Table of Contents
Advertisement
Design Integration Considerations
Electrical Interface
Electrical Interface
X72 RF Outputs
RF Output Impedance versus Frequency
The active rf output impedance for the X72 is a nominal 50 Ω at 10 MHz, but is a
widely varying impedance at other frequencies. you must take this into
consideration if the X72 rf output is driving a non-buffered filter. Note that the X72 is
specified as delivering 0.5 Vrms (typical) into a 50 Ω resistance load. Loads that
present complex impedance or resonances affect signal amplitude. The X72 can
drive a properly terminated transmission line of 50 Ω characteristic impedance.
Note: A 390 pf capacitor from the 10 MHz sine output to the sine
return is required within four inches of the output connector. It can be
located across the 50 Ω terminator (see
If the sine output port is disabled its characteristic impedance remains the same.
AC-Coupled RF Load
The X72 is designed to tolerate an ac-coupled rf load without waveform distortion
provided the coupling capacitor is low impedance at 10 MHz (for example, a
0.01 µF capacitor has an X
output impedance). The rf output stage of the X72 uses an ac-coupled design.
Symmetricom recommends that you use a ceramic coupling capacitor with an X7R
or NPO dielectric.
Transformer-coupled RF Load
The transformer-coupled rf load is used to break up ground loops. It can also be
used to provide some bandpass filtering. However, it also attenuates the rf signal,
making it difficult to provide a tight tolerance on the rf output level. It is also difficult
to obtain an inexpensive, off-the-shelf wide operating temperature range SMT rf
transformer.
X72 1 PPS Input and Output
The 1 PPS output signal of the X72 unit is positive-edge triggered and gated with
the rising edge of the clock. Its duration for a 10 MHz unit is 400 ns ±10%. Rise/fall
time is 4 ns.
The 1 PPS is exactly divided from the XO frequency.
Lock Signal
The LOCK signal indicates that the internal Voltage Controlled Crystal Oscillator
(VCXO) is locked to the atomic transition. If the Lock signal is LOW once warm-up is
completed, the output frequency is locked to the Rb atomic clock.
36
X72 Desiger's Reference and User's Guide
Figure
of 1.6 Ω, which is small compared to the 50 Ω nominal
c
2-1).
097-10603-01 Rev. A – November 2004
Advertisement
Table of Contents
