ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

AM Detectors

Updated on November 1, 2012

AM Detectors


AM Detection is a basic requirement for radio reception and well as many other applications such as field strength and SWR meters. It is about the simplest thing you could want to do in terms of signal processing but it turns out to present a few complications. Often it is the weakest link in an otherwise great circuit. A variety of simple AM detector options are reviewed here.


Figure 1

A collection of AM detector circuits.
A collection of AM detector circuits.

Detector Options



Figure 1 shows a collection of circuits that may be used to detect radio frequency voltages. Due to the basic semiconductor physics involved all the circuits have a number of regions of operation. The first is where the input is too low to produce any output or the output is buried in the noise floor. The second is a square law region where doubling the input produces approximately four times the output. All except the transistor square law circuit and the FET drain bend circuit then move into a third region where the output becomes linearly related to the input.

Figure 1a shows the commonly used single diode circuit. At input signal levels above 600 or 700 mV the output level has a relatively linear correspondence with the input level. Between about 300mV and 600mV it operates as square law detector. Below 300 mV it becomes hopelessly inefficient.

Nevertheless this circuit can be used in AM radios where an Automatic Gain Control (AGC) circuit keeps the input to the diode detector at a reasonably high level. Obviously there is going to be considerable audio distortion from such a circuit even with AGC. The typical distortion level is about 10% which is acceptable for general listening.

A useful improvement in performance can be obtained by forward biasing the diode with a constant current source (Figure 1b). This extends the usable square law detection region down below 40 mV. In the square law region changes in the dynamic resistance of the diode rather than actual switching cause detection. The biased diode detector is useful because the input impedance can be adjusted to suit the application. A high value resistor connected to a relatively high voltage source can approximate a constant current source for this circuit.

Figure 1c is a FET infinite impedance detector. This has a high DC input impedance and an input capacitance that is typically around 30pf. On it’s own it is not much of an improvement over a biased diode circuit because the transconductance of a typical FET is somewhat low. It’s efficiency and linear region can be improved greatly improved by incorporating an additional transistor (Figure 1d). Such high input impedance circuits are useful because they can be directly connected to a tuned LC resonant circuit without causing excessive loading which would otherwise cause a loss of sensitivity and selectivity.

A similar circuit using a standard transistor is shown in figure 1e. This is a very efficient linear detector but in contrast to the FET circuit the input impedance at radio frequencies is very much lower. You can use the circuit successfully in applications where low impedance input signals are available such as in SWR meters.

The two remaining circuits are very sensitive square law only circuits.



Figure 2

The efficiency of detectors 1a to 1e compared to an ideal diode detector for radio frequency inputs between 0 and 1000 mV.
The efficiency of detectors 1a to 1e compared to an ideal diode detector for radio frequency inputs between 0 and 1000 mV.

Comparisons


Figures 2 and 3 show the relative efficiency of each of the detector circuits compared to what would be obtained with an ideal diode.

It is clear that all of the detectors will give a high level of distortion at low input levels. A detector operating in the square law region will give 25% audio distortion with a 100% modulated AM signal. Fortunately the amount of distortion decreases for more usual levels of AM modulation. The resulting audio quality is generally acceptable for a communications use. All the circuits may can be used for measuring radio frequency signal strength as well.


Figure 3

The efficiency graphs for circuits 1f and 1g.
The efficiency graphs for circuits 1f and 1g.

Comments

    0 of 8192 characters used
    Post Comment

    No comments yet.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://hubpages.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)