Variations of AM

Bandwidth Efficiency Power Efficiency Complication Single Side Band Vestigial Side Band Power Efficiency Reduced Carrier Suppressed Carrier Complication Circuits: generation, tuning, detection Filter, tone variation, carrier recovery

Double Side Band Suppressed Carrier (DSBSC) It is a technique where the sidebands are transmitted without the carrier (carrier is being suppressed/cut) Characteristics: Power content less Same bandwidth Disadvantages - receiver is complex and expensive.

Frequency spectrum of DSB-SSC

Phasor diagram of DSB-SC

We know that, the total power transmitted in AM is

If the carrier is suppressed, then the total power transmitted in DSB-SC-AM is P t = PLSB+PUSB

Single Side Band (SSB) Improved DSBSC and standard AM, waste power and occupy large bandwidth. SSB is a process of transmitting one of the sidebands of the standard AM by suppressing the carrier and one of the sidebands Advantages: Saving power Reduce BW by 50% Increase efficiency, increase SNR Disadvantages Complex circuits for frequency stability

Frequency spectrum of SSB

Vestigial Side Band (VSB) VSB is mainly used in TV broadcasting for their video transmissions. TV signal consists of Audio signal – transmitted by FM Video signal – transmitted by VSB A video signal consists a range of frequency and fmax = 4.5 MHz. If it transmitted using conventional AM, the required BW is 9 MHz (BW=2fm). But according to the standard, TV signal is limited to 7 MHz only So, to reduce the BW, a part of the LSB of picture signal is not fully transmitted.

The frequency spectrum for the TV signal / VSB:

GENERATION OF AM WAVES

Generation of Amplitude Modulation It is broadly divided into two types namely (i) Linear modulator or large signal modulator. (ii) Non Linear modulator or small signal modulator Linear modulator or large signal modulator: In this type of modulators the devices are operated in the linear region of its transfer characteristics thus the relation between the amplitude of the modulating signal and the resulting depth of modulation is linear. It can be divided into two types (i) Transistor modulator (ii) Switching modulator

In this linear modulator the pair of non-linear elements having (as near as possible) the same characteristics in a balanced circuit cancel out the unwanted components. ` Non-linear Modulator or Small signal modulator These modulators makes use of non-linear V-I characteristics of the devices and are, in general suited for use at low voltages. The important type is: Square law modulator In simple words it can be defined as the devices used in these modulator are operated in non linear region of its characteristics.

Square law modulator A square law modulator requires to add up the carrier and modulating signal to obtain AM with carrier. Thus a square law modulator has three features. 1. Summer-To add carrier and modulating signal 2. A non-linear (active) element. 3. Bandpass filter for extracting desired modulating products.

Basic Square Law modulator

Sqr. Law modr. using Diode

A. Modulating Signal B. Carrier C. Sum of carrier and modulating signal D. Diode current E. AM output across tuned circuit

Square law modr. using FET

The FET is biased in a non-linear region of its transfer characteristics, to obtain the desired output. The output tank circuit RLC is tuned to the carrier frequency to select the desired modulating components. Mathematical Analysis The current of a non-linear element suitably biased in its non-linear region is given by the square law equation i.e., io = a1V1+a2V12 +...........

Where V1 = input voltage applied to the FET V1 = Am sin ωmt +Ac sin ωct Substituting above equation in prev. eqn.

Neglecting the second order terms,

When the bandpass filter is tuned to the carrier frequency it allows only wc , (ωc − ωm) and (ωc + ωm) terms and it eliminates all other terms. Hence we obtain,

Product Modulators Used for generation of DSB-SC signals. Here, a DSB-SC signal is obtained by simply multiplying the modulating signal Vm(t) with carrier signal cos ωct.

Let the modulating signal be Vm (t) = Am sin ωmt and the carrier signal C(t) = Ac sin ωct When multiplying both the carrier and message signal, the resulting signal is the DSB-SC-AM signal. S(t)DSBSC =Vm (t)C(t) Therefore S(t)DSBSC = Am sin ωmt Ac sin ωct

Balanced Modulators (practical) for DSB-SC generation: There are two ways of generating DSB-SC (i) Balanced modulator. (ii) Ring modulator. Balanced modulator: a pair of symetric non-linear elements are used. They cancel out the unwanted components

Diode balanced Modulator:

The carrier voltage is applied in parallel to diode D1 and D2 the modulating voltage in push-pull to the diodes.

A bandpass filter is that type of filter which allows to passband of frequencies. Since the bandpass filter is centred around ωc it will pass a narrow band of frequencies BPF is centred at ωc with a small bandwidth of 2ωm to preserve the sidebands. Therefore, the output of BPF centered around ωc is given by Which is the expression for a DSB-SC signal.

Switching or Ring or Chopper modulator.

A ring modulator uses four diodes in ring form. diodes are controlled by a square wave carrier signal applied through a center-tapped transformer. If the carrier is large enough to cause the diodes to switch states, then the circuit acts like a diode switching modulator The modulating signal is inverted at the carrier rate. This is essentially multiplication by ±1.

When the carrier is +, outer diodes will conduct. When the carrier is - , inner diodes will conduct. Thus carrier switches the diodes on / off. Bandpass filters are used to extract the frequency of interest.

Diode conduction for + and – carrier:

Generation of SSB-SC AM SSB-SC waves can be generated in two ways: (i) Frequency discrimination or filter method (ii) Phase discrimination method Phase discrimination method itself can be divided into two types. (a) Phase shift method (b) Modified phase shift-weavers method.

Filter method of SSB generation:

first a DSB-SC signal is generated simply by using an ordinary product modulator or a balanced modulator. After this from the DSB-SC signal one of the two sidebands is filtered out by a suitable Band pass filter (BPF). The design of band pass filter is quite critical and puts some limitations on the modulating or baseband and carrier frequencies.

Limitations of frequency discrimination method. is useful only if the baseband signal is restricted at its lower edge due to which the upper and lower sidebands are non-overlapping. the base band signal must be appropriately related to the carrier frequency (design of the BPF becomes difficult if ωc >> ωm.

Phase shift method of SSB generation :

The phase shift method avoids filter. makes use of two balanced modulators and two phase shifting networks. One modulator receives the carrier with the phase shift of 90◦ and the modulating signals directly. The other modulator receives modulating signal with the phase shift of 90◦ and the carrier signal directly.

For BM 1 Vm (t) = Am sin (ωmt) Vc (t) = Ac sin (ωct +90◦) For BM 2 Vm (t) = Am sin (ωmt +90◦) Vc (t) = Ac sin ωct

Output of balanced modulator M1 will contain sum and difference frequencies

Output of balanced modulator M1 will contain sum and difference frequencies