Re: [Discuss-gnuradio] AM receiver?

[tongaloa]

Suggest using a simple LC lowpass filter on input
C in series, L to ground
This will prevent electrostatic buildup on your antenna from
damaging the board and with appropriate component values, place an
upper limit on the frequencies reaching the board.
-Bob

Ben Gelb wrote:

> Not a whole lot to add over what Eric has said here, but AM does work 
> just fine. I connected my Measurement Computing 4020 board directly to 
> an antenna without any external filtering. It's not terribly elegant, 
> and you do leave yourself open to some problems with aliasing, but it 
> will work - you will hear plenty of stations.
>
> I also didn't need an amplifier of any sort. Local AM stations were 
> very stong as well as quite a few shortwave broadcast stations. I do 
> have a large (over 100') wire antenna that I use though, which helps.
>
> There is a screenshot here http://www.tjhsst.edu/~bgelb/techlab/wtop.html
> the biggest thing of interest is that there are lots of strong 
> signals! (Upper left FFT).
>
> You really don't want to see the code for that app though. It worked 
> but ... suffice to say its not fit for consumption. The first code I 
> wrote with GNURadio.
>
> I hope this is helpful.
>
> -Ben
>
>
> Eric Blossom wrote:
>
> > On Mon, Feb 02, 2004 at 11:27:50AM -0700, Jung Ko wrote:
> >
> > > Hi there,
> > >
> > > I'm new to GNURadio mailing list, so apologies if this question has 
> > > been asked/discussed before.
> >
> >
> >
> > Welcome!
> >
> >
> > > I'm interested in receiving AM radio signals. Here I have some 
> > > questions that hopefully someone will be able to answer me:
> > >
> > > 1. Since the frequency is not that high, I suppose it is enough to 
> > > hook up
> > > the antenna directly to the ADC?
> >
> >
> >
> > You'll need at least some minimal bandpass filtering to restrict what
> > the A/D sees.  If you've ever grabbed a scope probe with your fingers
> > you've probably noticed a huge 60 Hz signal!
> >
> >
> > > 2. If 1) is true, then is it still necessary to write software 
> > > frequency
> > > mixer to shift down the frequency to IF (around 455Khz)?
> >
> >
> >
> > Assuming you're using a fast enough A/D (approx 4 MS/s for broadcast
> > AM [540 to 1700 kHz]), no mixer will be required.  You'll need the
> > bandpass filtering and probably some kind of amplifier.  If you're
> > looking for a minimalist solution you can probably get by with a
> > simple R/C or R/L/C filter and a single transistor amplifier.  A FET
> > input op-amp would probably work too.  If you use a long enough piece
> > of wire for your antenna and you're close to a station, and your A/D
> > has a high input impedance, you might not need the amp.
> >
> > With this configuration your ability to listen to weak stations in the
> > presence of strong stations will be lousy.   Strong station reception
> > should be OK.
> >
> >
> > > 3. Which parts of the AM demodulation are already implemented in 
> > > GNURadio? Does it use envelope detection or synchronous detection? 
> >
> >
> >
> > I don't know that anyone has pieced it all together, but most of it
> > should be there.  Use GrFreqXlatingFIRFilterFCF to extract the station
> > of interest and translate it to baseband.  Envelope detection would be
> > absolute value (GrMagnitude<VrComplex,float>) followed by a low pass
> > filter (GrFIRfilterFFF).
> >
> > Sounds like a great example.  Let us know what it takes to make it
> > work.  Nothing like building a crystal radio with a couple of thousand
> > dollars worth of computer hardware ;-)
> >
> > Eric
> >
> >
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>
>
>
>
>
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