Does the FSK demodulator work?

[Josephchiang]

Hello everyone,

My research topic is related to biomedical field.

The modulation technique including amplitude shift keying(ASK), frequency shift keying(FSK) and binary phase shift keying(BPSK) was normally used in wireless transmission.

Some material told me that only ASK modulation technique could achieve with non-coherent demodulation, the others two need coherent demodulation.

In the other word, the FSK and PSK demodulator need the Phase lock loop(PLL) to achieve demodulation.

The principle of FSK modulation is that using two different frequency to distinguish the "H" "L" data.

My question is why don't we just hire a counter and a precise clock to count the input frequency. We could infer the transmission data from the counter output result.

I think that this method is more power saving that conventional one.

Does it work?

Any suggestion is deeply appreciate.

 

[D.A.(Tony)Stewart]

A counter is a chain of; async ripple counters or sync flip-flops . A PLL phase comparator is only an XOR gate with a VCO, so has fewer gates.

The counter method requires high CNR from narrowband BPF to prevent glitches and extra or missing counts.

Group delay distortion of the channel filter andSNR will limit bit rate.

A detector which integrates all of the signal energy over the bit interval with a receiver bandwidth matching that signal content, will give the performance of an ideal "Matched Receiver"

once the clock is phase locked, the integrator can be dumped or initialized and start integrating the phase error all over again to determine the next bit. However typical tpe II detectors use Pump up/ Dn output that otherwise float or hold until next bit intervalphase meaurement to generate the VCO error voltage.

The integration of a step in frequency (FSK) is a ramp in Phase rate of change.

Synchronous detection is ideal for both ASK and FSK using integrate and dump relative to the reference (AGC amplitude or PLL frequency )

But both can and have been done with or without synchronous detection. FSK once used the edge of a BPF to detect amplitude shift with frequency and no clock required.

a "matched receiver" is preferred for SNR < 20.

For FSK there is a signal improvement factor from carrier deviation ratio. So from CNR to SNR , improvement increases with log of deviation ratio.

what are your channel specs for signal and, noise?

 

[FvM]

I don't agree that FSK needs a PLL or other "coherent demodulation§ methods. It can work with any kind of frequency discriminator, may be a frequency counter. But practically, a PLL FSK detector has many advantages, particularly for small frequency shifts.

 

[Josephchiang]

To SunnySkyguy
Thanks for your billion explanation.
But please forgive me that my knowledge is limited.
"A counter is a chain of; async ripple counters or sync flip-flops . A PLL phase comparator is only an XOR gate with a VCO, so has fewer gates." <= Does it means PLL based may cousume less power than counter base?

What does the term "CNR" stand for?
Why narrowband BPF could prevent glitches or missing counts?

I don't have any channel or noise specs?
In my case, the signal will be transmitted via air by two coupling coils which be separated by few centimeter
How could I evaluate the spec from the application?


To FvM
Beside detects small frequency shifts , have any advantages of coherent demodulation?
Thanks a lot.

 

[D.A.(Tony)Stewart]

Does it means PLL based may consume less power than counter base? I think so.

What does the term "CNR" stand for? (Carrier/Noise Ratio)
Why narrowband BPF could prevent glitches or missing counts? ( Narrow band rejects Noise and presents high Q signal )

I don't have any channel or noise specs?
In my case, the signal will be transmitted via air by two coupling coils which be separated by few centimeter
How could I evaluate the spec from the application?
Signal Bandwidth = ?
Channel Bandwidth = ?
Receiver Bandwidth = ?
Receiver Noise Threshold =?
Path Loss =?
Desired BER =? thus required worst case SNR = ?


To FvM
Beside detects small frequency shifts , have any advantages of coherent demodulation?
Thanks a lot.[/QUOTE]