IEEE-488 connector allows a computer to control the SR844
via the IEEE-488 (GPIB) instrument bus. The address of the instrument is
set with the
Setup
key. The default address is 8.
From left to right, the BNC connectors are
TRIG IN This TTL input may be used to trigger internal data storage and/or to
start data acquisition. Data storage is available only via the remote
interfaces. If Trigger Start is selected, then a rising edge will start data
storage. If the sample rate is also Triggered, then samples are recorded at
the first and every subsequent trigger. The maximum sample rate is
512 Hz with a 2 ms trigger to sample latency.
TTL OUT This output is a TTL output (0–5 V nominal) at the reference frequency.
It is only available for reference frequencies below 1.56 MHz. This
output can drive a 50
Ω
load.
AUX OUT 1 This is an auxiliary DC output voltage. The range is
±
10.5 V and the
resolution is 1 mV. The output impedance is
<
1
Ω
and the current is
limited to 10 mA.