SRS Labs SR625 User manual

MODEL SR625
RUBIDIUM TIMEBASE
/PRESCALER
1290-D Reamwood Avenue
Sunnyvale, CA 94089 U.S.A.
Phone: [phone], Fax: [phone]
email: [e-mail] • www.thinkSRS.com
Copyright © 1994 by SRS
All Rights Reserved
Revision 2.1 (07/2001)

1
Safety and Preparation for Use
This instrument may be damaged if
operated with the LINE VOLTAGE
SELECTOR set for the wrong ac line
voltage or if the wrong fuse is installed.
LINE VOLTAGE SELECTION
The SR625 operates from a 100V, 120V,
220V, or 240V nominal ac power source
having a line frequency of 50 or 60 Hz.
Before connecting the power cord to a power
source, verify that the LINE VOLTAGE
SELECTOR card, located in the rear panel
fuse holder, is set so that the correct ac input
voltage value is visible.
Conversion to other AC input voltages
requires a change in the fuse holder voltage
card position and fuse value. Disconnect the
power cord, open the fuse holder cover door
and rotate the fuse-pull lever to remove the
fuse. Remove the small printed circuit
board and select the operating voltage by
orienting the board so that the desired
voltage is visible when it is pushed firmly
back into its slot. Rotate the fuse-pull lever
back into its normal position and insert the
correct fuse into the fuse holder.
LINE FUSE
Verify that the correct line fuse is installed
before connecting the line cord. For
100V/120V, use a 1 Amp slow blow fuse
and for 220V/240V, use a 1/2 Amp slow
blow fuse.
LINE CORD
The SR625 has a detachable, three-wire
power cord for connection to the power
source and to a protective ground. The
exposed metal parts of the instrument are
connected to the outlet ground to protect
against electrical shock. Always use an
outlet which has a properly connected
protective ground.
WARNING: Dangerous voltages, capable of causing death, are present in this instrument.
Use extreme caution whenever the instrument covers are removed.

2
SPECIFICATIONS:
2.0 GHz Prescaler
Input
Impedance 50 Ω
Maximum Input level +23dBm
Frequency Range 50 MHz - 2.0 GHz
Sensitivity See Figure below
Output
Expected Load 50 Ω
Frequency 1/10 of the input frequency
Waveform square wave, 700 mV pk-pk, +500 mV DC
-40.00
-30.00
-20.00
-10.00
0.000
10.00
20.00
Input amplitude (dBm)
1000 1200 1400 1600 1800
Input frequency (MHz)
Prescaler Sensitivity (Typical)
Maximum amplitude
Minimum amplitude
2000
Rubidium Timebase (see Operation Manual of Rb Oscillator)
Frequency 10 MHz
Accuracy at Shipment ± 5 E-11
One Day Stability 4E -11/day
Long Term Drift ≤5E -11/month, ≤5E -10/yr
Short Term Stability 1 Second (Allan Variance): 1 E -10
10 Second (Allan Variance): 3.16 E -11
100 Second (Allan Variance): 1 E -11
Warm-up time 10 minutes to meet short term stability

3
INTRODUCTION
The SR625 Rubidium Timebase and
Prescaler is designed to extend the frequency
measurement range of the SR620 Time
Interval Counter to 2.0 GHz and to provide
an extremely stable timebase. The prescaler
is a divide by 10 circuit with variable
sensitivity adjustment for
measuring signals up to 2.0 GHz. The
timebase is an atomic-resonance (Rubidium)
standard that provides a stable 10 MHz
signal once locked. The 10 MHz output
from the Rb timebase can be used to
increase the resolution and accuracy of the
of the SR620 time interval counter.
Figure 1 – SR625 Front Panel
FRONT PANEL FEATURES
POWER indicator is lit when the SR625 is
powered from line voltage.
LOCK indicator is lit when the 10 MHz
oscillator is locked to the Rubidium atomic
resonance (Note: this light may turn on
momentarily after power is turned on.)
RF INPUT BNC accepts signals from
50 MHz to 2.0 GHz (see Figure 3 for
sensitivity curve.) The input impedance is
50 Ω. The maximum allowable input level is
+23dBm (3.15 Vrms into 50 Ω). The shield
of the prescaler input is connected to
earth ground.
PRESCALE OUTPUT BNC provides an
output signal that is one-tenth (÷10) the
frequency of the input signal. When
terminated into a 50 Ωload the output is a
700 mV pk-pk square wave with a 500 mV
DC offset. This output is connected to the
input of the SR620 to measure frequencies
from 50 MHz to 2.0 GHz. The SR620 input
(channel A or B) should be DC coupled, 50
Ωimpedance, and the trigger level set to
autolevel. The shield of this output is
connected to earth ground.
PRESCALE ADJUST knob adjusts the
amplitude necessary to trigger the prescaler.
The threshold is closest to the zero crossing
voltage (i.e. the prescaler is most sensitive)
when the knob is pointing straight up.
Turning the knob changes the threshold and
makes the prescaler less susceptible to false
triggers due to noise.

4
REAR PANEL FEATURES
Figure 2 – SR625 Rear Panel
POWER ENTRY MODULE contains the
fuse and line voltage selector of the
prescaler/timebase. Use a 1 amp slow blow
fuse for 100/120 volt operation, and a 1/2
amp fuse for 220/240 volt operation. To set
the line voltage selector for the correct line
voltage, first remove the fuse. Then, remove
the line voltage selector card and rotate the
card so that the correct line voltage is
displayed when the card is reinserted.
Replace the fuse.
10 MHz OUTPUT should be connected to
the 10 MHz timebase input on the back
panel of the SR620 with a standard BNC
cable. The Rb oscillator can be also be
independently used as a high stability source
through this output. The timebase will
operate independently of the SR620 as long
as power to the module is applied. The
output is a 1 V pk-pk 10 MHz sine wave (50
Ωimpedance). The shield of this output is
connected to earth ground.
CONFIGURING SR620
The SR620's default timebase is its own
internal source. To use the Rb timebase the
configuration of the SR620 must be changed.
To change the settings of the SR620 press
the SEL button in the CONFIG area of the
front panel (right hand side) until the "CAL"
menu flashes. Now press the SET button in
the CONFIG area until "CLOCK SOURCE"
is displayed. Press either the up or down
arrow key in the SCOPE and CHART area
to toggle the clock source from "INT"ernal
to "REAR" panel. The 10 MHz Rb output
from the prescaler/timebase must be
connected to the rear connector of the
SR620. Pressing the SET key again should
show that the clock frequency is 10 MHz. If
it is not use the arrow keys in the SCOPE
and CHART section to toggle the value to
10 MHz. To exit the configuration menus
press any other key outside the configuration
area of the keyboard. The unit is ready for
operation once the locked light on the front
of the SR625 module is lit.

5
CIRCUIT DESCRIPTION
Prescaler-
The combination of R18,R19 and R20 attenuate the input signal to provide input protection for
the circuit. U9 amplifies the input signal. U5 frequency divides the signal by two. P1 adjusts the
threshold at which the differential clock of U5 will trigger. Components U4, U7 and U8 are
configured to act as a synchronous divide by five counter. Transistor Q1 drives the output signal.
R10 provides reverse termination of 50Ω.
PARTS LIST
REF. SRS part# VALUE DESCRIPTION
C 1 5-00170-509 6800U Capacitor, Electrolytic, 50V, 20%, Rad
C 2 5-00100-517 2.2U Capacitor, Tantalum, 35V, 20%, Rad
C 3 5-00100-517 2.2U Capacitor, Tantalum, 35V, 20%, Rad
C 4 5-00100-517 2.2U Capacitor, Tantalum, 35V, 20%, Rad
C 5 5-00030-520 2200U Capacitor, Electrolytic, 16V, 20%, Rad
C 6 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 7 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 8 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 9 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 10 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 11 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 12 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 13 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 14 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 15 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 16 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 17 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 18 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 19 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 20 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 21 5-00298-568 .01U Cap, Ceramic 50V SMT (1206) +/-10% X7R
C 22 5-00219-529 .01U Cap, Monolythic Ceramic, 50V, 20%, Z5U
D 1 3-00001-301 1N4001 Diode
D 2 3-00001-301 1N4001 Diode
D 3 3-00001-301 1N4001 Diode
D 4 3-00001-301 1N4001 Diode
D 5 3-00001-301 1N4001 Diode
D 6 3-00001-301 1N4001 Diode
D 7 3-00001-301 1N4001 Diode
D 8 3-00001-301 1N4001 Diode
D 9 3-00424-307 GREEN LED, T-3/4
D 10 3-00424-307 GREEN LED, T-3/4
D 11 3-00203-301 1N5711 Diode
P 1 4-00445-447 SR620-48 Pot, Single Control
PC1 7-00577-701 SR625 SIDECAR Printed Circuit Board

6
REF. SRS part# VALUE DESCRIPTION
Q 1 3-00570-360 MMBR536L Integrated Circuit (Surface Mount Pkg)
R 1 4-01464-461 240 Thick Film, 5%, 200 ppm, Chip Resistor
R 2 4-01479-461 1.0K Thick Film, 5%, 200 ppm, Chip Resistor
R 3 4-01459-461 150 Thick Film, 5%, 200 ppm, Chip Resistor
R 4 4-01475-461 680 Thick Film, 5%, 200 ppm, Chip Resistor
R 5 4-01464-461 240 Thick Film, 5%, 200 ppm, Chip Resistor
R 6 4-01457-461 120 Thick Film, 5%, 200 ppm, Chip Resistor
R 7 4-01457-461 120 Thick Film, 5%, 200 ppm, Chip Resistor
R 8 4-01457-461 120 Thick Film, 5%, 200 ppm, Chip Resistor
R 9 4-00833-406 24 Resistor, Thick Film, (SMT1206) 5% 1/8W
R 10 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 11 4-00031-401 100 Resistor, Carbon Film, 1/4W, 5%
R 12 4-00169-407 249 Resistor, Metal Film, 1/8W, 1%, 50PPM
R 13 4-00169-407 249 Resistor, Metal Film, 1/8W, 1%, 50PPM
R 14 4-00169-407 249 Resistor, Metal Film, 1/8W, 1%, 50PPM
R 15 4-00187-407 4.53K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 16 4-00241-407 787 Resistor, Metal Film, 1/8W, 1%, 50PPM
R 17 4-00187-407 4.53K Resistor, Metal Film, 1/8W, 1%, 50PPM
R 18 4-01459-461 150 Thick Film, 5%, 200 ppm, Chip Resistor
R 19 4-01459-461 150 Thick Film, 5%, 200 ppm, Chip Resistor
R 20 4-01444-461 36 Thick Film, 5%, 200 ppm, Chip Resistor
R 21 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 22 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 23 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 24 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 25 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 26 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 27 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 28 4-01448-461 51 Thick Film, 5%, 200 ppm, Chip Resistor
R 29 4-00021-401 1.0K Resistor, Carbon Film, 1/4W, 5%
R 30 4-00062-401 270 Resistor, Carbon Film, 1/4W, 5%
R 31 4-00062-401 270 Resistor, Carbon Film, 1/4W, 5%
R 32 4-00021-401 1.0K Resistor, Carbon Film, 1/4W, 5%
R 33 4-01464-461 240 Thick Film, 5%, 200 ppm, Chip Resistor
R 34 4-01455-461 100 Thick Film, 5%, 200 ppm, Chip Resistor
R 35 4-00851-407 340 Resistor, Metal Film, 1/8W, 1%, 50PPM
R 36 4-00021-401 1.0K Resistor, Carbon Film, 1/4W, 5%
RX0 4-00056-401 22 Resistor, Carbon Film, 1/4W, 5%
RX2 5-00313-552 1P Capacitor, Chip (SMT1206), 50V, 5%, NPO
U 2 3-00384-329 LM350T Voltage Reg., TO-220 (TAB) Package
U 3 3-00149-329 LM317T Voltage Reg., TO-220 (TAB) Package
U 4 3-00568-360 MC10EL35D Integrated Circuit (Surface Mount Pkg)
U 5 3-00573-360 MC10EL32D Integrated Circuit (Surface Mount Pkg)
U 6 3-00149-329 LM317T Voltage Reg., TO-220 (TAB) Package
U 7 3-00568-360 MC10EL35D Integrated Circuit (Surface Mount Pkg)
U 8 3-00568-360 MC10EL35D Integrated Circuit (Surface Mount Pkg)
U 9 3-00569-360 MAR6SM Integrated Circuit (Surface Mount Pkg)
X 0 1-00321-165 10 PIN SOLDER Connector, D-Sub, Female

7
REF. SRS part# VALUE DESCRIPTION
X 0 1-00322-100 COAX INSERT Connector, Misc.
Z 0 0-00000-000 UNDECIDED PART Hardware, Misc.
Z 0 0-00014-002 6J4 Power_Entry Hardware
Z 0 0-00025-005 3/8" Lugs
Z 0 0-00042-010 4-40 HEX Nut, Hex
Z 0 0-00089-033 4" Tie
Z 0 0-00100-040 1/4X1/16 Washer, Flat
Z 0 0-00109-050 1-1/2" #18 Wire #18 UL1007 Stripped 3/8x3/8 No Tin
Z 0 0-00128-053 4" #24 Wire #24 UL1007 Strip 1/4x1/4 Tin
Z 0 0-00129-053 5" #24 Wire #24 UL1007 Strip 1/4x1/4 Tin
Z 0 0-00150-026 4-40X1/4PF Screw, Black, All Types
Z 0 0-00153-057 GROMMET2 Grommet
Z 0 0-00208-020 4-40X3/8PF Screw, Flathead Phillips
Z 0 0-00209-021 4-40X3/8PP Screw, Panhead Phillips
Z 0 0-00211-020 4-40X5/8PF Screw, Flathead Phillips
Z 0 0-00212-021 6-32X2PP Screw, Panhead Phillips
Z 0 0-00228-052 17" #22 GREEN Wire #22 UL1007
Z 0 0-00231-043 #4 SHOULDER Washer, nylon
Z 0 0-00241-021 4-40X3/16PP Screw, Panhead Phillips
Z 0 0-00243-003 TO-220 Insulators
Z 0 0-00251-004 SR620-49 Knobs
Z 0 0-00256-043 #6 SHOULDER Washer, nylon
Z 0 0-00263-052 3" #22 Wire #22 UL1007
Z 0 0-00264-052 9-1/2" #22 RD Wire #22 UL1007
Z 0 0-00266-052 8-1/2" #22 BLK Wire #22 UL1007
Z 0 0-00306-026 4-40X3/16PP Screw, Black, All Types
Z 0 0-00317-000 40MM 24V Hardware, Misc.
Z 0 0-00418-000 CLIP, CABLE Hardware, Misc.
Z 0 0-00419-056 RG178B/U Cable, Coax & Misc.
Z 0 0-00421-032 SOLDR SLV RG178 Termination
Z 0 0-00440-052 2-1/2" #22 RED Wire #22 UL1007
Z 0 0-00493-021 10-32X1/2"PP Screw, Panhead Phillips
Z 0 0-00523-048 5-5/8" #18 Wire, #18 UL1015 Strip 3/8 x 3/8 No Tin
Z 0 0-00854-031 PEM 4-40 Standoff
Z 0 1-00003-120 BNC Connector, BNC
Z 0 1-00218-120 227699-2 Connector, BNC
Z 0 1-00220-170 3" BNC Cable Assembly, Multiconductor
Z 0 1-00227-172 Y JUNCTION Line Cord
Z 0 1-00228-172 USA 3' Line Cord
Z 0 4-00541-435 130V/1200A Varistor, Zinc Oxide Nonlinear Resistor
Z 0 6-00004-611 1A 3AG Fuse
Z 0 6-00167-610 SR625 Transformer
Z 0 7-00571-709 SR625 Lexan Overlay
Z 0 7-00572-720 SR625 Fabricated Part
Z 0 7-00573-720 SR625 Fabricated Part
Z 0 7-00574-720 SR625 Fabricated Part
Z 0 7-00576-720 SR625 Fabricated Part
Z 0 7-00581-721 SR625-6 Machined Part

8

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