Quadrature Null Effect Research Paper

Quadrature null effect (QNE) is a phenomenon in amplitude modulation (AM) demodulator circuit arise if carrier signal generated from local oscillator is not synchronized in phase with input signal. Our main motive is to synchronize phase in this effect by various synchronizing techniques so as to reconstruct input modulated signal completely, are scrutinized in this paper.
Keywords— AM, DSB-SC, SSB-SC, QNE, Synchronization Techniques.

I. 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

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.
III. 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.
A. 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