Filter Fiasco: Chapter 2

For those of you that missed it, Chapter 1 of this saga can be found here.

We left off with the initial simulations looking peachy keen and me feeling hopeful that the hardware implementation would work. However, as these things usually go, the first time I fired the filter up nothing worked right. From Figure 1 below you can see nothing really met spec other than the response was bandpass shaped.  The center frequency for a single stage was 9 MHz rather than 10 MHz, the attenuation was much less steep then expected, and where the heck did the gain go?

Figure 1: Frequency Response of 1st BPF Stage

Clearly something was wrong and I spent a few days trying to figure out what exactly that was.  Going back to my simulation I started to “beat on it” as one of my professors would say.  I stuck small capacitors to ground (~10 pF) on various nodes to simulate board parasitics and watched the Bode response change. Sure enough, the input nodes to the op amps  were very sensitive to parasitic capacitance and I was able to reproduce the terrible response I was seeing on the spectrum analyzer.  Being somewhat stupid and inexperienced I had unfortunately decided to go against the datasheet’s suggestion on page 20 to remove ground planes from beneath the amplifier.

I didn’t have the option of ordering a new revision of the board so in order to fix the problem I had to get creative.  Eventually I went to ask the director of the surface mount soldering lab if he had any suggestions on the best way to partially remove my unwanted ground plane.  His suggestion, mill it out by hand using the small drill press in the lab.

Figure 2: My Operating Table

With sweaty palms and a beating heart I carefully began to mill out the ground plane beneath each of the three filter stages on my board.  Did I mention that I left the components on while do this? Well I did and the whole time I was working I was terrified that I’d drill straight through an op amp ruining the both the part and the PCB. Probably not the smartest thing I’ve ever done but after a few minutes I emerged victorious. Now to retest the filter and see if it would cooperate.

Figure 3: PCB Bottom Post Drilling

Sadly, things were still pretty crappy the second time around. I forgot to grab a scope capture of the response but it was pretty similar to Figure 1 only the passband was a few dB lower than seen there. It appeared my efforts had been for naught and having exhausted all ideas I could think of I decided to hit the books to see what was really going on in my circuit.

Stay tuned for the Chapter 3 for the stunning conclusion. I promise I will have it out much sooner than it took to get Chapter 2 out.

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