INTRODUCTION:

QNE is phenomenon seen in heterodyning where a coherent detection mixture operates like a Hilbert transformer : that is two signals reaching the RF- and the LO- ports of the mixture are of same frequency but of 90o offset in phase, causing the mixture to generate a minimal IF signal output.

The main idea behind this work is to recover original message signal by demodulator circuit using synchronous detection technique.

The phase and frequency of the locally generated carrier in synchronous detector is extremely critical. Precision in phase and frequency control of the local carrier needs an expensive and a complicated circuitry.

Various methods to synchronize this effect are:

1. Pilot Carrier

2. Costa’s Receiver

3. Squaring Loop

AM DEMODULATION:

AM is a modulation process in which amplitude of carrier is varied in accordance with the instantaneous value of modulating signal.

In amplitude modulation, the sidebands contain the signal. The power in the sidebands is the only useful power. The power carried by the side bands is only 33.3% even when there is 100% modulation. So, the amplitude modulation has low efficiency.

An AM system produces two sidebands corresponding to each frequency component in modulating signal is termed as Double Sideband Suppressed Carrier (DSB-SC). In this modulation, unlike AM, the carrier wave is not transmitted; thus, a great percentage of power that is dedicated to it is distributed between the sidebands, which imply an increase of the cover in DSB-SC, compared to AM, for same power used. This is basically an AM wave without carrier therefore reducing power wastage, giving it a 100% efficiency rate.

Demodulation is extracting the original information bearing signal from a modulated carrier wave.

To demodulate AM signal Synchronous or Coherent or Homodyne technique is used.

This detection process needs a local oscillator. The frequency and phase of the locally generated signal and the carrier signal at the transmitter must be identical with reference to both frequency and phase, else the detected signal will be distorted.

The demerits of this technique is that it requires an additional system at the receiver to ensure that locally generated carrier is synchronized with the transmitter carrier, making the receiver complex and costly.

Fig. 1: Coherent Detector

When there is only phase error, i.e. ∆f = 0, but ɸ ≠ 0

s(t)= [Ac(1+Ka m(t))cos2πfct]

c(t)=[ Ac cos(2πfct+ɸ)]

v(t)=[Accos2πfct+Accos2πfctKam(t)][Ac cos(2πfct+ɸ)]

v(t)=(Ac2/2)[cos(4πfct+ɸ)+cosɸ]+(Ac2/2)Kam(t) [cos(4πfct+ ɸ)+cos ɸ]

When this signal is passed through a low pass filter with a cut-off frequency fm, the terms centered around ± fc are filtered out, and the filter output is given as-

(LPF)output = (Ac2/2)Kam(t) cos ɸ

the above equation shows that the output is multiplied bycosɸ. When ɸ is time independent, there is no distortion; rather, there is only attenuation. The output is maximum when ɸ =0, and minimum when ɸ =90o.

QUADRATURE NULL EFFECT:

The detected output is zero when ɸ =90o. This is called as QNE, because the output signal is zero when the local carrier is in phase quadrature with the transmitted carrier.

Methods to synchronize QNE:

1. Pilot Carrier:

A small amount of carrier signal is transmitted along with modulated signal from transmitter, known as pilot carrier. This carrier is separated at receiver by an appropriate filter, is amplified, and is used to phase lock the locally generated carrier at the receiver. The phase locking provides the synchronization.

2. Costa’s Receiver:

The system has two synchronous detectors- one detector is fed with locally generated carrier which is in phase with the transmitted carrier. This detector is known as In phase Coherent Detector or I- channel. The other synchronous detector employs a local carrier which is phase quadrature with the transmitted carrier, and is known as Quadrature phase coherent detector or Q- channel. Combined, the two detectors constitute a negative feedback system which synchronizes the local carrier with transmitted carrier.

Fig. 2: Costa’s Receiver

The Operating principle of Costa’s Receiver is discussed below. Let us assume that the local carrier is synchronized with the transmitted carrier, and ɸ =0. As in fig. 2 , the output of the I-channel is the desired modulating signal (cos ɸ =1), but the output of Q-channel is zero (as sin ɸ =0) due to QNE.

Now, assuming that the local oscillator-frequency drifts slightly, i.e., ɸ is a small non zero quantity, I-channel output is almost unchanged, but Q-channel output is now not a zero, rather some signal will appear at its output, proportional to sin ɸ with a polarity same as I-channel for one direction of phase shift in local oscillator, and opposite for other direction of phase shift. The phase discriminator provides a d.c. control signal which may be used to correct local oscillator phase error. The local oscillator is a voltage controlled oscillator (VCO). Its frequency can be adjusted by an error control d.c. signal.

Limitation: This receiver ceases phase control when there is no modulation. The phase control re-establishes itself on reappearance of modulation.

3. Squaring Loop-

The received signal is squared by a squaring circuit.The output of the squarer is given as,

[Am(t) coswct]2 = A2 m2(t) cos2wc(t)

m(t) = coswmt

Then output of squarer becomes,

= (A2/4)[1+ cos2wmt + cos2wct + cos2wmt cos2wct]

The term cos2wct may be obtained by using a notch filter centered at ±2wc. This frequency ± 2wc is kept constant by tracking through a phase locked loop (PLL). The VCO output is frequency divided by 2, to yield a synchronized local carrier of frequency wc. The local carrier is used in syncronous detector. The frequency division may be accomplished by using bistablemultivibrator.

Fig. 3: Squaring Loop

SSB-Modulation:

All of the above mentioned techniques are used only to synchronize QNE, but this effect cannot be eliminated completely, so another modulation is preferred in which QNE is absent, known as Single Sideband-Suppressed Carrier (SSB-SC).

In SSB-SC we only transmit only one of the sidebands either Lower Sideband (LSB) or Upper Sideband (USB) with suppressed carrier.

Fig. 4: SSB-SC

The SSB-SC signal is:

S(t)= (Ac/2)[m(t)cos2πfct ± m^(t)sin2πfct]

Where, m^(t) is obtained by giving a phase shift (-90o) to each frequency component present in m(t) , actually represents the Hilbert Transform of input message signal.

As in DSB-SC detection our carrier was not synchronized to input signal i.e.,

C(t) = Ac cos(2πfct+ ɸ)

In demodulation of SSB-SC by synchronous detector, when signal is passed through Low pass filter, output is

(LPF)output= (Ac2/4)[m(t)cos ɸ ± m^(t)sin ɸ]

If ɸ is 0o, we get = (Ac2/4)m(t)

If ɸ is 90o, we get = (Ac2/4)m^(t)

In both of the above cases, output signal is never zero and reconstruction of message signal is easy. This shows that no QNE has occurred. Therefore, SSB is preferred over other AM techniques.

CONCLUSION:

QNE is a phenomenon occurred due to difference in phase and frequency of carrier generated by local oscillator and input signal. Various receivers are used to synchronize phase and frequency which leads to complex circuitry. Hence, to avoid this situation in DSB-SC, SSB-SC is used where QNE does not occur.

REFERENCES:

1. R.P.Singh and S.D.Sapre, “Analog and Digital Communication”, TMH 2007.

2. Herbert Taub and D.L Schilling, “Principle of Communication”, McGraw Hill, 2nd ed.

3. Simon Haykin,”Communication Systems”, Wiley, 3rd ed.

4. The Costas Reciever is named in honor of its inventor; see the paper by Costas (1956).

5. Leon W.Couch,II, “Digital and Analog Communication Systems”, Pearson, 6th ed, pp. 303-312.

6. Lathi B.P.,”Communication Systems”, Wiley Eastern, 2000.

7. Shanmughan K.S., “Digtal and Analog Communication Systems”, John Wiley, 1979.

8. P.Ramakrishna Rao,”Analog Communication”, TMH, 2011.

Received on 01.10.2016 Accepted on 05.02.2017

Int. J. Tech. 2017; 7(1): 11-14

DOI:10.5958/2231-3915.2017.00003.7