Thursday, March 27, 2014

Coaxial Cavity Filter - Unit #3

This attempt differs from the previous in two important ways.  First it uses a DB9 pin on the SMA post with some heat shrink tube.  Second it uses threaded brass rod to ease fixing the final tuning.  The rest is the same as unit #2 (1/8" thick brass plate, 1.25" x 3" as the base, 1" copper pipe for outer tubes at 2.25" long).  The loops are set at 3/8" from the base plate and in the case of DB9 female connector this is about at the top of the pin where the soldered wire starts. (Recall the idea with the DB9 pin is to provide mechanical attachment between the loop and the SMA connector while soldering it to the base).  The SMA connectors are 063 board edge with the board pins broken off and soldered flush with the base.  The following shows the SMA loop prior to insertion and the SMA loop inserted into the already soldered tubes.  The loop between the cavities is just coax through a hole drilled between the tubes, insulation left for the inter-tube hole, and the ends passing through the base plate.

This worked quite well.  Care has to be taken with the SMA loop as it may provide tension and push the post off the center of the hole (the heat shrink tube helps minimize this).  The filter loops were slightly tuned and the posts inserted and filter center tuning begun.  The following picture shows the final assembly.  A top plate was not needed for fine tuning at this point (may be attached in the future with tuning screws to fine tune the response).

The original section tuning was done using an A-B-C stack with the Si application.  This is good enough to adjust the inner tubes.  As noted by others, use of threads on the tuning rods does cause some problems.  The inner tubes do not always make good contact with the base thus creating times when the filter response drops out.  This isn't a problem when the jam nuts afix the rods in final tuning.  If you apply a slight amount of pressure good contact is made while tuning.  My plan was to solder the rods in place (the threads were only intended to hold the rods firmly while upside down to prevent any movement while soldering), however, the nuts have proven to be quite stable so far and provide the ability to modify the tuning in the future so I've elected to stay with them for the time being.

The following is the response as traced by the Si application (previous post on unit #1 includes block diagram). The blue is the response of both sections while the gold (its faint) is a memory of the response with no filter attached. Not shown, is the unattenuated loop where the C-Aux is input to the B RF input directly yields a ~ -50 dB level.  This indicates a roughly 8dB insertion loss.
There are a couple of issues going on here.  First I could not get both filter sections to fully overlap - I always ended up with the double peak response.  This may be due to over coupling between sections (need to go back and do some reading).  The other issue is the roll off is not as expected for a two section filter.  Part of the concern is the response without the filter (gold line).  To investigate this, I used a spectrum analyzer to conduct a manual filter response.  Those results compared to the Si results are below.
The baseline response scan (no filter) seems to have a lobed character when you scan out several hundred MHz.  It appears there is some kind of bleed through of the C board RF into the B board keeping the base power level high.  Before fine tuning this filter or constructing another 2 section filter to cascade I need to spend some time with the Si response.

All in all this approach is better than previous attempts.  Using 1/8" brass plate might sound nice and sturdy but it creates a lot of thermal ballast which makes soldering tricky.  The material I wish I had read *prior* to starting this is from Sam Wetterlin.  He has two documents far down the page that go into great detail on variations on cavity filter construction.

Friday, March 21, 2014

Coaxial Cavity Filter - Unit#2

The second try at building a cavity filter takes a slightly different approach.  This time I used 1/8" brass plate and the SMA connectors are mounted on the outside rather than the inside.  This avoids the problem with shortening the cavity with the SMA base protruding into it and the solder paste and flux running down the connector threads clogging them up. 

This filter is also constructed as a two section filter with each built independently and inter-section coupling done externally.  The following pictures capture the construction.  Starting at upper left and moving clock wise they are: a) second section SMA loop connections prior to mounting the outer tube, b) inside the filter after loops have been bent around and soldered, c) outside view of both sections, d) end view of filter.
The SMA connectors were soldered first, the loops constructed, then the outer tube soldered in place, tuning and then inner tube soldering.  The first section worked fine.  Indirect heat was used on the outer tube to minimize heat to the base and SMAs.  Amazingly enough this worked well.

The second section was a different story.  After initial tuning and alignment with the first section, the inner tube slipped during final soldering.  While trying to correct this one of the SMA connectors shifted and shorted the post against the side of the insertion hole.  At some point the filter was dropped breaking the solder on the outside tube of section 1. (hint: whats the visual difference between hot and cold brass - none, other than the blister after touching it)  When it was all said and done both sections were corrected and worked, unfortunately their response centers were too far apart to be pulled together using 1" 6-32 tuning screws. [ being off by 1/16" inner tube length difference is roughly 75MHz and the furthest I can pull with the fine tuning screws is about 28MHz ].

The SMA board edge connectors on the outside appear viable, proper soldering and construction order should help, and tapping the center tube will avoid some of the final assembly and adjustment problems.

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.

Friday, March 7, 2014

BBB Performance Notes

As part of this activity some of the BBB processor and network performance have been measured, investigated and characterized.  One of basic points is that 1MSPS (treated as 16 bit samples) is readily processed by the ARM processor and transported over the network.  In the SDR server application a down conversion and integer FIR filter is applied to the samples and sent via UDP.  This takes on the order of 87% of the processor (measured via “top” application).  While the processing is minimal, it is sensitive and on the edge.  If "–O3" is not used in compiling the FIR filter, the processed sampling rate drops from meeting 500k complex samples/second to fluctuating around 300k complex samples/second.

One of the open questions which I had no data on was the FFT performance of the ARM processor.  The Si (spectral investigation) application was designed to conduct all processing except display on the BBB.  This allows for thin java clients for control and presentation only (e.g. tablet or phone).  The FFT performance is an important aspect of spectral evaluation in the application (at this point I haven’t moved to poly-phase filter but wanted to focus on FFT based processing as a start).  The original work used a simple FFT in C from a reference text.  This approach was intended to be instructive, not high performance.  This was then updated to use the FFTW package.  The table below captures the measured FFT performance with a magnitude squared calculation on the ARM comparing both implementations.  Note: These are double implementation FFTs (not integer – which will be evaluated later if need be, wanted to start simple), also the mag squared operation appears to take a very small fraction of the time.
FFT Size
Time (uS) Reference
Time(uS) FFTW
The impetus for focusing on this metric is that in a spectrum analyzer like application, one of the key metrics is the refresh rate at a given frequency span.  Based on the hardware at hand this translates into frequency stepping speed.  The driving aspect of this is the collection of samples.  Based on previous noise measurements, a good starting point seems to be around an 8k FFT.  At 1MSPS the collection of 8k samples will require 8mS [i.e. (8E3 sample)/(1E6sample/sec)=8E-3 seconds ].  Sample collection can be overlapped with power spectrum estimate calculation (i.e. FFT) and transmission of results.  So the bottom line is the target is 8mS per 8k FFT which is not being met based on the data above.  There are a couple of options including switching to an alternate power spectrum estimate technique or evaluating integer FFT performance. (FFT3.3.3 includes ARM NEON support).  This will be deferred until further hardware characterization is complete.  An interim target of ~30 steps per second appears readily achievable which if we use 250kHz per step yields a sweep rate of 7.5MHz per second.