https://urgentcomm.com/wp-content/themes/ucm_child/assets/images/logo/footer-new-logo.png
  • Home
  • News
  • Multimedia
    • Back
    • Multimedia
    • Video
    • Podcasts
    • Galleries
    • IWCE’s Video Showcase
    • IWCE 2022 Winter Showcase
    • IWCE 2023 Pre-event Guide
  • Commentary
    • Back
    • Commentary
    • Urgent Matters
    • View From The Top
    • All Things IWCE
    • Legal Matters
  • Resources
    • Back
    • Resources
    • Webinars
    • White Papers
    • Reprints & Reuse
  • IWCE
    • Back
    • IWCE
    • Conference
    • Special Events
    • Exhibitor Listings
    • Premier Partners
    • Floor Plan
    • Exhibiting Information
    • Register for IWCE
  • About Us
    • Back
    • About Us
    • Contact Us
    • Advertise
    • Terms of Service
    • Privacy Statement
    • Cookie Policy
  • Related Sites
    • Back
    • American City & County
    • IWCE
    • Light Reading
    • IOT World Today
    • Mission Critical Technologies
    • TU-Auto
  • In the field
    • Back
    • In the field
    • Broadband Push-to-X
    • Internet of Things
    • Project 25
    • Public-Safety Broadband/FirstNet
    • Virtual/Augmented Reality
    • Land Mobile Radio
    • Long Term Evolution (LTE)
    • Applications
    • Drones/Robots
    • IoT/Smart X
    • Software
    • Subscriber Devices
    • Video
  • Call Center/Command
    • Back
    • Call Center/Command
    • Artificial Intelligence
    • NG911
    • Alerting Systems
    • Analytics
    • Dispatch/Call-taking
    • Incident Command/Situational Awareness
    • Tracking, Monitoring & Control
  • Network Tech
    • Back
    • Network Tech
    • Interoperability
    • LMR 100
    • LMR 200
    • Backhaul
    • Deployables
    • Power
    • Tower & Site
    • Wireless Networks
    • Coverage/Interference
    • Security
    • System Design
    • System Installation
    • System Operation
    • Test & Measurement
  • Operations
    • Back
    • Operations
    • Critical Infrastructure
    • Enterprise
    • Federal Government/Military
    • Public Safety
    • State & Local Government
    • Training
  • Regulations
    • Back
    • Regulations
    • Narrowbanding
    • T-Band
    • Rebanding
    • TV White Spaces
    • None
    • Funding
    • Policy
    • Regional Coordination
    • Standards
  • Organizations
    • Back
    • Organizations
    • AASHTO
    • APCO
    • DHS
    • DMR Association
    • ETA
    • EWA
    • FCC
    • IWCE
    • NASEMSO
    • NATE
    • NXDN Forum
    • NENA
    • NIST/PSCR
    • NPSTC
    • NTIA/FirstNet
    • P25 TIG
    • TETRA + CCA
    • UTC
Urgent Communications
  • NEWSLETTER
  • Home
  • News
  • Multimedia
    • Back
    • Video
    • Podcasts
    • Omdia Crit Comms Circle Podcast
    • Galleries
    • IWCE’s Video Showcase
    • IWCE 2023 Pre-event Guide
    • IWCE 2022 Winter Showcase
  • Commentary
    • Back
    • All Things IWCE
    • Urgent Matters
    • View From The Top
    • Legal Matters
  • Resources
    • Back
    • Webinars
    • White Papers
    • Reprints & Reuse
    • UC eZines
    • Sponsored content
  • IWCE
    • Back
    • Conference
    • Why Attend
    • Exhibitor Listing
    • Floor Plan
    • Exhibiting Information
    • Join the Event Mailing List
  • About Us
    • Back
    • About Us
    • Contact Us
    • Advertise
    • Cookie Policy
    • Terms of Service
    • Privacy Statement
  • Related Sites
    • Back
    • American City & County
    • IWCE
    • Light Reading
    • IOT World Today
    • TU-Auto
  • newsletter
  • In the field
    • Back
    • Internet of Things
    • Broadband Push-to-X
    • Project 25
    • Public-Safety Broadband/FirstNet
    • Virtual/Augmented Reality
    • Land Mobile Radio
    • Long Term Evolution (LTE)
    • Applications
    • Drones/Robots
    • IoT/Smart X
    • Software
    • Subscriber Devices
    • Video
  • Call Center/Command
    • Back
    • Artificial Intelligence
    • NG911
    • Alerting Systems
    • Analytics
    • Dispatch/Call-taking
    • Incident Command/Situational Awareness
    • Tracking, Monitoring & Control
  • Network Tech
    • Back
    • Cybersecurity
    • Interoperability
    • LMR 100
    • LMR 200
    • Backhaul
    • Deployables
    • Power
    • Tower & Site
    • Wireless Networks
    • Coverage/Interference
    • Security
    • System Design
    • System Installation
    • System Operation
    • Test & Measurement
  • Operations
    • Back
    • Critical Infrastructure
    • Enterprise
    • Federal Government/Military
    • Public Safety
    • State & Local Government
    • Training
  • Regulations
    • Back
    • Narrowbanding
    • T-Band
    • Rebanding
    • TV White Spaces
    • None
    • Funding
    • Policy
    • Regional Coordination
    • Standards
  • Organizations
    • Back
    • AASHTO
    • APCO
    • DHS
    • DMR Association
    • ETA
    • EWA
    • FCC
    • IWCE
    • NASEMSO
    • NATE
    • NXDN Forum
    • NENA
    • NIST/PSCR
    • NPSTC
    • NTIA/FirstNet
    • P25 TIG
    • TETRA + CCA
    • UTC
acc.com

Call Center/Command


Crystal clear

Crystal clear

Almost every radio has a crystal oscillator — here's how they work.
  • Written by Urgent Communications Administrator
  • 1st October 2009

Radio receivers and transmitters both require a precise frequency reference, and this reference almost always is provided by a crystal oscillator. A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal to generate a sinusoidal electronic signal at a very precise frequency. The translation by certain materials of a a mechanical impulse to an electrical oscillation is called the piezoelectric effect. The most common type of piezoelectric resonator is the quartz crystal and electronic circuits designed around them are called crystal oscillators.

Piezoelectricity was discovered by Jacques and Pierre Curie in 1880. The first crystal-controlled oscillator was constructed at Bell Laboratories in 1917 by Alexander Nicholson using a crystal of Rochelle salt. Walter Cady, a professor at Wesleyan University, built the first quartz crystal oscillator in 1921. By 1926, quartz crystals were used to control the frequency of AM radio broadcast transmitters and were widely used by amateur radio operators.

To illustrate the application of crystal oscillators to land-mobile radio, we will consider the Harris 800 MHz MASTR II repeater. The MASTR II repeater was manufactured from the mid-1980s through 1993. It is still used in public safety and private LMR systems, but 800 MHz rebanding in the United States caused many MASTR IIs to be replaced because the crystal oscillator in the receiver was no longer supported and no reliable substitute was readily available. Modern receivers do not have this problem because a single frequency oscillator (usually 10 MHz) is used in all receivers and the tuned frequency is generated by a frequency synthesizer using the the fixed oscillator as a reference. Although the MASTR II is obsolete, the problem with the receiver oscillator reveals the temperature and aging variations that apply at some level to all crystal oscillators.

Virtually all LMR receivers, including the MASTR II, employ a superheterodyne receiver that requires a particularly accurate and pure local oscillator (LO). In the MASTR II receiver, two local oscillators are used with mixers to step down the radio frequency signal from a frequency in the receive band (806-824 MHz) to each of two intermediate frequencies, where effective filtering and demodulation can take place. The first LO is generated by an Integrated Circuit Oscillator Module (ICOM) — not to be confused with the radio manufacturer of the same name. The ICOM uses a precisely cut quartz crystal and a matched temperature compensation circuit to generate the LO frequency. A MASTR II repeater cannot be retuned without replacing the ICOM or replacing the crystal in the ICOM.

A block diagram of the MASTR II receiver is shown in Figure 1. Note that the ICOM is the subassembly that generates the first LO frequency. In the MASTR II receiver, the LO is 45 MHz below the receive frequency.

The ICOM is a metal package that consists of a single quartz crystal and associated circuitry that generates a pure sine wave at the desired frequency. The precise physical dimensions of the quartz crystal determine its frequency. When a voltage is applied across the quartz crystal, the crystal will flex at its resonant frequency.

Due to internal losses within the crystal, these small oscillations would normally decay and at some point oscillation would stop. To ensure the oscillations persist, an external amplifier is used to overcome the losses and reinforce the oscillation. The term given to the combined crystal and amplifier circuit is the oscillator. Typically, the resonant frequency of the crystal is lower than the desired LO frequency and a multiplier circuit is used to raise the crystal frequency to the LO frequency. For example, the MASTR II receiver uses a multiplier factor of 48 and the multiplier circuit is external to the ICOM. Because the MASTR II LO is 45 MHz below the radio frequency (806.0125-823.9875 MHz) and the multiplier factor is 48, they receive ICOM crystal frequencies varying from 15.854427 MHz to 16.228906 MHz. Because the multiplier is fixed, a different crystal is needed for each of the more than 800 frequencies between 806.0125 and 823.9875 MHz.

The oscillator’s developed voltage is a sine wave whose frequency is directly related to the physical dimensions (resonant frequency) of the crystal mass. Changing the oscillator’s frequency requires one to make dimensional changes to the crystal mass. That is, crystals must be custom tuned by cutting or grinding to exact dimensions and tolerances to achieve a highly accurate desired frequency.

Although crystals once ground are, in a general sense, dimensionally stable, their instantaneous resonant frequency is affected by environmental changes, especially temperature.

Temperature control and temperature compensation. Crystal stability can be improved through the use of thermostatically controlled ovens (where the temperature is set to an artificially high, precisely-controlled level) or with electronic compensation circuitry whose temperature characteristics are selected to be exactly opposite of those of the crystal. Oven-controlled oscillators are denoted by the acronym OCXO while temperature-controlled oscillators are denoted by the acronym TCXO. The ICOM is a TCXO. When a crystal and compensation network are properly paired, the oscillator’s frequency is, for all practical purposes, independent of temperature variations.

The ICOM in the MASTR II base station incorporates three critical elements: a precisely tuned (ground-to-frequency) crystal, an amplifier and a custom-optimized automatic temperature compensation circuit.

The temperature compensation circuit is designed to maintain the ICOM frequency within precisely-established FCC requirements over a temperature range of -30 to +85° C.

Due to structural differences between crystals and the grinding process itself, each temperature compensation network for 800 MHz ICOMs used in MASTR II base stations was individually optimized specifically for the installed crystal.

Temperature compensation circuits operate at two extremes: low temperatures and high temperatures. At the low end, temperature compensation circuits activate at temperatures below 0° C (32° F). When the temperature drops below 0°C, the circuit is activated. As the temperature decreases, an equivalent resistance decreases and the compensation voltage increases. An increase in compensation voltage decreases the capacitance of the varactor in the oscillator, thereby raising the output frequency of the ICOM.

At the high end, temperature compensation circuits for the 800 MHz ICOM operate at temperatures above +55° C (131° F). When the temperature rises above 55° C, the first branch of the compensation circuit activates. At temperatures above 70° C, the second branch of the circuit activates so that both branches are now operating simultaneously. At temperatures above each of these activation points, the equivalent resistance decreases, thereby decreasing the compensation voltage. The decrease in compensation voltage increases the capacitance of the varactor, thereby lowering the output frequency of the ICOM.

The ICOM has a specified frequency accuracy of 1 part per million (ppm) over its specified temperature range of -30° C to +85° C, and 0.2 ppm at an ambient temperature of 27.5°C (81.5° F).

Frequency drift in crystal oscillators and the importance of aging. A characteristic of crystals is that they age over time and some of this aging occurs after the crystal is put into service. Aging causes frequency drift in the ICOM crystal, which is multiplied by the 48X multiplier circuit. While each crystal is unique in the way it ages, one can count on some drift from all crystals. It is generally understood in the industry that the original manufacturer, General Electric, aged ICOMs for 60 days. This aging process included temperature cycling of the ICOM while it was under operating conditions. Thus, the crystal and its matched temperature compensation network were aged and tested in the factory for an extended period to minimize drift once in the field. Once a non-defective crystal is fully aged, it is stable in time and temperature, provided the temperature compensation circuit is properly matched to the crystal. Like a good wine, no crystal oscillator should be sold before its time.

Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm located in Colorado Springs, Colo. He holds bachelor’s and master’s degrees in electrical engineering from Virginia Tech and Cornell University, respectively, and has more than 25 years experience as a radio-frequency engineer.

Tags: Call Center/Command Test & Measurement

Most Recent


  • Panel at SBC event examines significant economic, policy challenges facing in-building communications
    Ensuring that public safety can communicate while responding to emergencies inside buildings is a clear need, but identifying the proper solutions and making them affordable to building owners is a significant challenge, according to panelists exploring the topic during a recent Safer Buildings Coalition (SBC) event. Mike Baltrotsky, assistant chief and technology manager for Montgomery […]
  • Indoor 5G and how to solve it
    We all know the problem: mobile phone service is working fine … until you move indoors. Then, sometimes it works (you don’t notice), sometimes it sort of works (meh) and sometimes it clings on by one tiny bar or drops out entirely (argh). Diagnosing the problem is relatively simple. The loss of mobile service indoors […]
  • Honeywell releases cybersecurity, smart-building tools
    Honeywell has released a series of new tools aimed at improving operational efficiency and sustainability, including an operational technology (OT) cybersecurity tool and a smart building management system. The new cybersecurity tool, Cyber Insights, collects information from multiple OT data sources to identify vulnerabilities and threats.  “Organizations should leverage technology to address worker shortages, while […]
  • Small-cell focus still largely on densification, but private-network opportunity growing
    Some big ambitions for small cells were on display during the annual Small Cell World Summit in London this week. But it’s worth remembering that on the ground, their deployment is still at a relatively early stage. Dean Bubley of Disruptive Analysis, who chaired a panel addressing the issues of value creation and monetization, summarized […]

Leave a comment Cancel reply

To leave a comment login with your Urgent Comms account:

Log in with your Urgent Comms account

Or alternatively provide your name, email address below:

Your email address will not be published. Required fields are marked *

Related Content

  • New Orleans-area 911 center inks multiyear APEX deal with Carbyne to replace call-handling system
  • Crystal clear
    Newscan: Feds recover millions from pipeline ransom hackers, hint at U.S. Internet tactic
  • Cyber is the new Cold War, and AI is the arms race
  • Private wireless networks in the US start going public

Commentary


Updated: How ‘sidelink’ peer-to-peer communications can enhance public-safety operations

  • 1
27th February 2023

NG911 needed to secure our communities and nation

24th February 2023

How 5G is making cities safer, smarter, and more efficient

26th January 2023
view all

Events


UC Ezines


IWCE 2019 Wrap Up

13th May 2019
view all

Twitter


Newsletter

Sign up for UrgentComm’s newsletters to receive regular news and information updates about Communications and Technology.

Expert Commentary

Learn from experts about the latest technology in automation, machine-learning, big data and cybersecurity.

Business Media

Find the latest videos and media from the market leaders.

Media Kit and Advertising

Want to reach our digital and print audiences? Learn more here.

DISCOVER MORE FROM INFORMA TECH

  • American City & County
  • IWCE
  • Light Reading
  • IOT World Today
  • Mission Critical Technologies
  • TU-Auto

WORKING WITH US

  • About Us
  • Contact Us
  • Events
  • Careers

FOLLOW Urgent Comms ON SOCIAL

  • Privacy
  • CCPA: “Do Not Sell My Data”
  • Cookie Policy
  • Terms
Copyright © 2023 Informa PLC. Informa PLC is registered in England and Wales with company number 8860726 whose registered and Head office is 5 Howick Place, London, SW1P 1WG.