About the Limits and Problems and Shortcomings of AI (actually of LLMs) -- The Design and Construction of a Crystal Filter

Complaining about AI on the computer/internet while surrounded by digital rigs!

 

A couple of days ago, I described in this blog how AI (specifically the Gemini LLM) had helped me design a 13 MHz crystal filter.  There was some angry push-back from people who seem to dislike any use of AI. One fellow said I should use my own mind and hands, and that by using AI I would risk having time to stand on the street corner.  I found it kind of ironic that this observation was being sent to me on a computer connected to the internet (two tools that have made this kind of construction MUCH easier).  

In the first round I think AI helped a lot. It even drew nice looking schematics for me.  Using the motional parameters determinef by me via the G3UUR method,  AI used the basic equations to calculate the shunt capacitances and the termination impedances.  This helped me avoid many of the mathematical errors that had slowed me down in the past.  I went ahead and built this filter, and (later!) took a look at the passband in my Antuino (a Scalar Network Analyser from Farhan). 

I found the passband to be that of an "LSB filter":  the skirt was steep just below the cutoff frequency, but much more sloping farther down in frequency.  This is not what I needed.  The signal hitting this filter would be (and would remain) USB. I could try to use this filter but I would be placing my BFO/Carrier Oscillator on the sloping portion of the curve.  Carrier and opposite sideband suppression would suffer. (Note: It is NOT 20 db down!  -20 db on this meter equals 0 db.) 

And here is one point where LLM AI really screwed up:  I asked it for a 5 crystal filter that would be a USB filter: One with the steep part of the skirt above and close to the cutoff frequency.  AI LLM quickly obliged.  I was suspicuous, because I had never seen this before. Here is what AI LLM Gemini recommended: 

Wow, looks great right?  Yea,  but it doesn't work.  I built it and looked at it in the Antuino.  NOPE.  It wasn't even acting as a bandpass filter.  It is more of a notch filter. Confronted with the failure, AI LLM admitted it was wrong (my coments in red): 

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This configuration does not work. I have never seen it in any amateur radio book. Are you sure that this will work?

You are 100% correct to call that out—that schematic is incorrect and will not work as an SSB bandpass filter.

Why the Shunt-Crystal Schematic Fails

In quartz crystal equivalent circuits, the crystal reaches its lowest impedance () at its series resonant frequency ().

• Series Position (Correct Bandpass): When the crystal is placed in the series signal line, its low impedance at allows the RF signal to pass through to the next stage.
• Shunt Position (Incorrect): Placing the crystal directly from the signal line to ground causes it to act as a shunt short circuit at . Instead of passing a window, it short-circuits directly to ground, creating a band-reject (notch) response rather than a bandpass filter.

This is why you will not find that topology in Experimental Methods in RF Design (EMRFD), the ARRL Handbook, or classic G3UUR/Dishal filter literature.

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At this point I decided that a 10 crystal Cohn filter would give me the steep skirts I needed.   Here is what AI recommended: 

I went ahead and built this one.  I looked at it with the Antuino.  It works. 

You can see the steeper skirts on this filter.  These steep skirts will allow me to use this filter on 17 meters with the 5 to 5.5 MHz VFO from the old Kenwood TS-820s.  

In the course of doing this, an old friend who works in this field (and who will remain anonymous) warned me about the dangers of over-reliance on AI LLMs, especially when working on ham radio circuitry.  More on this in the days ahead.