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202
Circuit Description
amplification at U105 and discrimination at U112. The logic-level reference is then
converted into a fast differential transition using current-mode logic from U109, and then
level-shifted and slew-rate limited with U102 before being driven differentially through
R105, R108 to the shielded twisted pair interconnect J101 leading to the motherboard.
Chip selects, clock gating, and relay control is provided by U201 through U206. dc
power is re-regulated from ±16V to ±5V with linear regulators U207 and U208.
Sine Output
The SR865A sine output module provides filtering, attenuation, and dc level generation
for the differential sine output. A passive 7th order differential Cauer low pass filter is
implemented with components L102–L107, C101–C107, C112–C114. The signal is
amplified by U101 (and optionally U102) before being launched through R109–R116 as
a 100Ω impedance differential signal. Programmable attenuators are switched in or out
using steering relays K102–K109. The dc level control is generated by DAC U105 and
amplifier U106, and then driven with 100Ω impedance either directly or through an
attenuator divider selected by K110.
The ac signal is then wire-added to the 100Ω impedance dc level to create 50Ω
impedance signals at the output BNCs. Chip selects, clock gating, and relay control is
implemented with U201–U711. All power rails for the circuit board are re-regulated from
the main power supply ±16V to ±15V and +5V with U212–U214 linear regulators.
Auxiliary I/O
The rear-panel auxiliary I/O circuit supports the 4 AUX OUT and 4 AUX IN analog
signals. Outputs are received differentially (referenced to system ground) from the
motherboard on header J1, through difference amplifier circuits U1A–U1D. Analog
inputs are buffered with amplifiers U1A–U1D, and presenting 1 MΩ input impedances.
The TTL compatible TRIG IN signal is also on this board, but separately interconnected
to the motherboard via J11.
Linear Power Supply
The SR865A uses a low noise linear power supply. A multi-tap "universal primary"
toroidal transformer can be configured at the power entry module for either 100VAC,
120VAC, 220VAC, or 240VAC. Multiple secondary windings, most of which are center-
tapped, provide dedicated power to all sections of the instrument. Multiple independent
grounds are left isolated from each other within the power supply to avoid creating
ground loops; each circuit, however, is ultimately ground referenced to chassis ground.
Bridge rectifiers on the lower power supply board convert 50 Hz / 60 Hz to dc; some of
these components (particularly D6) run quite hot under normal conditions. Unregulated
dc power is routed up from the rectifier board to the upper (regulator) board by the 20-pin
header J2 located near the rear panel of the SR865A.
Linear regulators on the upper power supply board establish the voltage rails for
distribution across the instrument. Most regulators are clamped onto the channel heat sink
on the center of the board. Please note that the DB-9 connections for the Reference In
module (J6) and the Sine Out module (J8) are not interchangeable — these subassemblies
have different power requirements and must be cabled correctly. 24 VDC is tapped from
SR865A DSP Lock-in Amplifier
Appendix H
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