Face the TRUTH! LOOK at Your Signal with an Antuino!

At first, I didn't want to believe it.  I was in denial.  I wanted to shoot the messenger (in this case, the Antuino). How could my beloved HB 40 meter DIGITIA transciever have an output that was so.... so DIRTY!  Everyone tells me it sounds great.  But the little Antuino screen told a different story.  Strong spurs up at 9 MHz and down at around 5.4.  And lots of places in between.  (In these display images, the center frequency is 7.2 MHz and each division to the left or right is 1 MHz.) 

Farhan tried to get me to face the truth: "The frequency domain viewing of RF Signals is the opening of the third eye. Once you start seeing signals as a bunch of simultaneous  sines, you will always be wary of the waveforms on the scope. In fact, time domain readings make little sense."

At first I blamed strong VHF RFI and my somewhat hay-wire test set up.  My homebrew Rube Goldberg 20 db attenuator was probably picking up some of the VHF RF. But as I looked more closely at the output of the transceiver in the frequency domain, I gradually accepted that it was true.  There were a lot of spurs.   I have a general coverage receiver in the shack, and with it I could hear the little devils. And after some adjustment I could see them in the FFT display on my Rigol o'scope.  An exorcism was definitely needed. 

But first came a tightening up of the test setup.  Pete advised me to do this.  I had in the shack some really nice dummy load/attenuators from the HP8640B Signal Generator that Steve Silverman had given me (and that Dave Bamford had hauled across New York City for me).  I ordered the necessary N connectors and adapters and soon my test setup improved a lot. 

All this got me thinking about spurs.  I consulted EMRFD and was reminded of a really great program in the LADPAC software pack that came with the book.  The SPURTUNE program predicts spurs and tells you what to look out for.  It is really illuminating.  Try SPURTUNE. 

Through this, I gained a better appreciation of the importance of the bandpass filter in an SSB transceiver.  I'd always thought of it as something that allowed the other mixing product to be eliminated while passing the one you want.  But I came to realize that it does a lot more than that -- it also helps get rid of spurs.   If it is designed right.  Mine was not.  I had plucked it out of an old QST article and had not paid much attention to it. All it needed to do was knock down the unwanted mixing product, right?  And in my transceiver (9 MHz IF,  VFO running 16.0 - 16.3) MHz that unwanted product would be way up at 25 MHz.  It wouldn't take a lot of selectivity to knock that down.  But I'd forgotten about the closer-in spurs.   Antuino reminded me of them.  And SPURTUNE explained where they came from.  

For the exorcism, I decided to use the bandpass filter design from Farhan's BITX-40 Module.  I had made the BP filter on this rig "plug-in" so it was easy to build a new filter.

I even checked out the filter design in a simulator.  For this I use ELSIE.   Another very useful program.  Here is what ELSIE predicted for Farhan's BITX40 Module filter: 

I plugged the new filter into the DIGITIA and... SUCCESS!  The big spurs that were bothering me were gone and the remaining spurs were all below 50db down from the main signal. Here is what it looks like now:  

The Antuino is a very useful device.  You can learn a lot from it, but you have to realize that this is not plug and play radio.  You have to think about what you are testing, make sure you have the test gear set up properly, think about the circuit you are looking at, and be careful not to put too much RF into the device.  

Three cheers for Farhan and his new Antuino!  More on this soon.  And we will talk about this on the next podcast.