{"id":882,"date":"2020-11-19T17:56:18","date_gmt":"2020-11-19T17:56:18","guid":{"rendered":"http:\/\/bassett.me\/mike\/blog\/wordpress\/?p=882"},"modified":"2020-11-19T17:56:18","modified_gmt":"2020-11-19T17:56:18","slug":"a-really-good-notch-filter","status":"publish","type":"post","link":"https:\/\/bassett.me\/mike\/blog\/wordpress\/shed\/a-really-good-notch-filter\/","title":{"rendered":"A Really Good Notch Filter"},"content":{"rendered":"\n<p>When you think notch filter you usually think twin T:<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" width=\"1024\" height=\"883\" src=\"http:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Twin-T-1024x883.png\" alt=\"\" class=\"wp-image-884\" srcset=\"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Twin-T-1024x883.png 1024w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Twin-T-300x259.png 300w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Twin-T-768x662.png 768w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Twin-T-680x586.png 680w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Twin-T-740x638.png 740w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Twin-T.png 1294w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><figcaption>Twin T filter<\/figcaption><\/figure>\n\n\n\n<p>In this R1 = R2 and C2 = C3, C1 is 2 * C2 and R3 is R1 \/ 2.  The notch is at 1\/2piRC.<br><\/p>\n\n\n\n<p>My favourite though is the Bridged differentiator:<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" width=\"1024\" height=\"542\" src=\"http:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Bridged-differentiato-1024x542.png\" alt=\"\" class=\"wp-image-886\" srcset=\"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Bridged-differentiato-1024x542.png 1024w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Bridged-differentiato-300x159.png 300w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Bridged-differentiato-768x406.png 768w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Bridged-differentiato-680x360.png 680w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Bridged-differentiato-740x392.png 740w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/Bridged-differentiato.png 1474w\" sizes=\"(max-width: 1024px) 100vw, 1024px\" \/><figcaption>Bridged Differentiator<\/figcaption><\/figure>\n\n\n\n<p>The reason for this is that it much easier to adjust.  With the twin T you have to adjust three components in perfect harmony, a nearly impossible task.  <br>With the Bridged differentiator R1 determines the Q and needs to be exactly 6 times R2 + R3 to get a good notch.  To adjust the frequency you vary the ratio between R2 and R3.<br>The capacitors have to be perfectly matched to get a good notch but I found a set of three on e-bay that my 4-digit capacitance meter couldn&#8217;t tell apart.<br>The frequency of the notch is determined by the formula:<\/p>\n\n\n\n<figure class=\"wp-block-image size-large is-resized\"><img loading=\"lazy\" src=\"http:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/bridged-1.png\" alt=\"\" class=\"wp-image-892\" width=\"234\" height=\"72\" srcset=\"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/bridged-1.png 383w, https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-content\/uploads\/2020\/11\/bridged-1-300x92.png 300w\" sizes=\"(max-width: 234px) 100vw, 234px\" \/><\/figure>\n\n\n\n<p>For my mains hum filter the capacitors were 100nF, R1 was a 430k plus a 100k trimmer, R2, R3 were a 1k and a 56k and a 20k trimmer between them with the wiper to ground,<br>You only need to adjust R1 once.  with the right R2, R3 this can go from 30Hz to 150Hz and beyond.<br>This functions just as well as a twin T and is lot more friendly.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>When you think notch filter you usually think twin T: In this R1 = R2 and C2 = C3, C1 is 2 * C2 and R3 is R1 \/ 2.&nbsp;[&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[3],"tags":[],"_links":{"self":[{"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/posts\/882"}],"collection":[{"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/comments?post=882"}],"version-history":[{"count":2,"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/posts\/882\/revisions"}],"predecessor-version":[{"id":894,"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/posts\/882\/revisions\/894"}],"wp:attachment":[{"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/media?parent=882"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/categories?post=882"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/bassett.me\/mike\/blog\/wordpress\/wp-json\/wp\/v2\/tags?post=882"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}