{"id":407,"date":"2019-03-14T20:13:20","date_gmt":"2019-03-14T20:13:20","guid":{"rendered":"https:\/\/geekblog.febo.com\/?p=407"},"modified":"2019-03-14T22:38:25","modified_gmt":"2019-03-14T22:38:25","slug":"peak-vs-average-power-in-complex-waveforms-and-gnuradio","status":"publish","type":"post","link":"https:\/\/blog.febo.com\/?p=407","title":{"rendered":"Peak vs. Average Power in Complex Waveforms (and Gnuradio)"},"content":{"rendered":"

One of my Gnuradio projects is a tool that records IQ data covering 375 kHz<\/A> — which is enough to capture an entire HF ham band (except for 80\/75M). A neat trick is to retransmit such a recording to allow local receivers to tune in The Signals From The Past (of course, with very low power and tiny antenna so as to prevent interference). For example, I can take a recording of 14.000 – 14.350 MHz and play it back from 29.000 to 29.350 MHz. I use an Ettus USRP-1 to do this; it’s capable of about 0 dBm output.<\/p>\n

If you think of the 275 kHz wide recording as a single waveform, it’s interestingly complicated. All sorts of signals of various amplitudes and frequencies appear and disappear, their phase vectors adding and subtracting. Such a waveform can have a very high peak-to-average power ratio. I decided to do some tests to see how much head-room is necessary to avoid distortion.<\/p>\n

I played back the recording with the USRP feeding a spectrum analyzer through a 20dB pad. I can adjust the level of the digital stream, and I can also inject a single CW tone at the center of the band. Here is a screenshot of three conditions captured using the “max hold” trace function:<\/p>\n

\"PEAK_AVG\"<\/a><\/p>\n

The orange trace shows the maximum power CW signal by itself — about -3 dBm. The “pedestal” at the bottom of the screen might include some distortion products, or might just be phase noise. If I deliberately overdrove the CW level, I got a very wide response that doesn’t appear here, so I think this is below the clipping level.<\/p>\n

The yellow trace shows the playback with a gain value of 200. It’s very clean with no distortion. There’s enough headroom for the maximum instantaneous signal to avoid clipping.<\/p>\n

The blue trace shows what happens when the gain is increased to 300 — clipping occurs and the occupied spectrum increases tremendously, though only for an instant at a time. The max hold function captures this; a real time spectrum analyzer would do a better job.<\/p>\n

The spectrum analyzer can also do channel power measurements. This capture shows the occupied bandwidth, and power within that bandwidth, of the unclipped waveform:<\/p>\n

\"CLEAN_CH_PWR\"<\/a><\/p>\n

Apparently no one is at the extreme top or bottom of the band, as the 99% bandwidth is 312 kHz (I’ve seen it show up to 350 kHz). The average power in that bandwidth is -22.75 dBm (remember, there’s a 20 dB pad). Assuming that there’s not much room to increase the drive before clipping sets in, that means there’s about a 20 dB difference between the average clean power, and the maximum CW power. <\/p>\n

Here’s what happened when I cranked the drive way up:<\/p>\n

\"CLIPPED_CH_PWR\"<\/a><\/p>\n

Average power is up by about 10 dB, but the occupied bandwidth has grown to 460 kHz. And if we could capture instantaneous values, it would be wider than that.<\/p>\n

This is why it’s so important to use clean amplifiers with lots of headroom for video and complex digital waveforms.<\/p>\n","protected":false},"excerpt":{"rendered":"

One of my Gnuradio projects is a tool that records IQ data covering 375 kHz — which is enough to capture an entire HF ham band (except for 80\/75M). A…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_lmt_disableupdate":"","_lmt_disable":"","ngg_post_thumbnail":0,"footnotes":""},"categories":[4,9],"tags":[49,13,50,48],"class_list":["post-407","post","type-post","status-publish","format-standard","hentry","category-ham-radio","category-sdr","tag-fieldfox","tag-gnuradio","tag-hamsci","tag-usrp"],"_links":{"self":[{"href":"https:\/\/blog.febo.com\/index.php?rest_route=\/wp\/v2\/posts\/407","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.febo.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.febo.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.febo.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.febo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=407"}],"version-history":[{"count":7,"href":"https:\/\/blog.febo.com\/index.php?rest_route=\/wp\/v2\/posts\/407\/revisions"}],"predecessor-version":[{"id":417,"href":"https:\/\/blog.febo.com\/index.php?rest_route=\/wp\/v2\/posts\/407\/revisions\/417"}],"wp:attachment":[{"href":"https:\/\/blog.febo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=407"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.febo.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=407"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.febo.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=407"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}