[islandlabs] Low Pass Filter for the $5 Transmitter

Burns, William burns at cshl.edu
Thu Aug 26 18:32:22 EDT 2010

I'm not sure I follow your capacitor logic but:
If you put a capacitor in series, it acts as a high-pass filter
(blocking low frequencies)
... but we're looking for a low-pass filter, so I was wrong to think
there would be a capacitor in series ...
I you put a capacitor in parallel (across 2 wires in a differential
circuit, or between signal and a ground reference) it would act as a
low-pass filter because the higher frequencies would be short-circuited
through the capacitor.
re: filtering out lower frequencies:
It's certianly possible to filter out low frequencies, but we have no
need of that, since all the harmonics would be higher that our primary
AFAIK, there should be be no lower frequency noise to filter out.
Re: enclosures:
I noticed that the filter was built in a box made of double-layer PCB.
I assume this acts as shielding, preventing signal from escaping from
our circuit before it reaches the filter...
That would mean that we'd also want to put the signal generator inside
the shielded box.
Is that right?


	From: list-bounces at islandlabs.org
[mailto:list-bounces at islandlabs.org] On Behalf Of Jonathan Dahan
	Sent: Thursday, August 26, 2010 1:12 PM
	To: Island Labs main mailing list
	Subject: Re: [islandlabs] Low Pass Filter for the $5 Transmitter
	With an inductor, the induced magnetic field resists alternating
current at a strength directly proportional to the frequency being
applied, so high frequencies are attenuated more, and low frequencies
are allowed to pass.

	With a capacitor, a low frequency provides enough time for the
capacitor to fill with charge, essentially letting the signal pass after
a short time, while high frequencies will not fully charge the
capacitor, and therefore attenuate more.

	I am not sure if this is the correct explanation, but it makes
sense in my head. Wikipedia confirms inductor for high filter, capacitor
for low filter.

	Combining low and high filters gets a band pass, so if we want
we can add some LC circuitry to filter out frequencies under say 28Mhz,
just for giggles, though (can a HAM confirm this?) its more important to
not be spewing harmonics over the spectrum than to create a nice
band-pass filter for our use.

	<3 wikipedia:
	- Jonathan
	On Thu, Aug 26, 2010 at 11:02 AM, Burns, William
<burns at cshl.edu> wrote:

		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
		> As Bill requested at last night's meeting, here is the
		> 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
		> -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
		> distant past (foil between microscope glass slide
		> foil rolled between a length of kraft paper, etc), but
		> going to look for some more modern techniques.
		> Unless, of course, this is going to cost more in
		> than simply buying them mail-order....
		> Joe
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