{"id":5592,"date":"2021-11-05T10:11:00","date_gmt":"2021-11-04T23:11:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2021\/11\/05\/the-importance-of-keeping-the-noise-flat\/"},"modified":"2025-07-22T18:04:30","modified_gmt":"2025-07-22T08:04:30","slug":"the-importance-of-keeping-the-noise-flat","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2021\/11\/05\/the-importance-of-keeping-the-noise-flat\/","title":{"rendered":"The Importance of Keeping the Noise FLAT"},"content":{"rendered":"<\/p>\n
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A few days ago I put up a blog post about using a noise generator (in my case my cheap FeelTech sig generator) and my TinySA spectrum analyzer to look at the passband of a crystal filter. I was using the 9 MHz filter used by Dean KK4DAS and the Vienna Wireless Makers Group. The idea is simple: insert broadband noise into the input. The filter should pass more of the noise that falls within its passband. The TinySA should let you see this. <\/b>At first, I was pleased that I could clearly see the passband. <\/b> I thought I had succeeded. See above. <\/b><\/div>\n

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But I was bothered by something. Look at that bump in the passband. It should be close to flat across the top. <\/b><\/div>\n

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I decided to take a look at the same filter with my NanoVNA. Here I was not using a noise generator. The NanoVNA sweeps the filter using and looks at output in the Log-Mag mode. Here is what it looked like (below): <\/b><\/div>\n
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That was much better. But why the difference? Tony Fishpool G4WIF suggested that my noise source might not be putting out noise at the same level on all frequencies. I took at look at the noise output of the FeelTech sig gen in the range of the filter passband (with some above and below frequencies for reference) and I found that the flatness of this noise depended a lot on what frequency I had the sig gen set to. I tuned it around a bit until I found a setting that produced a flat noise output in the desired frequency range. Then I went back and swept the filter with the noise and the TinySA again. Here is what it looked like with the “flat” noise: <\/b><\/div>\n

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Better, I think. Closer to the passband displayed by the NanoVNA.<\/b> <\/div>\n
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Tony points out that these Chinese sig gens don’t really put out random noise — they give us predictable noise. Dean said “Predictable Noise” would be a good name for a rock group. I said they could open for my favorite: “The Ceramic Spurs.” <\/a><\/b><\/div><\/p>\n","protected":false},"excerpt":{"rendered":"

A few days ago I put up a blog post about using a noise generator (in my case my cheap FeelTech sig generator) and my TinySA spectrum analyzer to look at the passband of a crystal filter. I was using the 9 MHz filter used by Dean KK4DAS and the Vienna Wireless Makers Group. The … Continue reading “The Importance of Keeping the Noise FLAT”<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":5593,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[16,329,191,71,226],"tags":[],"class_list":["post-5592","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-filters","category-fishpool-tony","category-nanovna","category-test-gear","category-tinysa"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/5592","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=5592"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/5592\/revisions"}],"predecessor-version":[{"id":5596,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/5592\/revisions\/5596"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/5593"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=5592"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=5592"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=5592"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}