HP 4194A User Manual

Ihnl HEWLETT
PACKARD
Application Note 339-9
Negative Impedance Measurements
of Crystal Oscifetors
- HP 4194A Impedance/Gain-Phase -
Introduction
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
Impedance/Gain-Phase Analyzer and the HP
41941A/B Impedance 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 Impedance
Analyzers have the capability to perform low-
grounded measurements, but these instruments
limit the ability to fully characterize oscillators
for the following reasons.
-R1
Cl
• Oscillator Circuit
X: Reactance of crystal
R: Resistance of crystal
Cl: Load capacitance of oscillator
-R1 : Negative resistance of oscillator
Figure 1. Equivalent Circuit of a Crystal
a Crystal Oscillator
Figure 2. Example of Crystal Oscillator

HP 4194A Specifications

General

Brand	HP
Model	4194A
Category	Measuring Instruments
Language	English

Summary

Introduction to Negative Impedance Measurements

HP 4194A Impedance/Gain-Phase Analyzer Capabilities

Vcc Effects on Oscillator Input Impedance

Explains how Vcc affects oscillator input impedance and how to evaluate Vcc margins.

Oscillator Input Impedance Measurement Functions

Details measuring oscillator input impedance versus frequency and input signal level.

Auto Sequence Program (ASP) for Evaluation

Describes the ASP function for automatic oscillator evaluation and presents an example program.

Crystal Oscillator Measurement Procedures

Crystal Impedance Characteristics Measurement

Measures crystal impedance characteristics to identify the design oscillation frequency.

Oscillator Level for 1mW Drive Determination

Determines the OSC level needed for a 1mW crystal drive by measuring R/Power vs. OSC Level.

Oscillator Input Impedance vs. Frequency

Measures frequency characteristics of oscillator input impedance to ensure oscillation.

Oscillator Input Impedance vs. Input Level

Measures input level characteristics of oscillator input impedance for cancellation.