Stanford Research Systems SR844, User Manual

Page: 176 / 246

6-10 Circuit Descriptions
SR844 RF Lock-In Amplifier
to the ground for the rest of the circuitry. This avoids undesirable contamination of the
DC signals.
Daughter Card
Comparators U870 and U871 reconstitute both the TTL output signal (
TTL1
) and a
signal at twice the frequency (
TTL2
) from low-level inputs
TL1
±
and
TL2
±
from the
divider chain 84DVC. The low-level signals are
±
200 mV.
TTL2
is used to clock
output registers U872 and U873 which are wired all 16 lines in parallel in order to be
able to drive a 50
Ω
load. N872, N873 provide current-limiting and short-circuit
protection on the outputs, as does R872.
84MBD: Motherboard
The motherboard 84MBD occupies the front half of the instrument. Besides providing
power supplies and interconnects for the various other circuit boards, the motherboard
contains the following circuitry: (a) the Platform Interface, which is the interface
between the host ‘186 processor and the rest of the instrument, (b) the Synthesizer, used
in internal reference mode (c) the Range Select circuitry.
Document Number Sheet Schematic
MBDMAIN 1 84MBD Main
MBDXRF 2 84XRF Power, Connectors
MBDDVC 3 84DVC Power, Connectors
MBDCMX 4 84CMX Power, Connectors
MBDSIG 5 84SIG Power, Connectors
MBDIFN 6 84IFN Power, Connectors
MBDDSP 7 84DSP Power, Connectors
MPLAT 8 Platform Interface
MSYNT 9 Synthesizer
MRNGS 10 Range Select
Power Supplies
Regulated (–7.7V, +5V, +8V, +25V) and Unregulated (
±
17V nom.) power comes in
from the CPU/Power Supply board 844C on connector J4. Most of the power is re-
regulated and distributed to the various boards. Distribution of input power is shown on
page MBDMAIN, while the (secondary) regulators are grouped along with the
interconnect to the board they service on separate pages.
Power supplies are decoupled from each other with capacitors and either beads,
inductors or resistors. Separate circuit sections generally have independently regulated
power. FR47 is a small surface-mount bead with 47
Ω
impedance at 100 MHz, FR95 is a
bigger surface-mount bead with 95
Ω
impedance at 100 MHz.