[islandlabs] Low Pass Filter for the $5 Transmitter

Burns, William burns at cshl.edu
Thu Aug 26 11:02:18 EDT 2010


I assumed that a low pass filter would have a capacitor in series with
the signal.
This filter is putting inductors in series instead.
It's interesting, but I don't know nearly enough about LC circuits to
figure out why multiple inductors are being used and what the component
values should be.

Just 'cause Joe outlined the use of PNP transistors for turning the
keycam on+off via a microcontroller..
Here's some good/interesting info on PNP transistors.

It looks like we've got all the technology pieces required to build this
transmitter, read a pressure gauge, and transmit that info back to the
Flyable APRS and/or GPS could be 'round the corner too. 


> -----Original Message-----
> From: list-bounces at islandlabs.org 
> [mailto:list-bounces at islandlabs.org] On Behalf Of Joe
> Sent: Thursday, August 26, 2010 6:48 AM
> To: Island Labs main mailing list
> Subject: [islandlabs] Low Pass Filter for the $5 Transmitter
> As Bill requested at last night's meeting, here is the Web 
> page I found that describes a low-cost low-pass filter for a 
> transmitter on the 10-meter band:
> http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12590
> "This simple 7 element, Chebyshev low pass filter, rolls off 
> at 30MHz. 
> Attenuation is better than -40dB at 60MHz, -65dB at 90MHz, 
> -80dB at 120MHz."
> As most of the current cost in building this transmitter is 
> now in the silver- mica capacitors, I'm going to start 
> researching how to build capacitors. I've done this in the 
> distant past (foil between microscope glass slide covers, 
> foil rolled between a length of kraft paper, etc), but I'm 
> going to look for some more modern techniques.
> Unless, of course, this is going to cost more in materials 
> than simply buying them mail-order....
> Joe
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