Friday, March 14, 2014

Cavity Filter

The A-B-C board combination can be re-configured via the SMA connectors to act as a simple spectrum analyzer with a tracking generator to measure filter responses.  That configuration is shown below.

The Si (spectral investigation) application steps the B board IF to be 10.75MHz above the  frequency of interest.  The C board synthesizer is set to the frequency of interest and the Aux output enabled.  The A board measures the power within a 250kHz window centered on 10.75MHz and converts to a dB 16 bit full scale.  This is repeated as the frequency of interest is scanned.  It isn't a true spectrum analyzer with tracking generator but it enables filter evaluation and tuning.  It takes a few seconds to scan a large bandwidth.

The figure below is the response of a home made single section coaxial cavity filter.  The gold line is a memory capture of the C Aux output directly into B RF input.  Not sure whats going on with the base response on the lower end (its stable and consistently like this - my guess is there is some kind of coupling going on between the boards in this frequency range).  The blue line is with the C board Aux connected to the filter.
This particular frequency response was taken prior to loop coupling adjustment so its very wide.  I have to admit that I've always been interested in constructing a cavity filter, however, never had the equipment to tune one.  This enables a reasonable tuning and response measurement.

The unit constructed was an initial test version to explore and understand the construction techniques outlined in "Quarter Wave Cavity Filters Using Copper Pipe, Mike Suhar, WB8GXB" and "Coaxial Cavity Filter for  Modularized Spectrum Analyzer, Scotty Sprowls".  The most significant difference is trying to use board edge SMA connectors rather than semi-rigid coax connectors and tubing (current price and availability). The pictures are prior to tuning (center tube not placed yet).  A single section is constructed for simplicity and learning.
Yes the top and bottom plate are a little wavy - they are copper pipe that was split in two and hammered flat.  When you've got the hack saw and propane torch out who can be bothered with waiting for the appropriate materials. The next step is to refine the construction techniques.

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