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
Ω
load 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
Ω
impedance, 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.