radio astronomy

Chris Lingard chris at stockwith.co.uk
Sat Jun 15 05:58:45 PDT 2002


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.

> 
> A what now?
> 

The whole system will probably be a double superhet;  The thing
on the dish converts the signal down from 10GHz to a more
sensible frequency.  You then need to listen across this band
of signals

> 
> My problem is I havent the first CLUE about RF electronics, let
> alone IF. all I know is Im going to need some EXPENSIVE transistors
> and be VERY careful about signal routing and shielding...
> 
> what exactly IS bandwidth? (no, I dont mean in the sense of 'how
> fast is my cable'.
> 

Bandwidth as in frequency range.  The bandwith for TV is MHz, for
FM music about 50KHz, speach down to 3KHz, (if you do not mind
sounding like Donald Duck). Morse 300Hz

> 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

FM  Frequency Modulation
The carrier frequency varies as the rate of the modulating signal;
but the amplituse remains constant

> 
Bandwidth as in frequency range
> and if altering the amplitude alters the frequency, surely the
> reverse is true?
> 
> so, surely then, amplitude modulation and frequency modulation are
> /the same thing/ ?
> 
> Also, what /is/ a down converter? how does it work?

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

A VHF filter would allow the 144MHz through.  You then need to tune
across this, as a whole band of signals has been selected.

> 
> 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.

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

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

> Is that a down conversion or direct conversion?

Direct conversion

Good, my download has finished.
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