Si5351 Phase Noise? A Tale of 3 Oscillators

Si5351 at 16 MHz

 

There is still a lot of talk about the supposedly horrible phase noise of the Si5351 chip. In a recent episode of a popular (and very good!) podcast about homebrewing, the podcasters talked about this in the context of some megawatt AM shortwave broadcast stations that had oscillator phase noise problems and were wiping out large portions of the HF spectrum.   I don't think those stations were running Si5351s, but the listener was left with the impression that these handy little chips are very noisy with lots of spurs and will inevitably produce horrible dirty, spectrally impure signals. 

 

This has not been our experience.   Following Pete's lead, several of us are using the Si5351 to generate both VFO and BFO signals in our transceivers, with good results. The receivers sound very good and we have not heard complaints of "broad" or "noisy" transmitted signals.

 

I decided to dig into this a bit.  This was also an excuse for me to use the FFT and screen capture features on my Rigol 'scope.  

 

I now have THREE BITX transceivers in the shack.   My BITX17 uses a VXO at round 23 MHz (IF at 5 MHz)/  My BITX20 uses a classic LC VFO running around 3.5 MHz (IF at 11 MHz).  Finally, my BITX40 (DIGI-TIA) uses the dreaded and much reviled Si5351 running at around 16 MHz (IF at 9 MHz).   I thought that these three rigs would provide a good opportunity to test the scurrilous claims about the Si5351.   

 

As a simple first test, I put my Rigol scope in FFT mode and just put the probe at the VFO Mixer's LO input.  The screenshot above is the FFT for the Si5351.  It looks pretty clean to me.  The 'scope is looking at 15 Mhz above and below the VFO signal.    

 



VFO at 3.5 MHz

 

Next I measured the output of the BITX20 VFO at the same point (input to the VFO mixer).  (I had to change the vertical range, but the horizontal was unchanged.)  Here you can see the second harmonic (just because at this low freq it is within the freq range setting of the 'scope).  It doesn't look much different than the Si5351.   



VXO at 23 MHz

 

Finally, here is the BITX17 VXO at 23 MHz, again at the input to the VFO mixer.   It looks remarkably similar to the Si5351, don't you think? 

 

More on this to come.  The ARRL Handbook (2002) has a good discussion on phase noise. I am digging into this and hope to do some more tests.   For now, I think we should reserve judgment on the utility (for us) of the Si5351.  



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BITX DIGI-TIA Update #4

 

Lots of progress over the long weekend.  Along the back you can see the three stages of the RF amplifier chain:  2N3904, 2N2218A, IRF510. I intend to add inter-stage shielding, and one long shield between the chain and the rest of the rig.   Front and center is the Si5351 board (code by LA3PNA).  Mic amp (which gave me a lot of trouble!) is to the right of the Si5351.   AF (RX) amp is to the left (2N3904 and LM386).  The big board in the center has the three Termination Insensitive Amplifiers (thanks Wes and Bob), the 9 MHz filter (thanks Steve Smith!) and the two SBL-1 mixers (thanks to Pete Juliano for the "mil-pad " boards).  And thanks to Farhan for the overall BITX concept and the TIA suggestion.  

 

In the upper right you will see where the plug-in low-pass filter will plug in (a better picture appears below).   I will use a similar plug-in for the band-pass filter.   Combined with the BFO/VFO flexibility of the Si5351, this plug in feature should allow me to cover many bands with this rig.    

 

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VK2YAC's Yard-long, In-The-Yard, Bidirectional, Backyard Si5351 Rig

Alf, VK2YAC, wrote to Pete thanking him for the inspiration provided by the LBS project that Pete and Ben launched via QRP Quarterly.   Alf has obviously taken to heart Pete's recommendation re using a BIG breadboard.  

On his web page Alf writes: "Build your homebrew station and join the revolution, it's lots of fun!"   We're with you Alf! 
Alf has more great pictures of his BBR-40 rig and short descriptions of the sub-circuits he has used.  Note Si5351 with Arduino Pro-Mini for BFO and VFO.

http://qsl.net/vk2yac/Page2.html 


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Once again, into the digital morass...

Well, it has happened to me yet again:  After a somewhat happy period of stubborn luddite rejection of digital technology, yesterday I decided to jump into the fascinating world of Si5351 PLL chips.  I don't have to tell you who is responsible for this decision.  But he is of Italian descent and lives in Southern California.

I'm working with an Arduino Uno board and a prototyping shield sent to me by the same Italo-Californian gentleman.  The physical wiring of the Si5351 was remarkably easy -- just four connections.  Wow, I thought, this is going to be easy.  WRONG. 

Lady Ada's sketch and library compiled and uploaded without difficulty and the little device was simultaneously spitting out 125 MHz, 13 MHz, and 10 kHz energy.  Pretty cool.

But I got into trouble when I tried to load some of the VFO circuits out there.  Most of the trouble was caused by my errors in managing all the libraries and such.  But as I was telling Billy, it really seems to me that the folks who work on this stuff are using another language. And the problem is that it is a NON-HUMAN LANGUAGE.   If it were French or even Chinese, it would probably be easier. But you end up getting error "messages" like "U-int-8 has not been declared!"   Oh, I see.  I found myself thinking back fondly to my recent problems on 40 meters with the screeching amp and the helpful ESSB guys.  Ah, the good old days.

So, I'm not going as far as declaring microcontrollers to be demonically possessed (remember the regens?)  But I do suspect extraterrestrial involvement in these little devices. 

In the end, I got several of the VFO sketches working.  You get a sense of how new the Si5351 VFO work is when you find yourself using code that was written last week or last month.  Thanks to Lady Ada, Pete N6QW, Jason NT7S, Tom AK2B, and Thomas LA3PNA.  All of these folks are doing great work, moving the radio art forward (even if they are working with ET). 


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The Slaughter of Inductors, Capacitors, and Crystals Continues...

 

Oh the humanity!  Pete Juliano has turned his digital attention to previously completed projects.  Perfectly fine rigs -- whose only crime was that they used LC or crystal (VXO) oscillators -- are now being ARDUINOED by the maniacal Dr. Juliano.  Not even Huff and Puffs will be spared.  This is like the French Revolution!   We hear that Pete has even mastered the art of cutting the square holes that are needed for his "displays," so there is no stopping him now.  

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A Box Filled with DDS

 

I put this in a box this morning.  Arduino + AD9850 shield by Paul M0XPD and Kanga UK.  Code by AD7C.  One stage amplifier by Pete Juliano N6QW.    Useful as a VFO or a sig generator.  Thanks to all involved.

 

Note the cool translucent feet!

 

 

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A Really Cool Idea: Use your BFO as your Arduino DDS Clock

(21)  If you add a microprocessor chip to augment some of the BITX 

functions, it might be advisable to use the 10 MHZ BFO signal to clock 

your uP.  This could help avoid unwanted 'birdies' from the uP oscillator 

getting into your BITX circuitry.
From a nice list of BITX mods: 

http://kambing.ui.ac.id/onnopurbo/orari-diklat/teknik/homebrew/bitx20/modifications/LIST%20(9-24-04).txt

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Pete and Ben's "Let's Build Something" Reference Page

Lots of tribal knowledge here!  That's the direct conversion receiver that forms phase 1 of Pete and Ben's "Let's Build Something" project.  Arduinos!  Si5351s!   AD9850s!  And it will morph into an SSB transceiver.  Check it out: 

http://www.jessystems.com/LBS_Detail.html

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DuWayne's Scalar Network Analyzer -- In an Altoids Tin!

DuWayne (KV4QB) has done something very cool here.  He's taken an Arduino Nano, a cheap AD9850 DDS board, a small screen, and a couple of log detectors, and he has built IN AN ALTOIDS TIN a scalar network analyzer that lets you see the bandpass of a filter. (We posted an earlier version of this here: http://soldersmoke.blogspot.com/2015/01/duwaynes-ad9850-arduino-tft-swr-scanner.html ) Wow.  I've been doing this by hand, changing the input freq at 100Hz increments, measuring the output, putting the results into a spread sheet, converting to log (db), creating a graph...  DuWayne makes it a lot easier.  DuWayne is being encouraged to write up the results, possibly for QRP Quarterly.


Hi Guys
Started playing around with the SWR scanner that I had been working on.  Waned to see how hard it would be to make a very simple scalar network analyzer out of what I had.  Really wanted something small to use for checking bandpass other filters.   Hoped to get about 30 db. of range ,which should be enough for most filters. I have a couple of  8307 log detectors, but was afraid that it would be a pain getting it working and shielded in an Altoids tin along with the rest of the circuitry.  Went with something even easier than the resistive SWR bridge I already had.  Replaced the bridge with two basic diode RF probes, and changed the amplifiers so I could adjust the gain.  I use one to measure the direct output of the 9850 DDS module, and the other for the output of the device under test.  Kept the same control function as in the SWR scanner.  A short push on the encoder button starts a sweep of the selected band.  Holding it down for over a second cycles through the bands.  Once a scan is done you can use the encoder to scroll through the sweep.  I display the frequency and iDUT value in db relative to the output of the DDS module.  The USB connector is available and different start and stop frequencies can be entered if needed when working with IF stages.

Well it worked much better than I had expected.  After a simple adjustment of the amp gains with the output looped directly to the input, I was getting nearly 50 db with the loopback removed.  Just using some standard value resistors, in a pi attenuator I got a very nice looking sweep that was within a couple db of the 40 db i had built it for.  Since I only used standard value resistors, I though this was good enough.

Then I used  ELSIE to design a 14mhz lowpass filter, again used standard values for L and C that I had on hand .  Really happy with the results I got.

Finally I grabbed 3 crystals out of a bag without checking frequency or other parameters, I threw together a basic crystal filter.   Used the USB interface to  set the sweep range, I was really really really pleased with the results I was able to obtain.

 

The software still needs a little tweaking and a couple of additional functions I want to add, but I think this will be a very nice tool.  Plan on giving it a try when I build Pete's Lets Build Something transceiver.  Amazing what you can stick in Altoids tins, even if you have to stack two so you can include a battery pack .

Attaching some pictures of the progress so far.  As you can see that with what I used to build the test fixtures, I am amazed that they even worked at all.
73 DuWayne

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Video of Colin's First Contact with his Homebrew Scratch-built BITX20

I love this video.  Colin finished his BITX a week or so ago and has been waiting for an opportunity to test it.  Over the weekend he braved the winter of Northern England and, with his son,  set up his new rig out in his snow-covered garden.   Appropriately for a first contact with scratch-built rig, the circuitry was unencumbered by any kind of case or box.  That's the way it should be done!  Well done Colin!  You are well and truly a member of the International Brotherhood of Electronic Wizards, and the diagnosis of "The Knack" has been confirmed (a severe case, it appears). 

Hi Pete and Bill,

It's been a lovely fine day here in West Yorkshire, so I took a table out into the garden and set up my BITX circuit on it. I set up my SOTA dipole on a 9m fishing pole.

I heard a strong German station calling CQ, so I gave him a call and hoped for the best!

How amazing to contact someone in another country using a rig and mic you've made yourself! Do I qualify as a REAL radio ham now? Do I have a confirmed case of the knack? :-)

Although I may appear underwhelmed in the video, (besides the air punches!), I did really get a kick out of the QSO.

73 and huge thanks to both of you for the encouragement and support.

Colin, M1BUU

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Back Into the Digital Morass

Using a very nice M0XPD AD9860 shield from Kanga-UK and software by Richard AD7C I put together another little DDS VFO.  This one went together without a lot of hair pulling.  I put the LCD, brightness control and rotary controller on one piece of vero board -- this will also serve as the front panel.  

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