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

Summary

Introduction to Negative Impedance Measurements

HP 4194 A 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 1 mW 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.

Overview

1. Function Description
2. Important Technical Specifications
3. Usage Features
4. Maintenance Features

The Hewlett-Packard HP 4194A Impedance/Gain-Phase Analyzer, in conjunction with the HP 41941A/B Impedance Probe Kit, is a sophisticated instrument designed for the precise measurement of negative impedance in crystal oscillators. This capability is crucial for the development, evaluation, and manufacturing of crystal and ceramic oscillators and resonators, as well as for electronic instruments that incorporate them. By providing a comprehensive solution for negative impedance measurements, the HP 4194A significantly reduces the cost and development time associated with these components.

Function Description

The primary function of the HP 4194A is to measure the negative input impedance of oscillator circuits under actual operating conditions. This is essential because an oscillator requires sufficient negative resistance to counteract the resistance of the crystal, enabling oscillation to begin and sustain. The analyzer helps verify that an oscillator circuit is performing as designed by characterizing its negative impedance.

The HP 4194A addresses limitations found in earlier impedance analyzers (HP 4191A, 4192A, and 4193A), which, while capable of low-grounded measurements, had restricted frequency ranges or measurement capabilities. The HP 4194A, with the HP 41941A/B probe, can measure grounded circuits and evaluate negative input impedance with enhanced features.

Important Technical Specifications

• Wide Frequency Coverage: The HP 4194A offers a broad frequency range from 10 kHz to 100 MHz. This wide coverage allows for detailed characterization of various types of oscillators.
• Accuracy: It provides an accuracy of 1.5% to 3% for Z-θ measurements, along with R-X measurements, ensuring reliable data for critical applications.
• Variable OSC Level and OSC Level Sweep: This feature allows users to vary the oscillator level and perform sweeps, which is vital for understanding how negative input impedance depends on the input signal level. This capability can also be used to characterize the output level of an oscillator.
• Low-Grounded Measurements: The instrument is designed to perform measurements on low-grounded oscillator circuits, which is a common configuration for these components.
• Automated Sequence Program (ASP) Function: The HP 4194A includes an ASP function that enables automatic evaluation without the need for an external computer. This streamlines the measurement process, making it more efficient and less prone to manual errors.

Usage Features

The HP 4194A offers several key usage features that facilitate comprehensive oscillator evaluation:

1. Vcc Effects on Input Impedance:

   • Users can confirm that the oscillator operates correctly at its designed Vcc value.
   • By varying the Vcc and observing the R vs. Vcc value on the display, Vcc margins can be evaluated. This helps in understanding the oscillator's performance under different power supply conditions.
   • To perform this, an external DC power supply (Vcc) is connected to the oscillator circuit (without a crystal installed), and the HP 41941A/B probe is connected to the location where the crystal would be installed.

2. Oscillator Input Impedance as a Function of Frequency:

   • This measurement is crucial for evaluating negative impedance characteristics in detail at frequencies around the designed oscillation frequency.
   • It helps determine the frequency range over which the input impedance remains negative, which is the range where oscillation can occur.
   • The ASP program can automatically measure the frequency characteristics of the oscillator input impedance, with a marker indicating the oscillation frequency and confirming that the oscillator's negative resistance is sufficient to cancel the crystal's resistance.

3. Oscillator Input Impedance as a Function of Input Level:

   • This feature allows users to substitute input level characteristics for output level characteristics of an oscillator.
   • The HP 4194A's oscillator level sweep function determines the dependency of negative input impedance on the input signal level.
   • The ASP program can automatically measure the input level characteristics of the oscillator input impedance, with a marker indicating the oscillation level and confirming that the oscillator's negative resistance is sufficient to cancel the crystal's resistance at that level.

4. Crystal Resonator Characterization:

   • The HP 4194A is also cost-effective for evaluating the resonators used in oscillator circuits.
   • It can measure the impedance characteristics of a crystal, identifying the design oscillation frequency.
   • It can determine the OSC level required to drive a crystal at a specific power (e.g., 1mW) by measuring the resistance R-OSC level characteristics and calculating power from OSC level and R.

Maintenance Features

While the manual does not explicitly detail maintenance features, the robust design and automated capabilities of the HP 4194A suggest a focus on reliability and ease of use, which inherently reduces the need for frequent manual intervention or complex maintenance procedures. The ASP function, by automating measurements and reducing human error, contributes to the longevity and consistent performance of the instrument by ensuring proper operational sequences. The modular nature of using the HP 4194A with the HP 41941A/B probe kit also implies that components can be serviced or replaced as needed, typical of high-precision test equipment.

HP 4194A Specifications

General

Brand	HP
Model	4194A
Category	Measuring Instruments
Language	English