am reception introduction am demodulation – reverse process of am modulation. basic understanding...
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AM RECEPTION
Introduction
AM demodulation – reverse process of AM modulation.
Basic understanding of the terminology commonly used to describe radio receivers & their characteristics.
SIMPLIFIED BLOCK DIAGRAM OF AN AM RECEIVER(RX)
Receiver (RX) Parameters
1. Selectivity2. Bandwidth improvement3. Sensitivity4. Dynamic range5. Fidelity6. Insertion Loss7. Noise temperature & Equivalent
noise temperature
1. 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. Give the receiver bandwidth at two levels of
attenuation. Eg: -3dB, -60dB The ratio of two BW ~ Shape factor
SF = B(-60 dB) / B(- 3dB)
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 ~ SF = 2 Example application for SF nearly 1
Satellite Microwave Two way radio Rx
2. Bandwidth Improvement (BI)
Thermal noise directly proportional to bandwidth. Reduce BW ~ reduce noise, improving system performance. Reducing BW = improving the noise figure of the RX
Where BRF = RF Bandwidth (Hz)
BIF = IF Bandwidth (Hz)
Noise figure improvement,
IF
RF
B
BBI
BINF log10
3. 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. Depends on:
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
4. 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. 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.
5. 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.
Forms of distortion that can deteriorate the fidelity of a communication system:- Amplitude Frequency Phase
Linear gain, 1-dB compression point, and third-order intercept distortion for a typical amplifier
6. Insertion loss (IL)
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.
in
outdB P
PIL log10)(
7. 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) Equivalent noise temperature, (Te) often used in low noise,
sophisticated radio receivers rather than noise figure.
Where T = environmental temperature (kelvin)
F = Noise factor
KB
NT
)1( FTTe
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. Non-Coherent Rx
Tuned Radio Frequency Rx Superheterodyne Rx
Non-coherent tuned radio frequency receiver (TRF Rx) block diagram
Cont’d… Earliest types of AM Rx. Figure shows the block diagram of a three stage TRF Rx. Consists of RF stage, detector stage and audio stage. Simple and high sensitivity. BW inconsistent & varies with the center frequency. Skin effect phenomenon.
Where Q is quality factor. TRF Rx is useful to single-channel, low frequency application.
Q
fB
AM superheterodyne receiver block diagram
Cont’d…
Non uniform selectivity of TRF led to the development of the Superheterodyne Rx.
Its gain, selectivity and sensitivity characteristics are superior to those of other Rx configurations.
Frequency conversion. High side injection, flo = fRF + fIF
Low side injection, flo = fRF – fIf
Image frequency; fim = fRF + 2fIF
Image Frequency Rejection Ratio;
im
RF
RF
im
f
f
f
fwhereQIFFR )1( 22
AM APPLICATION AM Radio broadcasting
Commercial AM radio broadcasting utilizes he frequency band 535 – 1605 kHz for transmission voice and music.
Carrier frequency allocation range, 540-1600 kHz with 10 kHz spacing.
Radio stations employ conventional AM for signal transmission – to reduce the cost of implementing the Rx.
Used superheterodyne Rx. Every AM radio signal is converted to a common IF
frequency of fIF = 455 kHz.
END OF AMPLITUDE MODULATION
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