radio astronomy

Ian Molton spyro at armlinux.org
Sat Jun 15 06:42:05 PDT 2002


On Sat, 15 Jun 2002 13:58:45 +0100
Chris Lingard <chris at stockwith.co.uk> wrote:

> Ian Molton wrote:
> 
> > On Fri, 14 Jun 2002 21:25:07 +0100
> > Chris Lingard <chris at stockwith.co.uk> wrote:
> > 
> >> Ian Molton wrote:
> > 
> >> 10GHz is near an amateur band so you will find plenty of designs
> >> at rsgb.org or arrl.com.  What sort of bandwidth would you want?
> > 
> > I havent the foggiest. whaty sort of bandwidth does radio astronomy
> > use? ;-) 
> 
> The signals do not come in modulated.  You will be looking at a
> spectrum of signals.  What comes down the cable if a whole band
> of signals, you then need to select whay frequencies you want.

I'd imagine a PC soundcard has a bandwidth of about 20kHz, so a 20kHz
band aroound 9.75GHz would be what I would be looking at.

> The whole system will probably be a double superhet;

Which is a ?

> The thing
> on the dish converts the signal down from 10GHz to a more
> sensible frequency.

do you know what a typical frequency would be ? Im told 900MHz, but
surely not all LNBs are the same?

> > as I understand it, if I have a 900MHz carrier and modulate it with
> > a 5KHz signal, I not only alter the amlitude, but also the
> > frequency, causing it to deviate either side of the 900MHz carrier,
> > although I dont fully understand WHY changing the amplitude affects
> > the frequency...
> 
> AM  Amplitude Modulation  also Ancient Modulation
> A 900MHz carrier with sideband 5KHz either side, amplitude of
> sidebands vary depending on loudness of signal

what is a sideband and why?
 
> FM  Frequency Modulation
> The carrier frequency varies as the rate of the modulating signal;
> but the amplituse remains constant

if modulating amplitude alters frequency, why does modulating frequency
not modulate amplitude?
 
> Bandwidth as in frequency range

> You could use an oscillator at 10.144MHz  Mixing this with 10GHz
> would make two signals at 144MHz and 20.144MHz, (sum and difference)

did you mean 10.144GHz?

and when you say 'at' 144MHz, what do you mean? will I have a band
144MHz wide starting from 144MHz?
 
> A VHF filter would allow the 144MHz through.  You then need to tune
> across this, as a whole band of signals has been selected.

across what? 144MHz is a frequency, not a band... (isnt it?)
 
> > if I have a 10GHz band to observe, and beat it against a 10GHz
> > oscillator, then a 10.001MHz signal would resolve to a 0.001MHz
> > signal?
> 
> So would a 9.999MHz signal, it is called the image frequency.
> You would always receive two signals.

so this is why I would beat against (say) 10.144GHz, and not 10GHz?

> Yes. You can do this too, If you make your oscillator variable
> you can tune accross the band.  Look for "phase locked loop"
> and "pumped oscillators" in an electronic book

how would this work?

> You can use one of those diodes that exhibit a negative resistance
> coupled with a tuned circuit

how? and what is a 'negative' resistance?

> > Is that a down conversion or direct conversion?
> 
> Direct conversion

so what is 'down' conversion?

also, if I do direct conversion, do I still need to have a detector and
tune into the band I want? or just plug into the soundcard?

> Good, my download has finished.

:-)
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