Simple I/Q transmitter test with Si5351 and amplitude modulating I and Q - Page 1
Forum section: electronics / rf-microwave

author: radio879
author_rank: Newbie
timestamp: December 06, 2019, 12:54:48 am
content: I just want to make sure I understand I and Q basics...From all i've read, I should be able to take the output from a sound card and GNU Radio, and modulate just the amplitude of the I and Q (was thinking maybe simple transistor amplifiers, modulate the collector) and it should work for all modulation types? (assuming everything is balanced enough)The Si5351 can output two signals one with a phase difference.. should be able to amplitude modulate just by having the sound card outputs modulate the voltage on transistor amplifiers.Just wanted to ask if i'm clear about this and this is technically all that's needed?

author: radiolistener
author_rank: Super Contributor
timestamp: December 06, 2019, 06:06:55 am
content: Quote from: radio879 on December 06, 2019, 12:54:48 amYes, quadrature IQ stream with any kind of modulation can be generated in software. So, on the output of your sound card you will have ready to use quadrature stream. In order to shift frequency up, you will needs quadrature LO and two mixers.Regarding to si5351, it's hard to generate quadrature output with precise phase shift, because si5351 has control register with limited phase shift resolution. But you can put the same in-phase frequency on two channels with enabled channel inversion for one channel. And then you can divide both output with high speed trigger and get very nice quadrature output with 90 degree phase shift.

author: mark03
author_rank: Frequent Contributor
timestamp: December 06, 2019, 05:17:49 pm
content: To understand this better, you might take a look at the "Softrock" series of receivers/transceivers. A Si5351-type chip is used to generate 4x the LO frequency, which is then divided into quadrature LO signals, and combined with the sound-card signals in a commutating mixer (often known as a Tayloe mixer). Simple and fun projects to build.

author: fourfathom
author_rank: Super Contributor
timestamp: December 06, 2019, 05:46:39 pm
content: Hans Summers (of QRP Labs) shows how to get very good quadrature outputs directly from the Si5351 in this presentation:https://qrp-labs.com/images/news/dayton2018/fdim2018.pdf. See pages 13-17. Apparently the precision is better than the typical "divide by four using an 'AC74" method.Hans explains why the supposed "333 ps" resolution of the Si5351 isn't an accurate description of the internal workings of the chip.

author: radiolistener
author_rank: Super Contributor
timestamp: December 06, 2019, 09:54:05 pm
content: Quote from: fourfathom on December 06, 2019, 05:46:39 pmThis method is well known, now it looks that it was invented by Hans. Thanks for the article.You're talking that it's precision is better than external dividers, but previously I was seen opposite information from someone other tests.In any case, this method has critical issues:1) It needs PLL reset on any frequency change, it will leads to terrible crackling sounds during smooth frequency tuning2) minimum output frequency is limited to 3.2 MHz, which don't allows to use it on 160 meters band

author: fourfathom
author_rank: Super Contributor
timestamp: December 07, 2019, 03:57:37 am
content: Quote from: radiolistener on December 06, 2019, 09:54:05 pmApparently not, if you change the frequency by changing the PLL feedback divider. This limits the frequency tuning range to about 1.5 : 1, beyond that you do need to change the output divider and do the reset. Hans mentions this in his document.Quote from: radiolistener on December 06, 2019, 09:54:05 pmYes this seems to be the case.

author: w2aew
author_rank: Super Contributor
timestamp: December 10, 2019, 12:02:14 am
content: Have you seen a few of my videos on IQ signals and IQ modulation?

author: radio879
author_rank: Newbie
timestamp: December 10, 2019, 12:29:44 am
content: QuoteYes! I've come across a lot of your videos...hours of watchingIt took me a lot of reading, and videos and more reading to really grasp fully what is going on, or enough where it doesn't seem like its some kind of magic. I really like how it works..A lot of what i was reading explained some of it, but still i felt like stuff was left out or it wasn't explained to me in the way my brain liked to process that kind of stuff. It still does seem a little magical.QuoteThanks for this, i read it and am saving it, very good stuff. Also I suppose if someone wanted to extend the freq range of a radio below 3.x mhz using the Si5351 there could be a cheap divider circuit etc switched into it for below 80 meters.The original question - the reason I was asking isn't necessarily to build a good radio but after reading about I/Q so long I am curious to do a quick test since I have a 5351 sitting here.

author: iMo
author_rank: Super Contributor
timestamp: December 10, 2019, 04:21:07 pm
content: There is the famous softrock series of kits you may tryhttp://www.wb5rvz.com/sdr/You may use the si5351 as the LO (at 4x the rx/tx freq).PS: I built several softrocks kits myself (long time back) and they worked really nice. You copy the same signals as with a bigger rig.