{"id":14543,"date":"2015-09-01T08:21:10","date_gmt":"2015-09-01T12:21:10","guid":{"rendered":"http:\/\/swling.com\/blog\/?p=14543"},"modified":"2015-09-01T08:21:10","modified_gmt":"2015-09-01T12:21:10","slug":"using-synchronous-detection-and-notch-filter-to-eliminate-het-noise","status":"publish","type":"post","link":"https:\/\/swling.com\/blog\/2015\/09\/using-synchronous-detection-and-notch-filter-to-eliminate-het-noise\/","title":{"rendered":"Using synchronous detection and notch filter to eliminate het noise"},"content":{"rendered":"
\"WinRadioExcalibur-VOG-Notch\"<\/a>

The lighter shaded side of the AM carrier indicates a lower sideband sync lock. (Click to enlarge)<\/p><\/div>\n

A few days ago, I tuned to 9,420 kHz and found a relatively strong signal from the Avlis transmitter site of the Voice of Greece. The broadcast was quite clear until a heterodyne (het) tone popped up out of nowhere.<\/p>\n

I checked the spectrum display of my Excalibur to find two steady carriers located about .5 kHz off each side of VOG’s AM carrier. I assume this may have been a faint digital signal centered on the same frequency as VOG.<\/p>\n

The noise was annoying,\u00a0but SDRs (and many tabletop radios) have tools to help mitigate this type of noise.<\/p>\n

The het tone was originating from both sidebands of the VOG AM carrier (see spectrum display above). I had planned to use my notch filter to eliminate the noise, but\u00a0I had two<\/em> carriers to notch out and only one<\/em> notch filter.<\/p>\n

\"VOG<\/p>\n

Synchronous detection to the rescue…\u00a0<\/strong><\/p>\n

The simple solution was to eliminate one of the carriers using my SDR’s synchronous detector which can lock to either the upper or lower sideband. In this case, it didn’t make any difference which sideband I locked to because both had similar audio fidelity and were otherwise noise free. In the end, I locked to the lower sideband, thus eliminating\u00a0the het in the upper sideband.<\/p>\n

Next, I enabled my notch filter and moved its frequency to cover the annoying het carrier in the lower sideband; I kept the notch filter width as narrow as I could to preserve VOG’s audio fidelity. You can see the notch filter location and width in the spectrum display above (the notch filter is the thin yellow line).<\/p>\n

I should note here that the great thing about using an SDR–or tabletop receiver with a spectrum display–is that you can see<\/em> where the noise is. I was using my WinRadio Excalibur<\/a>, but pretty much any SDR in my shack could have handled this task.<\/p>\n

The results? No het tone and I was able to preserve the great audio fidelity from the Voice of Greece broadcast!<\/p>\n

Here’s a 3.5 hour recording I made after cleaning up the signal. I believe at one point in the recording, I switched off the notch filter to demonstrate how loud the het tone was:<\/p>\nhttps:\/\/archive.org\/download\/VOG9.42MHz25AUG20151025Z\/VOG-9.42MHz-25AUG2015-1025Z.mp3<\/a><\/audio>\n","protected":false},"excerpt":{"rendered":"

A few days ago, I tuned to 9,420 kHz and found a relatively strong signal from the Avlis transmitter site of the Voice of Greece. The broadcast was quite clear until a heterodyne (het) tone popped up out of nowhere. I checked the spectrum display of my Excalibur to find two steady carriers located about […]<\/p>\n","protected":false},"author":15,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[373,627,3,746,43,158,1167],"tags":[2254,4160,3605,4159,4106,801,800],"class_list":["post-14543","post","type-post","status-publish","format-standard","hentry","category-am","category-how-to","category-news","category-recordings","category-shortwave-radio","category-software-defined-radio","category-whats-on-shortwave","tag-am-synchronous-detector","tag-how-to-use-notch-filter","tag-how-to-use-synchronous-detection","tag-notch-filter","tag-recordings","tag-vog","tag-voice-of-greece"],"jetpack_publicize_connections":[],"jetpack_shortlink":"https:\/\/wp.me\/pn3uc-3Mz","jetpack-related-posts":[{"id":7626,"url":"https:\/\/swling.com\/blog\/2013\/10\/am-sync-lessens-noise-in-this-the-voice-of-greece-broadcast\/","url_meta":{"origin":14543,"position":0},"title":"AM sync lessens noise in this The Voice of Greece broadcast","author":"Thomas","date":"October 18, 2013","format":false,"excerpt":"Sometimes, the Voice of Greece plays very little Greek music; October 10th was one of those occasions. \u00a0Nonetheless, I recorded that evening's broadcast. Using AM sync for sideband noise In the first hour of the 10\/10 VOG broadcast, you'll hear a pulsating noise from an unknown origin (possibly a jammer?).\u2026","rel":"","context":"In "AM"","block_context":{"text":"AM","link":"https:\/\/swling.com\/blog\/category\/am\/"},"img":{"alt_text":"TheParthenonAthens","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2013\/01\/TheParthenonAthens-300x210.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":12313,"url":"https:\/\/swling.com\/blog\/2015\/02\/reader-survey-comparing-the-tecsun-pl-680-and-pl-660-synchronous-detection\/","url_meta":{"origin":14543,"position":1},"title":"Reader Survey: Comparing the Tecsun PL-680 and PL-660 synchronous detection","author":"Thomas","date":"February 16, 2015","format":false,"excerpt":"[Update: Please note that this survey has been closed, but the audio samples--labeled Radio A and Radio B--will remain to allow others the opportunity to make an evaluation prior to reading the PL-680 review.] [Update 2: The PL-680 review and readers' survey results have now been posted! Check it out\u2026","rel":"","context":"In "New Products"","block_context":{"text":"New Products","link":"https:\/\/swling.com\/blog\/category\/new-products\/"},"img":{"alt_text":"PL-680-Sync-Detector","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2015\/02\/PL-680-Sync-Detector.jpg?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2015\/02\/PL-680-Sync-Detector.jpg?resize=350%2C200 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2015\/02\/PL-680-Sync-Detector.jpg?resize=525%2C300 1.5x"},"classes":[]},{"id":7060,"url":"https:\/\/swling.com\/blog\/2013\/08\/shortwave-radio-recordings-voice-of-greece-8-14-but-no-signal-last-night\/","url_meta":{"origin":14543,"position":2},"title":"Shortwave Radio Recordings: Voice of Greece 8-14, but no signal last night","author":"Thomas","date":"August 17, 2013","format":false,"excerpt":"I'm a little concerned: last night I couldn't hear the Voice of Greece on 9,420 kHz. Not even a carrier. Though I've noticed they've had intermittent transmitter problems in some broadcasts (you can even hear it in the first minutes below) they haven't gone off the air this long in\u2026","rel":"","context":"In "Broadcasters"","block_context":{"text":"Broadcasters","link":"https:\/\/swling.com\/blog\/category\/broadcasters\/"},"img":{"alt_text":"greece","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2013\/01\/greece.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":9913,"url":"https:\/\/swling.com\/blog\/2014\/06\/shortwave-radio-recordings-the-voice-of-greece-one-year-on\/","url_meta":{"origin":14543,"position":3},"title":"Shortwave Radio Recordings: the Voice of Greece, one year on","author":"Thomas","date":"June 12, 2014","format":false,"excerpt":"Last night, while learning the ropes of the new Elad FDM-S2, I noticed some great music on 9,420 kHz, the former\u00a0Voice of Greece\u00a0frequency. It was then that I realized yesterday (June 11) marked the one year anniversary of the day that\u00a0the Greek government shut down ERT\u00a0and the\u00a0Voice of Greece. If\u2026","rel":"","context":"In "Broadcasters"","block_context":{"text":"Broadcasters","link":"https:\/\/swling.com\/blog\/category\/broadcasters\/"},"img":{"alt_text":"GREECE-ECONOMY-MEDIA","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2013\/06\/greece-ert-replacement-protest.jpg?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2013\/06\/greece-ert-replacement-protest.jpg?resize=350%2C200 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2013\/06\/greece-ert-replacement-protest.jpg?resize=525%2C300 1.5x"},"classes":[]},{"id":8689,"url":"https:\/\/swling.com\/blog\/2014\/01\/shortwave-radio-recordings-radio-station-of-macedonia-vog\/","url_meta":{"origin":14543,"position":4},"title":"Shortwave Radio Recordings: Radio Station of Macedonia (VOG)","author":"Thomas","date":"January 30, 2014","format":false,"excerpt":"For your listening pleasure: 1 hour 29 minutes of The Radio Station of Macedonia (formerly Voice of Greece). This broadcast was recorded on January 28, 2014 around 1:50 UTC on 9,420 kHz. During much of the month of December (2013), the Avlis\u00a0transmitter on 9,420 kHz was off line. Since its\u2026","rel":"","context":"In "Music"","block_context":{"text":"Music","link":"https:\/\/swling.com\/blog\/category\/music\/"},"img":{"alt_text":"TheParthenonAthens","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2013\/01\/TheParthenonAthens.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":11165,"url":"https:\/\/swling.com\/blog\/2014\/10\/ert-open-former-voice-of-greece-moves-to-9415-khz\/","url_meta":{"origin":14543,"position":5},"title":"ERT Open (former Voice of Greece) moves to 9,415 kHz","author":"Thomas","date":"October 24, 2014","format":false,"excerpt":"Many listeners have noticed that the former Voice of Greece (ERT Open) has moved from 9,420 kHz to 9,415 kHz. This must be due to interference from the Islamic Republic of Iran Broadcasting (IRIB) who has been\u00a0transmitting\u00a0on 9420 kHz as well. Here in North America, even when IRIB was broadcasting\u2026","rel":"","context":"In "Broadcasters"","block_context":{"text":"Broadcasters","link":"https:\/\/swling.com\/blog\/category\/broadcasters\/"},"img":{"alt_text":"View from the town of Litochoro, in the foothills of Mount Olympus, Greece. (Source: Public Domain via Wikimedia)","src":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2014\/10\/Olympus_Litochoro.jpg?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2014\/10\/Olympus_Litochoro.jpg?resize=350%2C200 1x, https:\/\/i0.wp.com\/swling.com\/blog\/wp-content\/uploads\/2014\/10\/Olympus_Litochoro.jpg?resize=525%2C300 1.5x"},"classes":[]}],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/posts\/14543","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/users\/15"}],"replies":[{"embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/comments?post=14543"}],"version-history":[{"count":0,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/posts\/14543\/revisions"}],"wp:attachment":[{"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/media?parent=14543"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/categories?post=14543"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/swling.com\/blog\/wp-json\/wp\/v2\/tags?post=14543"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}