84Mbd: Motherboard - Stanford Research Systems SR844 User Manual

6-10 Circuit Descriptions
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 50Ω 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
MBDMAIN
MBDXRF
MBDDVC
MBDCMX
MBDSIG
MBDIFN
MBDDSP
MPLAT
MSYNT
MRNGS
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 47Ω impedance at 100 MHz, FR95 is a
bigger surface-mount bead with 95Ω impedance at 100 MHz.
SR844 RF Lock-In Amplifier
Sheet Schematic
1 84MBD Main
2 84XRF Power, Connectors
3 84DVC Power, Connectors
4 84CMX Power, Connectors
5 84SIG Power, Connectors
6 84IFN Power, Connectors
7 84DSP Power, Connectors
8 Platform Interface
9 Synthesizer
10 Range Select