Hi,
i am using a Bell 202 compatible modem chip (mx614 - www.mxcom.com) in the design I am
doing. Is this afsk? The datasheets I have only say fsk. I guess it is probably afsk
since the chip outputs 1200hz/2200hz audio tones. Is fsk carrier frequency shifting?
>This signal happens to be a 1200hz tone that has an
>arbitrary phase offset (it might be coherent fsk then no offset but that's
>abnormal on ham bands as far as I know).
Could you explain the above? I don't know what coherent fsk and arbitrary phase offset
are.
>Spend more time on making a transmitter that doesn't care about shaking
>and occasional power glitches.
I am using the alinco DJ-C4T http://www.alinco.com/Products/DJC1C4T.shtml
cheers,
Jamie
http://142.104.54.173
[EMAIL PROTECTED] wrote:
> On Fri, 14 Jul 2000, Jamie Morken wrote:
>
>
> > Is the set of N samples the analog
> > samples from the soundcard or is "fsk symbol" something else?
> Yes, the N samples are the raw samples from the soundcard that you think
> contain an fsk tone. For 1200baud afsk packet sampled at 11050hz, N is
> about 9 samples. Every 9 samples the tone may change and you want to use
> all 9 samples of the symbol to figure out what the tone was.
>
> > I see the ratio of
> > 1.8333333*1200 = 2200 but can you explain how you are checking for power at the
> > 1200 Hz frequency?
> Let's not worry about the fact that these are samples, just call S a
> function of t (time). This signal happens to be a 1200hz tone that has an
> arbitrary phase offset (it might be coherent fsk then no offset but that's
> abnormal on ham bands as far as I know).
>
> S[t] = A*sin( 1200*t*2*Pi+1.23456789 ) <--- A is the power we want
> to know
>
> Now when we do convolution(S[],Ia[]) we really mean integrate this
> S[t]*sin(1200*t*2*Pi)
> from zero to two Pi. That integral is
> A*cos(1.23456789). The convolution(S[],Qa[]) ends up looking like
> A*sin(1.23456789). So square both of those guys and add and you get:
>
> convolution(S[],Ia[])^2+convolution(S[],Qa[])^2 = A^2
> Tada, you have the power. Actually, you have the amplitude squared. Take
> the square root and divide by square root 2 if you really want power.
>
> >I was also wondering about the effect of doppler shift on the
> > radio signal. I am putting a radio in a rocket that will be travelling over mach 3
> > away from the groundstation so will the 1200/2200 fsk frequencies be shifted down
> > in frequency enought that I should be compensating for this?
>
> Well two points, normal packet is 1200baud afsk = audio frequency shift
> keying = frequency shifting frequency modulation. The signal your decoding
> will be immune to doppler, unless your going fast enough to invoke special
> relativity in which case your going to have trouble with the symbol timing
> pll and NASA.
>
> Straight FSK and PSK will doppler shift, but unless your planning on
> orbit your not going to get it going that fast. If your
> worried the shifted frequency will be:
> F=441Mhz*(1+(speedofrocket)/speedoflight)
>
> The real problem is tranceiver stability. That's why we use FM for voice
> and AFSK for data, they're very immune to cheap digital oscillators.
> Multimode 440Mhz rigs are $$$ because it takes work to make a good stable
> platform for PSK.
>
> > Also will the UHF
> > carrier be shifted a significant amount requiring frequency tuning?
>
> If you use normal packet, no problem. 9600baud fsk, psk or any exotic
> modes wil be a problem.
>
> Spend more time on making a transmitter that doesn't care about shaking
> and occasional power glitches.
>
> -------===Dustin Moore===---------
