January 3, 2012

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Homebrew Filter Design and Measurement Techniques  

In our 7th week, we now extend the mini-tutorial from the Design Considerations phase covered last time to the Design and Measurement phase. 

For those who would like to actively participate with us during the upcoming session, our Notes (below) lay out the basic design specification for the Low pass Filter.  The goal is to select the appropriate component based on those specs and actually build up the simple circuit.

We'll be doing this ourselves as well, of course, and the discussion topic will be the step-by-step design process used and the start of techniques for empirically measuring the results of the design.

~George N2APB & Joe N2CX

Audio Recording ... (Listen to MP3 recording)

Attendees ...
George N2APB, Joe N2CX, Bill W7WMB, Bob WB6KWT, Cecil K5NWA, Charles WC5MC, Chris KJ4GUU, Chris W0ANM, Dave AD7JT, Dave G1OYG, David KB4FXC, Douglas KA2UPWChuck WA6LTV, Fred WA8PGE, Howie K3HW, Larry W2HHV, Jim AF9A, John KJ6CVB, Joe WA2QZP, John K5JHF, Ken VA3KMD, Larry W2HVV, Milt W8NUE, Paul AK1P, Paul VA6PW, Randy WB0SMX, Ray K2ULR, Rick K3IND, Sid G3VBV, Terry WB4JFI

Text Log from tonight's session notes ...
"George - N2APB": http://www.njqrp.org/teamspeak/Jan%203.html
"George - N2APB": Thanks to Larry W2HHV for the correction last week on pass band attenuation/insertion loss.
"George - N2APB": AADE = Almost All Digital Electronics ... great little company ... http://www.aade.com/filter32/download.htm
"George - N2APB": KitsAndParts Toroid Winding Calculator ... http://kitsandparts.com/tcalc.html  ... Diz W8DIZ provides great service and parts for us homebrewers
"George - N2APB": Rolloff frequency is the highest frequency of the LPF that will be passed without any major attenuation.
"George - N2APB": The "pass band" are the frequencies below the rolloff frequency.
"George - N2APB": The "stop band" are the frequencies above the rolloff point.
"George - N2APB": The rate at which the roll off occurs is important in determining how well your filter is performing.
"Joe - N2CX": Manhattan type technique excellent for RF - has a solid ground plane and short leads.
"George - N2APB": For those looking at the web page live on the Internet, I have updated the page to include Joe's circuit diagram and plots. (For some reason they were omitted at first.)
"Joe - N2CX": AA908 is a handy platform using a microcontroller that can be used for a bunch of applications beyond simply antenna measurments.
"Chris - w0anm": When will the AA908 be available again, this summer?
"George - N2APB": Actually looking to be offering the Kit again later this month.  Lots of renewed interest by some, as well some add-on accessories (like the Sweeper Input Card, USB adapter, improved reflectometer, graphics display, and more.)
"Terry WB4JFI": Where does one get the island cutter?
"Joe - N2CX": I think that QRPME is now selling a cutter for the Island technique
"Ray K2ULR": Harbor Freight has a tool that will do the job.


Mini-Tutorial Notes ...



Desired performance:

Suggested free tools & design programs:


Preparation Notes for January 3rd online session:


Using the AADE Design software is pretty straightforward.  Essentially what we'll do is specify a design type, enter the design parameters, change some component values to conveniently-available ones, and then run an analysis plot to see if the design goals were met.


The two main windows we'll be dealing with are the Schematic and Analysis windows, as shown below and illustrated first in the very useful Help file for the design program.



Step 1: Specify the filter


First we specify that we want an eliptical (cauer) filter, and a window appears for us to enter the basic design parameters ..



Step 2:  First-pass schematic


Then a filter schematic appears with some starting values.  But you'll notice that some of the values - specifically the capacitors - will be pretty hard to find when it comes time to building the physical circuit.



Step 3:  Change to convenient (capacitor) values


So what we can do is right-click on each capacitor in the schematic and select "Change Value".  Then in the pop-up window that appears, enter the nearest standard capacitor value that is commercially available.  Shown below is a page from the Digi-Key catalog (http://dkc3.digikey.com/PDF/US2011/P1994.pdf ) for silver mica capacitors showing some standard values you might choose from ...




Step 4:  Final LPF Schematic


After changing the capacitor values to something that can be achieved, the LPF schematics are shown below.  N2APB and N2CX approached the component selection task a little differently and you can see that the results are similar, but the performance is going to be substantially better with N2CX's (on the right).



N2APB design utilizing the spec's 0.5 dB rippple spec                                             N2CX design utilizing 0.1 dB ripple spec

                            (L3 = 15T #28 on T37-2,   L5 = 14T #28 on T37-2)                                                      (L3 = 18T #26 on T50-2,   L5 = 17T #26 on T50-2)


Step 5:  Analyze the Design


Assuming that we can get close to the desired inductance when winding the toroids in the construction phase, let's take a look at what the resulting performance would be for this LPF design.  You can see the plot by selecting the Analyze --> Power Effective Gain


N2APB design  (0.5 dB ripple spec)

N2CX design (0.1 dB ripple spec)

Passband Response (N2CX design)



        And for further pondering, you can look to see what the impedance looks at the input across the range of frequencies, by selecting Analyze --> Input Impedance ...


Input Impedance (N2APB design)



       And lastly, it can be interesting to look at the phase plot by selecting Analyze --> Phase ...


Phase Response (N2APB design)



Step 6:  Toroid winding selection! 


Using the KitsAndParts design tool (http://kitsandparts.com/tcalc.html ), we see ...




Knowing that the iron powder core "mix 2" is good for the frequencies of interest for us (7 MHz), we'll pick a convenient size (0.37") ... and our resultant L3 indictor will be 18 turns on a T37-2 core. 


Follow the same logic for the remaining inductor L5 yields 17 turns on a T37-2 core. 


Using any of a number of other references, we find that winding the toroids with #26 magnet wire is ideal for us.  ("Data Book for Homebrewers and QRPers" by Paul Harden, NA5N".  You can also check this in the ARRL Handbook.)


Note that it would be a good check to actually measure the toroid inductance after first winding them to these number of turns.  By placing the toroid on your AADE "L/C Meter IIB" (you do have one of these, right?! ... http://www.aade.com/lcmeter.htm  ), you can fine tune the inductance by adding or removing a turn of of the wire through the toroid core.



Step 7:  Build the Low Pass Filter!


We used a popular technique called "Manhattan-style construction", using a piece of copper-clad pcb material and drilling "islands" using a 5mm end-mill.  (See the "Islander Amp" article from QRP Homebrewing magazine for details on this approach ...  http://www.njqrp.org/islanderamp/Islander%20Amp%20Manual%20v1.pdf )


(Click on photo for larger view)



Step 8:  Measuring the LPF performance --


The approach we'll use for measuring the LPF performance is to inject frequencies into the input port of the filter and measure the output port. 



At the N2CX RF Palace, Joe is using an HP8640B signal generator to deliver a stable, precise and adjustable RF signal to the input of his LPF.  For measuring the filter output, he is using a new and specially-designed RF input probe (called the "Sweeper Input Card") that measures RF signals ranging from -60 to +10 dBm using a shielded, low-noise log amp.


HP8640B signal source feeding LPF input, with the precision RF probe (the "SIC") on the LPF output.
DVM reads the DC representation of the RF signal.

(Click on photos for larger view)



At chez N2APB, George is using a Micro908 Antenna Analyst in "VFO" mode to deliver a stable, adjustable DDS-based signal to the input of the LPF.  For measuring the filter ouput, he has integrated the Sweeper Input Card into his Micro908 and added a capability in VFO mode to read the log amp output and display the result on the LCD.  This provides for convenient scanning and numeric display of the filter performance.  [The Sweeper Input Card will soon be available as an add-on accessory to any existing Micro908 instrument; and when coupled with an automated frequency sweep and measured data sent out the serial port to a PC, a nice "filter performance" chart similar to the Voltage Gain plot shown above may be achieved.]


Micro908 Antenna Analyst, with built-in precision RF probe (the "SIC),

generates a precision DDS RF signal while simultaneously measuring the response of our Device-Under-Test (i.e., the LPF)

and displaying the frequency and signal output level on the instrument's display.
(Click on photos for larger view)







Step 9:  Analyzing the measured data ...



[Coming next ... ]

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