Communication Receivers In radio communication systems, the transmitted signal is very weak when it reaches the receiver, particularly when it has traveled over a long distance. The signal has also picked up noise of various kinds. Receivers must provide the sensitivity and selectivity that permit full recovery of the original signal. The radio receiver best suited to this task is known as the superheterodyne receiver.
Topics Covered in Chapter 9 Basic Principles of Signal Reproduction Superheterodyne Receivers Frequency Conversion Intermediate Frequency and Images Noise Typical Receiver Circuits Receivers and Transceivers
Basic Principles of Signal Reproduction A communication receiver must be able to identify and select a desired signal from the thousands of others present in the frequency spectrum (selectivity) and to provide sufficient amplification to recover the modulating signal (sensitivity). A receiver with good sensitivity will isolate the desired signal and greatly attenuate other signals. A receiver with good sensitivity involves high circuit gain.
Selectivity Selectivity in a receiver is obtained by using tuned circuits and/or filters. LC tuned circuits provide initial selectivity. Filters, which are used later in the process, provide additional selectivity. By controlling the Q of a resonant circuit, you can set the desired selectivity. The optimum bandwidth is one that is wide enough to pass the signal but narrow enough to eliminate signals on adjacent frequencies.
Sensitivity A communication receivers sensitivity, or ability to pick up weak signals, is mainly a function of overall gain, the factor by which an input signal is multiplied to produce the output signal. The higher the gain of a receiver, the better its sensitivity. The more gain that a receiver has, the smaller the input signal necessary to produce a desired level of output. High gain in receivers is obtained by using multiple amplification stages.
Basic Receiver Configuration The simplest radio receiver is a crystal set consisting of a tuned circuit, a diode (crystal) detector, and earphones. The tuned circuit provides the selectivity. The diode and a capacitor serve as an AM demodulator. The earphones reproduce the recovered audio signal.
A Crystal Receiver
Tuned Radio Frequency (TRF) Receiver In the tuned radio frequency (TRF) receiver sensitivity is improved by adding a number of stages of RF amplification between the antenna and detector, followed by stages of audio amplification. The RF amplifier stages increase the gain before it is applied to the detector. The recovered signal is amplified further by audio amplifiers, which provide sufficient gain to operate a loudspeaker.
Superheterodyne Receiver Superheterodyne receivers convert all incoming signals to a lower frequency, known as the intermediate frequency (IF), at which a single set of amplifiers is used to provide a fixed level of sensitivity and selectivity. Gain and selectivity are obtained in the IF amplifiers. The key circuit is the mixer, which acts like a simple amplitude modulator to produce sum and difference frequencies. The incoming signal is mixed with a local oscillator.
RF Amplifier The antenna picks up the weak radio signal and feeds it to the RF amplifier, also called a low-noise amplifier (LNA). RF amplifiers provide some initial gain and selectivity and they are sometimes referred to as preselectors. Tuned circuits help select the frequency range in which the signal resides. RF amplifiers minimize oscillator radiation. Bipolar and FETs can be used as RF amplifiers.
Mixers and Local Oscillators The output of the RF amplifier is applied to the input of the mixer. The mixer also receives an input from a local oscillator or frequency synthesizer. The mixer output is the input signal, the local oscillator signal, and the sum and difference frequencies of these signals. A tuned circuit at the output of the mixer selects the difference frequency, or intermediate frequency (IF). The local oscillator is made tunable so that its frequency can be adjusted over a relatively wide range.
Amplifiers The output of the mixer is an IF signal containing the same modulation that appeared on the input RF signal. The signal is amplified by one or more IF amplifier stages, and most of the gain is obtained in these stages. Selective tuned circuits provide fixed selectivity. Since the intermediate frequency is lower than the input frequency, IF amplifiers are easier to design and good selectivity is easier to obtain.
Demodulators The highly amplified IF signal is finally applied to the demodulator, or detector, which recovers the original modulating information. The demodulator may be a diode detector (for AM), a quadrature detector (for FM), or a product detector (for SSB). The output of the demodulator is then usually fed to an audio amplifier.
Automatic Gain Control The output of a detector is usually the original modulating signal, the amplitude of which is directly proportional to the amplitude of the received signal. The recovered signal, which is usually AC, is rectified and filtered into a DC voltage by a circuit known as the automatic gain control (AGC) circuit. This DC voltage is fed back to the IF amplifiers, and sometimes the RF amplifier, to control receiver gain. AGC circuits help maintain a constant output level over a wide range of RF input signal levels.
Frequency Conversion Frequency conversion is the process of translating a modulated signal to a higher or lower frequency while retaining all the originally transmitted information. In radio receivers, high-frequency signals are converted to a lower, intermediate frequency. This is called down conversion. In satellite communications, the original signal is generated at a lower frequency and then converted to a higher frequency. This is called up conversion.
Mixing Principles Frequency conversion is a form of amplitude modulation carried out by a mixer circuit or converter. The function performed by the mixer is called heterodyning.
Diode Mixer The primary characteristic of mixer circuits is nonlinearity. Any device or circuit whose output does not vary with the input can be used as a mixer. One of the most widely used types of mixer is the simple diode modulator.
Diode Mixer Operation The input signal, directly from the antenna, is applied to the primary winding of the transformer. The signal is coupled to the secondary winding and applied to the diode mixer, and the local oscillator signal is coupled to the diode by way of a capacitor. The input and local oscillator signals are linearly added this way and applied to the diode, which produces the sum and difference frequencies. The output signals are developed across the tuned circuit which selects the difference frequency.
Bipolar Transistor Mixer The primary benefit of transistor mixers over diode mixers is that gain is obtained. Transistor bias is set to provide class AB. Bipolar mixers with their inherent high noise are often replaced with field-effect transistors (FETs).
IC Mixer The NE602, IC mixer, also known as a Gilbert transconductance cell or Gilbert cell, consists of a double balanced mixer circuit made up of two cross- connected differential amplifiers.
IC Mixer NE602
Local Oscillator and Frequency Synthesizer The local oscillator signal for the mixer comes from either a conventional LC tuned oscillator or a frequency synthesizer. The simpler continuously tuned receivers use an LC oscillator. Channelized receivers use frequency synthesizers.
LC Oscillator A local oscillator, which is sometimes referred to as a variable-frequency oscillator, or VFO is used for frequencies up to 100 MHz. An amplifier (e.g. FET) is connected as a Colpitts oscillator. Feedback is developed by a voltage divider made up of capacitors. The frequency is set by a parallel tuned circuit. The output is taken across an RFC and it is buffered by a direct-coupled emitter follower.
Variable Frequency Oscillator
Frequency Synthesizer Most new receiver designs incorporate frequency synthesizers for the local oscillator, which provides some important benefits over simple VFO designs. The synthesizer is usually of the phase-locked loop (PLL) design and the output is locked to a crystal oscillator reference which provides high stability. Tuning is accomplished by changing the frequency division factor in the PLL, resulting in incremental rather than continuous frequency changes.
Intermediate Frequency and Images The primary objective in the design of an IF stage is to obtain good selectivity. Narrow-band selectivity is best obtained at lower frequencies. At low frequencies, circuits are more stable. At low frequencies, image interference is possible. At higher frequencies, circuit layouts must take into account stray inductances and capacitances. At higher frequencies, there is a need for shielding.
By Definition… To reduce image interference, which is an RF signal two times the IF above or below the incoming frequency, high-Q tuned circuits should be used ahead of the mixer or RF amplifier. Noise is the static you hear in the speaker when you tune an AM or FM receiver to any position between stations.
Direct Conversion Receiver A special version of the superheterodyne is known as the direct conversion (DC) or zero IF (ZIF) receiver. DC receivers convert the incoming signal directly to baseband without converting to an IF. They perform demodulation as part of the translation. The low-noise amplifier (LNA) boosts the signal before the mixer. The local oscillator (LO) frequency is set to the frequency of the incoming signal. Baseband output is passed via a low-pass filter (LPF).
By Definition… A software-defined radio (SDR) is a receiver in which most of the functions are performed by a digital signal processor (DSP). Noise consists of an electronic signal that is a mixture of many frequencies at many amplitudes that gets added to a radio or information signal as it is transmitted from one place to another. The signal-to-noise (S/N) ratio indicates the relative strengths of the signal and the noise in a communication system.
External Noise External noise comes from sources over which we have little or no control. Examples include: Industrial noise Atmospheric (static) noise Extraterrestrial noise
Internal Noise Electronic components is a receiver such as resistors, diodes, and transistors are major sources of internal noise. Types of internal noise include: Thermal noise Semiconductor noise Intermodulation (distortion) noise
Expressing Noise Levels The noise quality of a receiver can be expressed as in the following terms. The noise factor is the ratio of the S/N power at the input to the S/N power at the output. When the noise factor is expressed in decibels, it is called the noise figure. Most of the noise produced in a device is thermal, which is directly proportional to temperature. Therefore, the term noise temperature (T N ) is used. SINAD is the composite signal plus noise and distortion divided by noise and distortion contributed by the receiver.
Typical Receiver Circuits Typical receiver circuits include: RF amplifiers IF amplifiers AGC AFC Special circuits
RF Input Amplifier The RF amplifier, also called a low-noise amplifier (LNA), processes the very weak input signals, increasing their amplitude prior to mixing. Low-noise components are used to ensure a sufficiently high S/N ratio. Selectivity should be such that it effectively eliminates images. The RF amplifier is typically a class A circuit that can be configured with bipolar or field-effect transistors.
RF Amplifier Circuit
IF Amplifier Most of the gain and selectivity in a superheterodyne receiver are obtained in the IF amplifier. If amplifiers are tuned class A circuits capable of providing gain in the 10- to 30-dB range. Usually two or more IF amplifiers are used to provide adequate receiver gain. Ferrite-core transformers are used for coupling between stages. Selectivity is provided by tuned circuits.
Two-Stage IF Amplifier
By Definition… Crystal, ceramic, or SAW filters are used for better IF selectivity. They are smaller than LC circuits and provide higher selectivity and require no tuning. In FM receivers, one or more of the IF amplifier stages is used as a limiter, to remove any amplitude variations on the FM signal before the signal is applied to the demodulator.
Automatic Gain Control Receiver gain is typically far greater than required for adequate reception. Excessive gain usually causes the received signal to be distorted and the transmitted information to be less intelligible. Manual gain control by using a potentiometer in RF and IF stages can be achieved. Receivers include volume controls in audio circuits. AGC circuits are more effective and give the receiver a very wide dynamic range, which is the ratio of the largest signal handled to the lowest signal.
AGC and Circuit Gain The gain of a bipolar transistor amplifier is proportional to the amount of collector current flowing. Two methods of applying AGC are as follows: The gain can be decreased by decreasing the collector current. This is referred to as reverse AGC. The gain can be reduced by increasing the collector current. A stronger signal increases AGC voltage and base current and, in turn, increases collector current, reducing the gain. This method of gain control is known as forward AGC.
AGC Applied to IF Amplifier
AGC Operation A common emitter IF amplifier with bias derived from voltage divider resistors can be configured. A DC voltage is fed by way of a resistor to the base of the common emitter amplifier. As the level of the signal amplitude increases, the negative DC voltage increases, decreasing the base current. The reduced base current, in turn, decreases the collector current and lowers the circuit gain.
Squelch Circuit A squelch circuit, or muting circuit, is found in most communications receivers. The squelch is used to keep the receiver audio turned off until an RF signal appears at the receiver input. In AM systems such as CB radios, the noise level is high and can be very annoying. Squelch circuits provide a means of keeping the audio amplifier turned off during the time that noise is received in the background and enabling it when an RF signal appears at the input.
SSB and Continuous-Wave Reception Communication receivers designed for receiving SSB or continuous-wave signals have a built-in oscillator that permits recovery of the transmitted information. A circuit called the beat frequency oscillator (BFO), is usually designed to operate near the IF. The BFO signal is applied to the demodulator along with the IF signal containing the modulation.
Beat Frequency Oscillator (BFO)
Integrated Circuits (ICs) in Receivers In new designs, virtually all receiver circuits are ICs. A complete receiver usually consists of three or four ICs. IC receivers include components such as coils, transformers, capacitors, and filters. Most modern receivers are contained on a single IC.
Integrated Circuits (ICs) in Receivers (Continued) IC receivers are typically broken down into three major sections: The tuner, with RF amplifier, mixer, and local oscillator The IF section, with amplifiers, demodulator, and AGC and muting circuits The audio power amplifier
VHF Receiver A typical VHF receiver is designed to receive two-way aircraft communication between planes and airport controllers. They have a typical frequency range of 118 to 135 MHz. Amplitude modulation is typical with these receivers. VHF receivers are designed to use a combination of discrete components and ICs.
Transceiver Most two-way radio communication equipment is packaged so that both transmitter and receiver are in a unit known as a transceiver. Transceivers range from large, high-power desktop units to small, pocket-sized, handheld units. Transceivers have a common housing and power supply. Transceivers can share circuits, thereby achieve cost savings, and in some cases are smaller in size.