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
    • Product Guides
  • 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
    • Product Guides
  • 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

content


The next level of intermodulation

The next level of intermodulation

Here's what to do when dealing with amplifier signals that are unequal
  • Written by Urgent Communications Administrator
  • 1st June 2007

In the April edition, we lightly touched on the subject of intermodulation (I-M) and small-signal radio frequency amplifiers. We discussed how two equal-level tones (RF signals), A and B, when applied to the amplifier input, are used to produce third-order intermodulation products at the output of the amplifier. We left the reader hanging, somewhat, with the question: “What if tones A and B are not equal?” In this article, we provide the answer.

First, a little background about interconnecting the test equipment and the device under test (DUT) is in order. Two signal generators labeled A and B are used to generate the two continuous wave tones, A and B. The outputs of the two signal generators are combined and fed to the input of the DUT (a small-signal RF amplifier in this case), and the frequency and output level of the two generators are adjusted to produce third-order I-M products at the output of the RF amplifier. The test setup is shown in Figure 1.

A hybrid combiner is preferred over a simple resistive combiner. A resistive combiner like the one shown in Figure 2, while providing proper impedance matching, offers little isolation between signal generators. A high degree of isolation between combiner ports A and B is necessary to prevent interaction between the signal generators. Don’t even think about using a simple Tee connector as a combiner for this test. A hybrid combiner is made up of an RF transformer and resistor network. The isolation between ports A and B is achieved by cancellation of signals at opposite ports.

For example, the signal at port A arrives at port B via two separate paths. One path is through a transformer, and the other path is through a resistor. The path through the RF transformer is 180° out of phase with the signal through the resistor. With the signals being equal — or nearly so — cancellation occurs. The same is true for the signals traveling from port B to port A. Referring to Figure 1, RF isolators can be used with hybrid combiners for additional isolation. But they must be used with resistive combiners in order to achieve the required degree of isolation.

Another important point to note is that having a good 50 ohm load at the sum (S) port is necessary to optimize the isolation between ports A and B. A mismatch at the S port will adversely affect the isolation between ports A and B, possibly resulting in undesirable interaction between the signal generators. An attenuator pad connected as shown can improve the isolation figure.

In order to check for interaction between signal generators, vary the output level of generators A and B by ±10 dB and observe the effect on the opposite signal on the spectrum analyzer. For example, increase and decrease the level of generator A by 10 dB while closely monitoring the level of signal B on the spectrum analyzer. If the level of signal B is affected by changes in the level of generator A, there is insufficient isolation between the two signal generators. Also, make sure that I-M signals are not being formed in the signal generators. You can check this by changing the attenuation in the pad between the DUT and the combiner. If you see a dB-for-dB change in the I-M signal on the spectrum analyzer, the I-M signal is being formed at a point prior to the attenuator pad. If you see a 3-for-1 dB change in the third-order I-M signal on the spectrum analyzer, the I-M signal is being formed at a point after the attenuator pad.

Suppose that the amplifier in Figure 1 has a third-order intercept point (TOIP) of +10 dBm. Signal generators A and B are set to 150 MHz and 152 MHz, respectively. The output levels of generators A and B are set to produce a level of -10 dBm for each tone (A and B) at the input to the DUT (amplifier). Since both of the fundamental tones are equal in amplitude, the two third-order intermodulation products also are equal in amplitude. In Figure 3, each horizontal scale division on the spectrum analyzer represents 1 MHz. Thus, the 2A-B intermodulation product is at 148 MHz, and the 2B-A intermodulation product is at 154 MHz. Notice that the levels of the I-M products are each at -50 dBm. Also note in Figure 4 that the fundamental tones, A and B, are not of equal level. This results in I-M products that are not of equal level. The frequency spacing remains unchanged.

Figure 5 on page 48 is a nomogram that can be used to determine the output level of the third-order I-M products, given the amplifier’s third-order intercept figure and the level of fundamental tones, A and B, at the output. First, a description of the scales of the nomogram is in order. The first scale on the far left is used to represent the level of the second-order term. The second scale is used to represent the level of the first-order term. In the expression 2A-B, A is the second-order term, and B is the first-order term. Conversely, in the expression 2B-A, B is the second-order term, and A is the first-order term. The third scale is used to adjust the TOIP of the amplifier. If the TOIP of the amplifier is 0 dBm, no adjustment is necessary.

Now, let’s do an example on the nomogram. Suppose that an amplifier has a TOIP of +25 dBm. The level of the fundamental tone A at the output is -20 dBm and the level of the fundamental tone B at the output is -30 dBm. Now, we want to determine the level of the two third-order I-M products at the output, 2A-B and 2B-A. First, we will determine the level of the 2A-B intermodulation signal. The blue dashed line on the nomogram in Figure 5 represents this procedure. Start on the left scale for the second-order term at -20 dBm, represented by point 1 on the blue-dashed line. Move straight over to the second scale representing the first-order term at point 2. Since the first-order term is -30 dBm, we move downward by three increments to point 3 on the second scale. Next, move directly over to the third scale representing the third-order intercept adjustment at point 4. If the TOIP was 0 dBm, we would not need to make any adjustment. But, since the third-order intercept point is +25 dBm, we must move downward by five increments to point 5 on the third scale. Then, move directly over to the fourth scale at point 6, which represents -120 dBm for the third-order I-M level. Thus, the level of the 2A-B intermodulation product is -120 dBm. The level of the 2B-A intermodulation product is determined the same way, as depicted by the red dashed line on the nomogram.

The nomogram in Figure 5 is very intuitive, and in studying it, several things become apparent. For example, notice that a 10 dB change in the second-order term will change the third-order I-M level by 20 dB. A 10 dB change in the first-order term will change the third-order I-M level only by 10 dB, while a 10 dB change in both terms (in the same direction) will result in a 30 dB change in the output I-M level (in that direction).

Note that on the third scale — the TOIP adjustment — each increment represents a 5 dB change and that a 5 dB change in the third-order intercept point will result in a 10 dB change in the third-order I-M level. For a given output level of the fundamental A and B tones, increasing the TOIP results in a decrease in the I-M signal level. This is why it is important to use an amplifier with a very high TOIP figure to keep the resultant I-M signal levels low. The formulas for calculating the third-order I-M levels can be found in Equation 1.

It is important to note here that this discussion centers on the output TOIP and uses the tone levels, A and B, at the output of the amplifier. Thus, the figure listed is the output TOIP. The input TOIP would be less than the output TOIP by an amount equal to the gain of the amplifier. (Typically, RF amplifiers are rated in terms of the output TOIP figure.)

It is important to remember, too, that the TOIP point is an extrapolated figure based on the point at which the third-order I-M product is equal to the level of one of the two equal fundamental tones, A or B. This point occurs at a point 15 dB (plus/minus a few decibels) above the 1 dB compression point. Since it is well beyond the point of compression, the TOIP can’t be reached in a practical amplifier. Still, the figure is useful in determining low signal-level performance of an RF amplifier.
Until next time — stay tuned!

Tags: content

Most Recent


  • The next level of intermodulation
    Newscan: Securing the Internet of Things is quite a challenge
    Also: EWA requests dismissal of 900 MHz applications; TIA names tech and policy priorities for 2014; IJIS Institute names Shumate Award winner; App makes bus waits more tolerable; a Blackberry comeback may be in the offing.
  • The next level of intermodulation
    Newscan: FCC certifies Carlson Wireless's white-space radio
    Also: Congress looks to revamp telecom law; Obama to place some restraints on surveillance; IEEE to study spectrum-occupancy sensing for white-spaces broadband; Major Swedish transport operator opts for Sepura TETRA radios; RFMD to partner on $70 million next-generation power grid project; NENA opens registratiuon for "911 Goes to Washington."
  • The next level of intermodulation
    Newscan: A look at the critical job of 911 dispatchers
    Also: NYC launches website for tracking 911 response times; Oregon implements 911 on pre-paid cell phones; LightSquared wants to keep spectrum assets; Harris receives multiple government orders; FCC extends rebanding financial reconciliation deadline; Zetron gear at core of communications system upgrade; Ritron debuts wireless access control system; EWA seeks policy review of VHF vehicular repeater system deployments.
  • The next level of intermodulation
    Newscan: Average peak data rates of 144 MB/s average realized in tests with CAT 4 LTE device
    Also: Verizon, T-Mobile to swap unused spectrum to improve coverage; Internet giants oppose surveillance--but only when the government does it; FCC Chairman says incentive auction will be delayed until middle of 2015; FCC chair announces staff appointments; Alcatel-Lucent names Tim Krause as chief marketing officer; New Jersey county deploys TriTech CAD system; Toronto airport deploys 26-position Zetron console system;

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

  • RugGear: Contributing to the future of mission-critical broadband communication review and market vision
  • Photo gallery: 2014 Communications Marketing Conference (CMC) in Tucson
  • The next level of intermodulation
    Top 5 Stories - Week of Sept. 22
  • The next level of intermodulation
    RCA plans to expand this year's Technical Symposium

Commentary


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

26th January 2023

3GPP moves Release 18 freeze date to March 2024

18th January 2023

Do smart cities make safer cities?

  • 1
6th January 2023
view all

Events


UC Ezines


IWCE 2019 Wrap Up

13th May 2019
view all

Twitter


UrgentComm

Phishers trick Microsoft into granting them ‘verified’ Cloud Partner status dlvr.it/Shqngn

2nd February 2023
UrgentComm

Shapeshifting robot can morph from a liquid to a solid dlvr.it/Shqk9K

2nd February 2023
UrgentComm

Automakers against stampede to BEV dominance dlvr.it/ShpX08

2nd February 2023
UrgentComm

FCC nominee Gigi Sohn headed for third Senate hearing dlvr.it/ShpDcZ

1st February 2023
UrgentComm

Sign up to learn how to successfully manage your Motorola ASTRO® 25 System: spr.ly/60143j8fp https://t.co/XcxiUwzN27

1st February 2023
UrgentComm

Hytera parent cites financial health, but unable to make royalty payment to Motorola Solutions dlvr.it/ShlrlM

1st February 2023
UrgentComm

NATE: Todd Schlekeway highlights organization’s safety, legislative initiatives dlvr.it/ShljHj

1st February 2023
UrgentComm

Cybercrime ecosystem spawns lucrative underground Gig Economy dlvr.it/ShkKbf

31st January 2023

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.