Amplitude Modulation 2-3 AM RECEIVERS CHAPTER 2 Amplitude Modulation 2-3 AM RECEIVERS

Introduction AM demodulation – reverse process of AM modulation. Demodulator: converts a received modulated-wave back to the original source information. Basic understanding of the terminology commonly used to describe radio receivers & their characteristics is needed to understand demodulation process

Simplified block diagram of an AM receiver (superheterodyne)

Receiver Parameters Selectivity Bandwidth improvement Sensitivity Dynamic range Fidelity Insertion Loss Noise temperature & Equivalent noise temperature

Selectivity Used to measure the ability of the receiver to accept a given band of frequencies and reject all others. Way to describe selectivity is to simply give the bandwidth of the receiver at the -3dB points. Not necessarily a good means of determining how well the receiver will reject unwanted frequencies.

Cont’d… Give the receiver bandwidth at two levels of attenuation. Eg: -3dB, -60dB The ratio of two BW , Shape Factor where SF – Shape factor B(-60dB) – BW 60dB below max signal level B(-3dB) – BW 3dB below max signal level

Cont’d… If both BW equal, the shape factor would be 1. Impossible to achieve in practical circuit Example application for SF nearly 1 Satellite Microwave Two way radio Rx

Bandwidth Improvement Thermal noise directly proportional to bandwidth. Reduce BW ~ reduce noise, improving system performance. Reducing BW = improving the noise figure of the RX

Cont’d… Bandwidth Improvement, BI where BRF = RF Bandwidth (Hz) BIF = IF Bandwidth (Hz) Noise figure improvement, NF = 10 log BI

Sensitivity The minimum RF signal level that can be detected at the input to the Rx and still produce a usable demodulated information signal. Usually stated in micro volts of received signal. Rx sensitivity also called Rx threshold.

Cont’d… Depends on: To improve ~ reduce the noise level The noise power present at the input to the Rx. Rx noise figure. AM detector sensitivity. BI factor of the Rx To improve ~ reduce the noise level Reducing the temperature or Rx BW or RX noise figure

Dynamic range The difference (in dB) between the minimum input level necessary to discern a signal and the input level that will overdrive the Rx and produce distortion. Input power range over which the Rx is useful.

Cont’d… A dynamic range of 100dB is considered about the highest possible. A low dynamic range can cause a desensitizing of the RF amplifiers and result in severe intermodulation distortion of the weaker input signal.

Fidelity A measure of the ability of a communication system to produce (at the output of the Rx) an exact replica of the original source information.

Cont’d… Forms of distortion that can deteriorate the fidelity of a communication system:- Amplitude Frequency Phase

Insertion loss IL is a parameter associated with the frequencies that fall within the passband of a filter. The ratio of the power transferred to a load with a filter in the circuit to the power transferred to a load without the filter.

Noise Temperature & Equivalent Noise Temperature Thermal noise directly proportional to temperature ~ can be expressed in degrees, watts or volts. Environmental temperature, T (kelvin) where N = noise power (watts) K = Boltzman’s Constant (1.38 X 10-23 J/K) B = Bandwidth (Hz)

Cont’d… Equivalent noise temperature, (Te) Te = T(F-1) where T = environmental temperature (Kelvin) F = Noise factor (unitless) Te often used in low noise, sophisticated radio receivers rather than noise figure (in dB).

AM RECEIVERS Two basic types of radio receivers. 1. Coherent Synchronous receivers The frequencies generated in the Rx & used for demodulation are synchronized to oscillator frequencies generated in Tx. 2. Non-coherent Asynchronous receivers Either no frequencies are generated in the Rx or the frequencies used for demodulation completely independent from the Tx’s carrier frequency. Non-coherent detection = envelope detection.

Coherent demodulation EXAMPLE OF COHERENT DEMODULATION: SSB The received signal is heterodyned /mixed with a local carrier signal which is synchronous (coherent) with the carrier used at the transmitting end. Coherent demodulation SSB X LPF cos wct

SSB Receivers As in the SSBSC transmission, where the carrier was (Coherent receiver) As in the SSBSC transmission, where the carrier was suppressed at the transmitter, a carrier must be reinserted again at the receiver side for proper intelligence detection.

The detector of SSB receiver is a balanced modulator which is driven by a beat-frequency oscillator (BFO). If the BFO is at the correct frequency and phase coordination between transmitter and receiver, these two signals mix together and produce the original modulating signal.

Non-Coherent Rx Tuned Radio Frequency Rx Superheterodyne Rx

Non-coherent tuned radio frequency receiver (TRF Rx) block diagram

Cont’d… Figure shows the block diagram of a three stage TRF Rx. Earliest types of AM Rx. Consists of RF stage, detector stage and audio stage. Simple and high sensitivity.

Cont’d… Disadvantages : 1) BW is inconsistent & varies with the center frequency due skin effect phenomenon. where Q is quality factor. The Q will remain constant eventhough frequency increases. Hence, low-freq tuned Rx for selectivity will be excessive for high frequencies.

Cont’d… 2) Large numbers of amplifiers causing to instability and oscillations resulting to severe distortion and malfunction. 3) The gains of the TRF receiver are not uniform over wide frequency range because of the non-uniform L/C tank circuit ratios in the RF amplifiers. TRF Rx is only useful to single-channel, low frequency application.

AM superheterodyne receiver block diagram

Cont’d… Non uniform selectivity of TRF led to the development of the Superheterodyne Receiver. Its gain, selectivity and sensitivity characteristics are superior to those of other Rx configurations. Heterodyne means mixing 2 different frequencies together in a nonlinear device.

Cont’d… Frequency conversion in the mixer. If High side injection, flo = fRF + fIF Low side injection flo = fRF - fIf

INTERMEDIATE FREQUENCY (IF) Mixers generate signals that are the sum and difference of the incoming signal or carrier frequency (fRF) and the frequency of the local oscillator (fLO). If fLO > fRF , high-side injection : fLO = fRF + fIF If fLO < fRF , low-side injection : fLO = fRF – fIF Common AM receivers using fIF = 455kHz and FM receivers using fIF = 10.7MHz.

Let’s say you need to select an AM radio station at 1.0MHz band. If considering your receiver is a high side injection for its IF, the local oscillator frequency fLO is tuned to produce 1455kHz. Whereas, if another receiver is a low side injection, its local oscillator frequency fLO should be producing 545kHz.

IMAGES An image (fIM) is an undesired signal that is separated from the desired signal frequency (frf) by two times the IF (fIF). fI = frf + 2fIF or frf - 2fIF Images interfere with the desired signal. Images can be eliminated or minimized by: Proper selection of the IF in design. Use of highly selective filters before the mixer. Use of a dual conversion receiver.

con’t’d… Image frequency fim = fRF + 2fIF Image Frequency rejection ratio IFRR = √ (1 + Q²ρ²) where ρ = (fim/fRF) –(fRF/fim)

AM APPLICATIONS AM Radio broadcasting Commercial AM radio broadcasting utilizes the frequency band 535 – 1605 kHz for transmission of voice and music. Carrier frequency allocation range, 540-1600 kHz with 10 kHz spacing.

Cont’d… Radio stations employ conventional AM for signal transmission (DSBFC) – to reduce the cost of introducing special and expensive Rx on the public side. Used superheterodyne Rx. Every AM radio signal is converted to a common IF frequency of fIF = 455 kHz.

Regenerative Receiver There is another kind of receiver that extensively being used as wireless doorbell, car alarm / central lock, toy walkie-talkie, radio-controlled car, remote control and etc. It is a very simple receiver with only one transistor in the regenerative circuit and often used as an AM detector. By adjusting the level of oscillation slightly below the RF oscillation, the receiver will detect normal AM signals. But if the circuit is adjusted to operate slightly above the RF oscillation, SSB reception is possible. Depending on the level of oscillation adjustments, the regenerative receiver would behave either as a non-coherent or a coherent receiver.

Regenerative Receiver Circuit Adjust to control level of oscillation