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My mistake...I haven't really got everything figured out yet.

I originally believed I could do a direct conversion (I and Q) followed by filtering, and then limiting, and finally quadrature detection. So the filtering would of course be at a fixed IF (zero). I do understand the difficulty in channel filtering at RF...My primary concern was the filtering since I understand I need to have an SNR of around 10dB prior to limiting amplifier, in order to avoid the threshold effect.

So my main question was should I filter the spectrum in between the mark and space prior to limiting in order to improve the SNR. I'm really actually trying to figure out what noise BW I should use in order to determine the required NF of the front end. I've concluded that the noise BW in question is just 4*Bmod where Bmod is the bandwidth of the modulating signal (assuming a wide tone separation).

After reading more about the mathematics of quadrature detection, however, I guess that quadrature detection should be done at an IF (not zero), right? In which case I would need some other type of detection.

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I'm concerned about when the SNR is around the threshold (10 dB). So does that rule out PSS/Pnoise?


thanks,
Aaron

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I'm reading the book, "Digital and Analog Communications Systems", by Leon W. Couch II, and his description of quadrature detection is that a quadrature component is first obtained by shifting the signal by 90 deg. So if,

V_in = V_Lcos[ω_ct + Θ(t)]

then

V_quad = K₁VLsin[ω_ct + Θ(t) +K₂dΘ(t)/dt]

I assume ω_c is a low-IF and not the RF carrier. After that Vquad and Vin are multiplied together and a signal proportional to dΘ(t)/dt results. Since we are using FSK, this product is proportional to m(t)..the original signal.

My thinking is that if you do direct downconversion, then the quadrature components which are produced by the down-conversion mixers do not give the additional component proportional to dΘ(t)/dt. So simply multiplying the I and Q components together will not produce m(t).

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I don't necessarily need to use low-IF. I'm just thinking I can't use a quadrature detector (the way used in the book i just mentioned).


thanks for the help,
Aaron

Hi Aaron,

Direct conversion FSK systems I know which do not use ADC or quadrature detection would use a variant (digital or analog) on the balanced quadricorrelator for demodulation. It is (i believe) the lowest power technique and has been was used in pagers for years (but may suffer too much implemenation loss when the modulation index is very low or frequency offsets are large). I seem to recall that the original paper by Gardner is in "Monolithic Phase-locked Loops and Clock Recovery Circuits: Theory and Design" a collection of papers by Razavi.
Anyway I have worked on a zero-IF FSK SoC which is in production and was published in ISSCC08 (paper 7.2) and the wireless part of it (including links to demod) in JSSC July 08 (1st paper). Perhaps it will give you ideas of what can be done in very lower power....
cheers
aw