Hello everyone,

Im pretty new to RF and have actuallly just begun working as an engineer. I have been looking around the internet for a while now and have been trying to figure out if there is a way to calibrate analog filters quickly using dsp? If there is I would seriously appreciate any advice on where and how to start off developing a method for calibrating a 4x4 array of bandpass filters. Thank you for any information you are able to offer!

Carneyc,

Can you provide more detail about what you mean by "a 4x4 array of bandpass filters"?  Also, how are the filters now tuned/calibrated?  Do they have variable inductors or variable capacitors or varactor diodes or...?

regards,

Neil

Analog Devices has some digitally controlled filters for RF applications if you haven’t looked there yet. I imagine other manufacturers have similar offerings as well, but I’m not super into the whole RF thing.  Seems like a good place to start.

The filters are designed by my company so sadly that isnt an option but I'll keep it in mind for reference if I figure out a way to do what I want! :)

Neil,

Thank you so much for replying! 

First off, I am an intern so I am not as well versed in the material as I would like to be so please excuse any statements that seem ignorant. I am also not the engineer designing the filter and RF is definitely not my strength but heres what I know. The filter is distributed element (using varactors) and is a bandpass filter that feeds into a notch filter, that configuration is then replicated in a 4x4 array product. Since we design all of our filters in house we have to calibrate them so I have been trying to find a faster way to calibrate them through dsp. So I honestly do not even know if Im accurately explaining what I am trying to do. Thank you so much for any advice!

Carneyc,

So presumably the varactors are controlled by DC outputs of some kind of DACs (e.g. sigma-delta DACs).  I imagine you have a DAC assigned to each varactor, and each DAC is fed from a register.  The register value thus determines the varactor's capacitance.

I also assume that you have a network analyzer that is used to measure the filter response (S11 or S21 or both).  If so, your task is to tune the varactors to get the specified frequency response.

Assuming I understand the problem, I'm not sure what is the best way to approach it.  It partly depends on the number of tuned elements (varactors).  If there are not too many varactors, you could start with the nominal voltage values, then do an iterative search as follows:

1. adjust DAC 1 voltage to optimize response.
2. adjust DAC 2 voltage to optimize response.
3. etc...
4.  adjust DAC1 again.
5. adjust DAC2 again.
6. etc...

You would need a definition of the response error, which could be the sum of the response errors in dB with respect to the spec limit at N frequency points of the response.  For frequency points that exceed spec, the error is of course zero.  

Also, you would want to keep optimizing beyond the published spec to have margin.

regards,

Neil