{"id":6400,"date":"2016-02-09T11:40:00","date_gmt":"2016-02-09T00:40:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2016\/02\/09\/back-to-divide-by-4-big-improvement-in-receiver-performance\/"},"modified":"2025-07-21T20:52:32","modified_gmt":"2025-07-21T10:52:32","slug":"back-to-divide-by-4-big-improvement-in-receiver-performance","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2016\/02\/09\/back-to-divide-by-4-big-improvement-in-receiver-performance\/","title":{"rendered":"Back to Divide by 4 — Big Improvement in Receiver Performance"},"content":{"rendered":"
<\/a><\/div>\n

Thanks for all the comments and advice. I have come to understand the wisdom of divide by 4 IQ circuits. <\/b>

<\/b> Fortunately it was very easy to convert the divide by two 74AC74 circuit described earlier to a version of the divide by 4 scheme seen above. (From the SDR Ensemble II Receiver: http:\/\/www.wb5rvz.com\/sdr\/ensemble_rx_ii_vhf\/04_div.htm<\/a>) <\/b>

<\/b> This change provided a great way to observe 1) the improvement in the output signals from the VFO and 2) the resulting improvement in receiver performance, especially opposite sideband rejection. <\/b>

<\/b> Here are some numbers. I was very pleased to discover that my Rigol scope will measure duty cycle and phase difference. Thanks Rigol! <\/b>

<\/b> AD9850 Divide by 4 : 7.212 MHz Duty cycle: 48.3 Phase Difference: 87-90 degrees<\/b>

<\/b> Si5351 Divide by 2: 7.212 MHz Duty Cycle 49.6 Phase Difference: 83 degrees<\/b>

<\/b> Si5351 Divide by 4 7.212 MHz Duty cycle 49 Phase Difference: 85-90 degrees <\/b>

<\/b> Additional improvement came when I switched the power supply to the IQ inverters and Flip Flops. I switched from 3.3 to 5 volts: <\/b>

<\/b> Si5351 Divide by 4 7.105 MHz Duty Cycle 49.7 Phase Difference: 90 degrees<\/b>

<\/b> When I took the VFO box and put it back in the receiver with the divide by 4 scheme and the 5 volt supply I immediately noticed a big difference in performance. It was obvious that opposite sideband rejection was back to what I had had with the AD9850, perhaps better. <\/b>

<\/b> I have a quick and dirty method of measuring opposite sideband rejection: I put an RF signal into the antenna connector. I put the ‘scope on the audio output. I tune (on the desired sideband) for 1kHz audio and I measure the output voltage. Then, with the audio gain and RF sig gen output in the same positions, I tune to the opposite sideband, again tuning for 1 kHz, again measuring audio output. With the divide by 4 scheme and the 5 volt supply, the opposite sideband was so weak I had trouble measuring it. I estimate the rejection to be at least 32 db — this is back in the range of what I had with the AD9850, and significantly better than I had with the divide by 2 scheme. <\/b><\/p>\n

Now I just need to figure out how to get the Si5351 VFO sketch to tune above 42.94 MHz. For some reason it quits at this point, switching down to 2 kHz output, and keeping me on 30 meters and below. <\/b>

<\/b> Thanks again to Todd VE7BPO for a lot of help with the hardware and to Tom AK2B for help with the Arduino code. <\/b><\/p>\n","protected":false},"excerpt":{"rendered":"

Thanks for all the comments and advice. I have come to understand the wisdom of divide by 4 IQ circuits. Fortunately it was very easy to convert the divide by two 74AC74 circuit described earlier to a version of the divide by 4 scheme seen above. (From the SDR Ensemble II Receiver: http:\/\/www.wb5rvz.com\/sdr\/ensemble_rx_ii_vhf\/04_div.htm) This change … Continue reading “Back to Divide by 4 — Big Improvement in Receiver Performance”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":6401,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[126,137,287,54,40,61],"tags":[],"class_list":["post-6400","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-campbell-rick","category-dds","category-gale-todd","category-phasing-rigs","category-sdr","category-si5351"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6400","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\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=6400"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6400\/revisions"}],"predecessor-version":[{"id":6402,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6400\/revisions\/6402"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/6401"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=6400"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=6400"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=6400"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}