Frequency Discrimination SSB Modulation Circuit

 In the last tutorial Understanding SSB Modulation it was explained how SSB(Single Sideband) modulation using frequency discrimination works. The SSB modulation was explained using the SSB modulation circuit diagram and SSB modulation formula. In this AM modulation tutorial it is shown how to actually implement the SSB modulation circuit that uses frequency discrimination.

SSB modulation circuit diagram 

The following shows the circuit diagram of SSB modulation with frequency discrimination circuit diagram.

In the above circuit diagram, two function generators are used to generate message signal and carrier signals, (m(t) = A_m cos(w_m t)) and  (c(t) = A_c cos(w_c t) respectively.

These two signals are fed into the DSB AM modulator which is implemented using the AD633 analog multiplier integrated circuit(IC). The DSB-SC AM signal is given by,

(s_{dsb}(t) = m(t)c(t))      ————->(1)

or,  (s_{dsb}(t) = A_m A_c cos(w_m t) cos(w_c t))

or.  (s_{dsb}(t) = frac{A_m A_c}{2} cos[(w_c+w_m)t] +  frac{A_m A_c}{2} cos[(w_c+w_m)t])  —->(2)

The DSB AM signal thus generated is coupled into the LC parallel resonant band pass filter consisting of L1,C3,L2,C4. The center frequency of this band pass filter depends upon whether we want upper sideband SSB signal or lower sideband SSB signal. 

From the above equation(2) we can write,

if  (s_{ussb}(t) = frac{A_m A_c}{2} cos[(w_c+w_m)t] )   —->(3)

and (s_{lssb}(t) = frac{A_m A_c}{2} cos[(w_c-w_m)t] )   —->(4)

Then equation(2) is,

(s_{dsb}(t) = s_{ussb}(t) + s_{lssb}(t) )    —->(5)

That is the DSB AM signal has upper and lower sideband signal. So we can either block the lower sideband signal to get upper sideband signal or block the upper sideband signal and get the lower sideband signal. If we want to generate upper SSB signal then we can build a band pass
filter with center frequency (w_c+w_m) which will block the lower
sideband of the DSB signal. Similarly if we want to generate upper SSB
signal then we can build a band pass filter with center frequency
(w_c-w_m) and remove the upper sideband of DSB signal. Since here we
have used filter to remove either one of the sideband of the DSB signal
to generate SSB signal this method is called frequency discrimination method of SSB modulation.

If say that the carrier signal is of 200KHz and that the modulating message signal has frequency of 10KHz then if we use a band pass filter of center frequency of 200KHz-10KHz=190KHz with suitable bandwidth then the lower side band signal is blocked and we get upper side signal at the output of the filter. For 190KHz center frequency we can use 7uH inductors and 0.1uF capacitor. The following shows the frequency spectrum of lower sideband SSB modulated signal.

 Similarly if we use band pass filter with center frequency of 210KHz using inductors of 5.75uH and capacitors of 0.1uF we get upper sideband SSB signal whose frequency spectrum is shown below.

The values of inductors and capacitors for design of passive band pass filter can be calculated using the online LC Parallel Resonant Circuit Calculator.

The SSB signal generated after the band pass filter is amplified using the non-inverting amplifier using LM358N op-amp.

The working principle of frequency discrimination based SSB modulation can be understood with the simplified SSB modulation block diagram as shown below.

SSB Modulation Circuit Video demonstration

The operation of the SSB modulation circuit shown above is demonstrated in the following video.