Dishal Filter Alignment with the NanoVNA

When aligning bandpass filters it only takes me a few minutes of twiddling before frustration sets in. I had previously used the method attributed to Dishal as set out in Solid State Design. It is a very effective method although there is some set up needed. 

Over the years I had seen some very vague, if not useless, explanations of how to do this with a VNA. So I set out to discover for myself how this was done.  I threw together a 2 section narrow band pass made with coaxial stubs, tuned with high quality beehive capacitors. In the process I learned not only how to tune these filters but also how to design them with coaxial stubs.

 

Here then is how I use the NanoVNA to align multistage filters. (apologies to keen eyed readers who might noticed the dates/times of these images are mixed up. I lost a few so I had to mix two filter tuning records I had)

1. I used OneofEleven's PC software to drive the nanoVNA but this guide should be near universal.
2. OSL calibration as required
3. Connect the filter to be aligned to the VNA. I'm going to use Channel 0 as the Input, Channel 1 as the output in this guide. 
4. Use a suitable frequency span and add a marker for the desired center frequency eg 146.5MHz
   
5.  Select a polar view of S11 and place a short to ground at Point A
   
   
   
   
   
   
6. Adjust the marker using the "e-delay (ns)" field so it lies on the X axis. This is for convenience, the Y axis would also work. 

   
   

   

   
   

   
   

   
   

   7. Move the short to Point B

   

   
    

   8. Adjust the tuning capacitor on the first resonator until the marker rotates to be on the X axis but opposite to the rotated starting point.

   

   
   9. Move the Short to point C and tune the second resonator
   

   

   
   Remove the short. That's it. The picture in point 9 makes an important point. The whole trace is now on one side of the plot. That's irrelevant. What is important is the marker is on the right hand side of the two points which intersect the X axis. It started on the left hand side in point 8. Just try this, it is easy once you have had a go. Your traces will look very different to mine but the procedure is the same.

   
   So how did it work? As expected:

   

   
   I then decided to rebuild this for a narrower bandwidth. My tuning caps were just a tad too small so I used a center frequency of 150Mhz. You will notice below the peak is at 149.5Mhz. That's because I used a piece of floating PCB onto which the coupling capacitors were soldered. It was getting mechanically depressed during tuning changing the stray capacitance around the filter. 
   

   

   
    
   

   73's