[hackpgh-ham] Re: LC Filter design website
- From: Ed Paradis <legomaniac@xxxxxxxxx>
- To: Doug Philips <douglas.philips@xxxxxxxxx>
- Date: Sun, 20 Mar 2011 21:54:36 -0400
The audio filtering is done all in software, but radios also have
filters on the antenna input as well. The Funcube Dongle has filters,
but they have to be super wide to allow signals through out the entire
range through.
Other radios with a very wideband receiver have the same problem. A
very well tuned antenna can act as a bandpass filter, so sometimes you
can get away with very little input filtering.
A kit receiver I built has electronically adjust able input bandpass
filters that change the pass frequencies depending on what band you're
listening to.
The Funcube Dongle could have done something similar, but with a
receive range from 64 MHz to 1200MHz, it'd have been a pretty tough
design to do cheap and small. Honestly, I'm not sure its possible
except to have several choices of bandpass filters swapped in and out
with relays.
Other software defined radios, like the Softrock kits, receive such a
narrow band in comparison that they have fixed band pass filters on
their input.
So the long and short of it is that you can get away without any
additional filtering on the FCD, but if you're planning on using it on
just a certain set of frequencies, you'll get better performance by
adding a band pass filter between the antenna and the FCD.
I should get mine soon and I'll do some experiments.
Ed
On Sun, Mar 20, 2011 at 1:41 PM, Doug Philips <douglas.philips@xxxxxxxxx> wrote:
> On Sun, Mar 20, 2011 at 12:18, Ed Paradis <legomaniac@xxxxxxxxx> wrote:
>>
>> If you have a Funcube Dongle, you might be noticing that you need some
>> filtering on the input because it is so broad band.
>
> Uhhh, wait, what? I haven't done the Funcube Dongle thing yet, but I thought
> the whole point was that all the filtering is done in software?
>
> -Doug
>
>
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