{"id":2942,"date":"2024-08-25T21:33:37","date_gmt":"2024-08-25T21:33:37","guid":{"rendered":"https:\/\/workhouse.sweetdishy.com\/?p=2942"},"modified":"2024-08-25T21:33:48","modified_gmt":"2024-08-25T21:33:48","slug":"fm-modulators","status":"publish","type":"post","link":"https:\/\/workhouse.sweetdishy.com\/index.php\/2024\/08\/25\/fm-modulators\/","title":{"rendered":"FM Modulators"},"content":{"rendered":"\n<p id=\"P1170\">The most frequently used modulator in FM systems is the reactance modulator, which incorporates some method of varying the reactance across the oscillator circuit. This can be done by incorporating a device which changes either its inductive or capacitive reactance, depending on the oscillator involved. With a Colpitts oscillator some type of capacitive modulator would be used; a Hartley oscillator would use an inductive modulator.<\/p>\n\n\n\n<p id=\"P1180\">The capacitance of a simple signal diode depends on the width of its depletion layer when forward or reverse biased. A particular diode, called a&nbsp;<em>variable reactance<\/em>&nbsp;diode (<em>varactor<\/em>&nbsp;for short), is fabricated in such a way that its capacitance is a function of the voltage applied across it.<\/p>\n\n\n\n<p id=\"P1190\">If a varactor diode is connected across the tuned circuit of an oscillator, and the voltage across the diode is varied, the variation of the diode capacitance will cause a variation in the oscillator\u2019s frequency. The circuit virtually functions as a voltage-to-frequency convertor.<a><\/a><\/p>\n\n\n\n<p id=\"P1200\">A typical diode FM modulator is shown in\u00a0Figure 19.15. In this diagram the modulating signal is fed to a transformer which is coupled to a Colpitts oscillator in this case. The varactor diode\u00a0<em>C<\/em><sub>4<\/sub>\u00a0is reverse biased by\u00a0<em>V<\/em><sub>g<\/sub>, as shown, to a practical point on its characteristics. This reverse bias voltage varies the modulating signal voltage, causing\u00a0<em>C<\/em><sub>4<\/sub>\u00a0to vary, thus varying the oscillator frequency.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/learning.oreilly.com\/api\/v2\/epubs\/urn:orm:book:9781856175289\/files\/images\/F000196gr15.jpg\" alt=\"image\"\/><\/figure>\n\n\n\n<p><strong>Figure 19.15<\/strong>&nbsp;Typical diode FM modulator<\/p>\n\n\n\n<p id=\"P1210\">The radio frequency choke is necessary so that the RF voltage across&nbsp;<em>C<\/em><sub>4<\/sub>&nbsp;is not shorted out by the sources&nbsp;<em>v<\/em><sub>m<\/sub>&nbsp;and&nbsp;<em>V<\/em><sub>g<\/sub>.&nbsp;<em>C<\/em><sub>3<\/sub>&nbsp;is used to block any DC voltage from the oscillator.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\" id=\"S0390tit\">Example 19.11<\/h5>\n\n\n\n<p id=\"P1220\">A reactance modulator is used in an FM transmitter. It consists of a Colpitts oscillator and audio injection circuit as shown in\u00a0Figure 19.16. The two varactor diodes have a tuneable range from 19.4 to 3\u00a0pF. Determine the tuning range of the modulator if the inductance value is\u00a0<em>L<\/em>=2\u00a0\u03bcH.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/learning.oreilly.com\/api\/v2\/epubs\/urn:orm:book:9781856175289\/files\/images\/F000196gr16.jpg\" alt=\"image\"\/><\/figure>\n\n\n\n<p><strong>Figure 19.16<\/strong>&nbsp;Diode FM modulator<\/p>\n\n\n\n<h5 class=\"wp-block-heading\" id=\"S0400tit\">Solution<\/h5>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/learning.oreilly.com\/api\/v2\/epubs\/urn:orm:book:9781856175289\/files\/images\/F000196si65.png\" alt=\"image\"\/><\/figure>\n","protected":false},"excerpt":{"rendered":"<p>The most frequently used modulator in FM systems is the reactance modulator, which incorporates some method of varying the reactance across the oscillator circuit. This can be done by incorporating a device which changes either its inductive or capacitive reactance, depending on the oscillator involved. With a Colpitts oscillator some type of capacitive modulator would [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":2943,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[424],"tags":[],"class_list":["post-2942","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-communications-systems"],"jetpack_featured_media_url":"https:\/\/workhouse.sweetdishy.com\/wp-content\/uploads\/2024\/08\/music.png","_links":{"self":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts\/2942","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/comments?post=2942"}],"version-history":[{"count":2,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts\/2942\/revisions"}],"predecessor-version":[{"id":2945,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts\/2942\/revisions\/2945"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/media\/2943"}],"wp:attachment":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/media?parent=2942"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/categories?post=2942"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/tags?post=2942"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}