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Radio was first postulated in 1873 by Maxwell, demonstrated in 1888 by Hertz, and used for practical communications in 1895 by Marconi. Radio is an electromagnetic phenomenon and radicates as photons . It belong to the family of radiation that includes X-rays, light and infrared (heat )waves. The different categories of radiation differ in frequency, as shown in Figure 1.1. They also differ in energy and ability to propagate those different media.
The name was derived from the word heterodyne, which means the beating of two signals together to produce the sum and difference frequencies. The tuning and signal amplification in the early super heterodynes was done at supersonic frequencies around 50 to 60 kHz (the heterodyne frequency), and hence the word "super". The heterodyne frequency is now known as the intermediate frequency or IF, and in modern receivers is well above the supersonic range typically 455 kHz or 10.7 MHz.
An extension of the superhet principle is the double conversation superhet, which is commonly used in most cellular receivers, as well as I other high-frequency domestic sets. This receiver uses two IF stages in series, the first being typically at 10.7 MHz and the second at 455 kHz. This will provide superior RF gain with stability and virtually eliminate the main shortcoming of the superhet, which is its image rejection. The heterodyne action will produce both the sum and difference of the incoming frequency and the oscillator. If the incoming frequency was, for example, 800 MHz and only the 455 kHz IF was used, amixer operating at 800.455 MHz will result in the required IF signal 455 kHz. However when the receiver is tuned for an incoming signal of 799.090, the oscillator frequency would be 799.545 MHz (800 MHz-455 kHz), both the original 800-MHz signal will produce a beat frequency at 455 kHz. The 800-MHz signal will do as a result the difference frequency between it and the oscillator . In other words, two oscillator frequencies very close together will produce a valid signal to the IF amplifier. This unwanted signal is known as an image frequency, because unless the RF filtering screens the unwanted signal completely, the tuning will result in their being two positions on the tuning dial at which the 800-MHz signal is found; the undesired position is known as an image.
Clearly it would be difficult to provide adequate RF tuning at 800 MHz to ensure a perfect filtering of signals only 910 kHz (2 * 455 kHz) apart and for this reason the first IF stage frequency will be higher, typically 10.7 MHz. Now, the same problem will still occur, but the frequency at which the receiver will be tuned to generate the image of an 800-MHz signal will 778.6 MHz (800 to 2*10.7 MHz) and the RF tuning needs only to be able to screen out the 800-MHz signal, which is 21.4 MHz away.
Additional stability can be provided by utilizing a second IF frequency, as it can be seen that a high-gain tuned radio frequency stage is a potential oscillator f the leakage from input to output becomes sufficient. Usually that second stage will operate at 455 kHz, since the 910 kHz margin to the image frequency is quite manageable at the down-converted frequency of 10.7 MHz.
The choice of the actual IF frequencies is limited to those that have by convention been set aside for this purpose. No transmitters operate on these frequencies and so they will be noise-free.
A final improvement that is characteristic of modern receivers is the addition of an RF stage which consists usually of a single stage, low-noise amplifier to improve the signal-to-noise performance of the set. Infact, it will be seen later in the chapter on noise performance that in a normally operating superhet it is the noise figure of that RF stage that will virtually determine the overall receiver performance.
A receiver incorporating all these elements will be found in the typical cellular receiver in both base stations and in mobile phones shown in Figure 1.4
The modulation system used in most cellular radio systems is known as FM (Frequency Modulation). In this type of modulation the frequency of the carrier is varied proportionally to the signal to be transmitted. A typical FM modulator is shown in Figure 1.5.Portable FM radio was first developed by Motorola for the US Army Signal Corps during World War 2. The first unit was a backpack weighing around 16 kilograms and having a range of about 30 kilometers.
The audio input varies the bias on the varactor ( a solid state variable capacitor shown in Figure 1.5), which in turn changes the frequency of the tuned circuit. The maximum amount that the frequency can deviate from its central carrier frequency is called the peak deviation.
The S/N performance of FM systems is very high, provided the noise level is reasonably low. FM systems with wide deviation have better S/N performance than those with narrow deviation.
Some systems use phase modulation, particularly for data transmission. Phase modulation is closely related to frequency modulation and can be derived from it by passing the signal through a simple differential circuit before frequency modulation.
(see Figure 1.6)
Frequency Shift Keying (FSK), at relatively low speeds, (for example, the Manchester code at 6.4 kbits is for TACS and 8 kbits for AMPS on the control channel) is often used for data, because it has better S/N performance than FM at low signal levels. This enhances signalling in areas of poor reception.
As the signal level increases, the quality of FM (in noise performance terms) rises fairly rapidly, whereas the quality of FSK doesnot. Because cellular systems are designed to operate in relatively high signal environments (low noise), FM is chosen for the voice path.
DYNAMIC CHANNEL ALLOCATION
Because cellular systems use from 180 to 2000 channels, it is necessary for the mobile to automatically switch to the correct channel. This is done by sending an instruction (data) that indicates the channel number required. The mobile system then switches to the channel indicated by using synthesized tuning, a system where the frequency of the oscillator is numerically compared to the required frequency and adjusted by a "phase-locked loop" until the two frequencies match.
NOISE AND SIGNAL-TO-NOISE PERFORMANCE
All radio systems are ultimately limited in range by noise. When the intrusion of noise is such that an acceptable signal can no longer be obtained , then the system is said to be noise-limited.
Medium wave (broadcast band) and shortwave broadcasts operate in a very noisy environment; background noise limits the performance in the broadcasts/shortwave bands. The VHF (Very High Frequency) and UHF (Ultra High Frequency) bands where cellular radio operates are relatively much quieter and most of the noise is generated in the radio frequency preamplifier of the receiver itself. Regardless of how well designed the receiver is, there is a theoretical noise power level which, at a given temperature, cannot be improved upon. This is because of the thermal noise generated by the movement of atomic particles ( in the receiver and most particularly in the first radio frequency amplifier). This noise is proportional to the operating temperature. Hence the antenna and RF amplifier stages will generate thermal noise continuously. For this reason high quality receivers, such as radio telescopes, operate their input stage RF amplifiers at liquid-nitrogen temperatures.
In order to perceive a relatively noise-free signal, the incoming signal must exceed the noise level by a respectable margin, known as the signal-to-noise ratio. For cellular radio systems, this level is usually regarded as 12 dB for marginal reception and 30 dB for good quality conversations.
Signal-to-noise ratio is usually expressed as:
S / N = Signal level / Noise level (usually expressed in dB)
Because modern receivers closely approach the theoretical noise limits for their operating temperatures, it can easily be deduced what minimum received signal level is required to achieve satisfactory signal-to-noise ratio.
Thus when we speak of a 39 dBmicrovolt/m boundary level for an AMPS system, this is equivalent to specifying the point at which the signal-to-noise ratio is regarded as satisfactory.
A "satisfactory" level of signal-to-noise for cellular systems is usually regarded as one where the noise is just noticeable. For PMR (Public Mobile Radio), a usable but noisy level is often regarded as satisfactory. In FM systems, the noise usually occurs as sharp clicks (sometimes known as "picket fence" noise because it is similar to the sound produced by dragging a stick along a picket fence).
Humans perceive power logarithmically. For example, doubling the energy level of a sound pulse produces only a 3 dB increase in the perceived level and that increase is only just noticeable. The term dB was introduced to define relative power levels logarithmically.
The term dB is used often in radio systems and can be a major source of
confusion to the uninitiated because of the large number of different units of dBs.
Essentially , the dB level is the log of a power ratio: dBm, the most common form of
dB, is the power of the system measured compared to 1 milliwatt. Mathematically, this can be expressed as:
Power dBm = 10 log [ Power (in watts) / (0.001) ]
Power dBm = 10 log [ Power (in milliwatts) / 1 ]
Thus 1 watt = 10 log 1 / 0.001 dBm = 30 dBm
dBmicrovolt/m is a unit of field strength which compares the measured level with 1 microvolt per meter.
Mathematically, this is :
dBmicrovolt/meter = 20 log[ field strength in microvolts per meter / 1 ]
Note: 20 is the multiplying factor here because the terms being used are voltage, not power. Voltage squared gives the power ratio.
Automatic Gain Control (AGC)
AGC was first implemented in radios because of the fading propagation is defined as the variations in the amplitude of the received signals, that requires the continuous adjustments in the receivers gain to maintain a relative constant output signal. This problem led to design the AGC circuits. The primary ideal function of the AGC is to maintain a constant signal level at the output, though there are the signal variations in the input of the system. Requires gain adjustment to prevent overload and to adjust the demodulator input level for optimum operation.
A simple method of gain control may involve the use of a variable attenuator between the input and first active stage. However the attenuator decrease the signal level, but it may also reduce the signal-to-noise ratio of the weakest acceptable signal. Gain control is normally distributed n number of stages, so that the gain in coming stages i.e IF amplifiers is reduced and then in the earlier stages gain is reduced only to assure a large S/N. If the RF gain is enough switching in or out of an attenuator at RF only for sufficient high signal levels then variable gain control for the later stages would operate from low signal levels. Variable gain amplifiers are operated electrically, but when attenuators are used in receivers they are often operated electrically either by variable voltages for continuous attenuators or by relays, diodes for fixed or stepped attenuators.
Fig: 1.1 AGC BLOCK DIAGRAM
The input signal is amplified by the variable gain amplifier where the gain is controlled by the external signal Vc. The output of the VGA is further amplified by second stage to generate tolerable level of Vo. The output parameters such as amplitude, carrier frequency, index of modulation or frequency are sensed by the detector and the undesired components are filtered out and the remaining signal is compared with the reference signal. This comparison generates the control voltage Vc and adjusts the gain of the VGA. An AGC circuit in the receiver provides the stable signal level to the demodulator independent of the input signal level.
Fig 1.2 AGC ideal transfer function
For low input signals AGC is disabled where the output is the linear function of the input, but when the output reaches a threshold value V1 the AGC becomes effective and maintains a constant output level until it reaches a second threshold value V2. In this stage AGC becomes inoperative again to prevent stability problems at high levels of gain. The steady sate change in the input is significantly reduced if the gain loop is much greater than 1.
Fig 1.3 AGC real transfer function
In the above figure A, B, C represents a system where there is n AGC is applied. The output increases linearly until point is reached with the input signal, where some elements in the signal overloads and becomes non-linear. In general the point B to C the output signal is distorted unless the input signal is reduced. The A, D, E represents a system where AGC is applied. The slope A, D is greater than unity that indicates the gain of AGC prior to the AGC detector.
The AGC system is basically a feedback amplifier that has a closed loop gain characteristics which is low-pass. AGC systems include reference voltage inside the control loop is considered as delayed AGC. Normally direct conversion receivers do not use AGC. The advantage of not using the AGC is simplicity and the purity of the signal that's been received where the weak received signals sound weak and strong signals sound loud. The improperly designed AGC system may introduce some distortion to a clean signal. Within the limits of noise and distortion of VGA, the AGC loop is set to provide both signal amplification and compression to provide the output signal within the limits. If AGC controls the receivers LNA then it may be critical to maintain the noise figure within the limits and this leads to the natural decrease the receivers noise figure, but by adding a delay diode in series with the AGC bias line, the start of gain control may be postponed slightly that leads to permit the LNA to maintain its noise figure and gain and the noise performance of the receiver.
AGC Detector Types:
The AGC detector operates normally in a linear mode, where the DC output being proportional to the RF input voltage.
Envelope Detector (Rectifier)
The output voltage of the envelop detector is proportional to the magnitude of the immediate RF input voltage. Suppose the low pass filtering is applied to the output to eliminate RF ripple, the detector produces a voltage proportional to the envelope amplitude of the RF signal. If the loops bandwidth is small to avoid significant gain pumping, the effect of the loop using the detector is to stabilize the average rectified voltage of the signal where the resulting power is dependent on the RF signal's envelope waveform.
This detector has an immediate output that is proportional to the square of the immediate RF input voltage where the output is proportional to input power. This behavior when integrated into an AGC loop of reasonable bandwidth makes the loops equilibrium average output power independent of the input waveform. In the envelope detector the output never goes to negative because of loop having the similar tendency toward slew rate limited behavior when reacting to abrupt decreases in input amplitude.
This detector comprises the square-law detector followed by a low-pass filter followed by a square-root function. The low-pass filter performs the "mean" operation related with root-mean-square (RMS) function and it should have sufficiently long time constant to smooth the output variations of the squaring detector or it may arise the reasonable modulation of the signal. Because of the square root element in the detector the average output is proportional to the signal voltage so that the loops response for abrupt increase or decrease of signal level should be the same as that for an envelope detector, provided that the added pole is located in a region of the signal path.
This detector produces an output proportional to the logarithm of the RF input voltage, as this type of behavior is complimentary to that of linear -in-dB VGA in the loop, the resulting loop dynamics are those of a linear system assuming that the signal level fluctuations during transients remain with in the measurement range of the log detector, where the AGC loop's response to large changes in the input level will not be slew-rate limited, and will be faster to recover from amplitude decreases.
Variable Gain Amplifier - VGA
Variable Gain Amplifiers are commonly used as feedback configurations as automatic gain control amplifiers where the amplitude of the output signal is kept constant for all input signal levels. The ratio of maximum to minimum input signal amplitude that can handle is called the dynamic range.
DIGITAL AUTOMATIC GAIN CONTROL :
DAGC is a modular design that consists of several pipelined blocks. RSSI signal ( Received Signal Strength Indication ) is generated in analog WLAN front end and indicates power of incoming signal. This signal is digitized by using a 10-bit parallel AD controller.
Fig Digital Automatic Gain Control
The power detector is determined by the input value of RSSI signal and the control signal, that averages the RSSI signal. The power calculation linearly estimates the power of the signal based on the averaged RSSI. The values of the parameters for the linear interpolation is stored in the control registers and it an be programmed externally i.e they are extracted from the measurement results. Power calculation Pn is calculated from the input signal Pn and the linearization coefficients beta and A0. With coefficient settings it is possible to get ideal fitting and to calculate output signal. Based on the calculated power and the LNA it is possible to calculate the next value of VGA and LNA in block VGA and LNA correction. To not to overload the digital receiver with over-amplified noise it is better to define the saturation level of VGA (VGA_max). This parameter helps to represent the value of gain signal where it is possible to receive the correct data.
AGC and Digital Communications :
AGC circuits are widely applied in digital communication systems, disk drive read channels and many other digital systems. A receiver for data signals will have a flat output requirement so that the data detection circuits have a constant-amplitude input.
Normally, error free recovery of data from the input signal cannot occur till the AGC circuit has adjusts the amplitude of the incoming signal. Such amplitude acquisition normally occurs during a introduction where known data is transmitted. The preamble duration must exceed the acquisition or settling time of the AGC loop, but its duration must be minimized for efficient use of the channel bandwidth.
In a digital communication receiver, strong signals fall outside the narrowband digital filter bandwidth, but if they fall inside the analog IF translator bandwidths could overload saturate the A/D converter that results in the generation of in-band products and can results in significant degradation of the desired signal. If the large signal levels are detected at the A/D converter, the receiver gain may have to be re-distributed by reducing the analog gain and increasing the digital gain to maintain the desired signal output level, this reduces the signal-to-quantization noise ratio.