ASK and FSK modulators/demodulators; PSK with envelope detector

ASK modulator

In Amplitude Shift Keying (on-off keying) the carrier is sent for bit 1 and switched off for bit 0: $s(t)=b(t)\,A\cos 2\pi f_ct$, with $b(t)\in\{1,0\}$.

 binary (unipolar NRZ) b(t) ---->(X)----> ASK
                                  ^
                     A cos 2 pi fc t (oscillator)

The product modulator (or simply a switch) passes the carrier only during 1s.

ASK demodulators

Coherent:

 ASK -->(X)-->[integrate 0..Tb]-->[compare with Vth]--> data
         ^
      cos wc t (carrier recovery)

Non-coherent (envelope detection):

 ASK ->[BPF at fc]->[envelope det]->[comparator]-> data

The output is high when the carrier is present (1) and near zero when it is absent (0); the threshold is about half the peak.

FSK modulator

In Frequency Shift Keying bit 1 is sent at $f_1$ and bit 0 at $f_2$.

 b(t) --------------->(X)<-- cos w1 t --+
   |                                    (+)--> FSK
   +-->[Inverter]---->(X)<-- cos w2 t --+

Equivalently, the polar NRZ data can drive a VCO, whose output frequency switches between $f_1$ and $f_2$ (giving continuous phase).

FSK demodulators

Coherent:

        +->(X)<-cos w1 t ->[int 0..Tb]--+
 FSK ---+                              (-)--> l --> l>0: 1
        +->(X)<-cos w2 t ->[int 0..Tb]--+           l<0: 0

Non-coherent:

        +->[BPF f1]->[envelope det]--+
 FSK ---+                           [compare]--> data
        +->[BPF f2]->[envelope det]--+

The branch with the larger envelope decides the bit. A PLL demodulator can also be used.

Can PSK be detected with an envelope detector?

No. In binary PSK, $s(t)=\pm A\cos 2\pi f_ct$. The amplitude is $A$ for both 1 and 0; only the phase differs by $180^\circ$:

$$\text{envelope of } +A\cos\omega_ct = \text{envelope of } -A\cos\omega_ct = A$$

An envelope detector responds only to amplitude, so its output is a constant $A$ for every bit and all information is lost. PSK must be detected coherently (multiply by a locally recovered carrier and integrate, the sign gives the bit), or in differential form (DPSK) by comparing each bit with the previous one.