HP 4194A Manual

hnl HEWLETT
ACKARD
Application Note 339-9
Negative Impedance Measurements
of rystal Oscifetors
- HP 4194A Impedance/Gain-Phase -
ntroduction
Negative impedance, one of the most im ­
portant parameters to be considered when
characterizing crystal oscillators and resonators,
can be measured easily by using the HP 4194A
mpedance/Gain-Phase Analyzer and the HP
41941A/B mpedance Probe Kit. This ability to
perform negative impedance measurements pro ­
vides a powerful tool for oscillator development
and evaluation.
The relative ease with which negative im ­
pedance measurements can be performed with
the HP 4194A and the HP41941A/B also means
you can significantly reduce oscillator and clock
generator cost and development time in crystal/
ceramic oscillator and resonator manufacturing
labs, and in the labs of electronic instrument
manufacturers.
Measurement Requirements and HP 4194A
Solutions
Figure 1 shows an equivalent circuit for a
crystal oscillator. Because an oscillator must
have enough negative resistance to compensate
for the resistance of a crystal for oscillation to
start, negative resistance measurements are
important when determining oscillator
characteristics.
Figure 2 shows an example of a crystal
oscillator chip. The negative input impedance
of an oscillator circuit should be measured
under actual operating conditions to verify that
the circuit is performing as designed. As a
rule, all oscillator circuits are low-grounded.
The HP 4191A, 4192A, and 4193A mpedance
Analyzers have the capability to perform low-
grounded measurements, but these instruments
limit the ability to fully characterize oscillators
for the following reasons.
-R1
l
• Oscillator ircuit
X: Reactance of crystal
R: Resistance of crystal
l: Load capacitance of oscillator
-R1 : Negative resistance of oscillator
igure 1. Equivalent Circuit of a Crystal
a Crystal Oscillator
igure 2. Example of Crystal Oscillator