3Topics Covered in Chapter 9 9-1: Basic Principles of Signal Reproduction9-2: Superheterodyne Receivers9-3: Frequency Conversion9-4: Intermediate Frequency and Images9-5: Noise9-6: Typical Receiver Circuits9-7: Receivers and Transceivers
49-1: Basic Principles of Signal Reproduction 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.
59-1: 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 selectivity will isolate the desired signal and greatly attenuate other signals.A receiver with good sensitivity involves high circuit gain.
69-1: Basic Principles of Signal Reproduction Selectivity: Q and BandwidthSelectivity in a receiver is obtained by using tuned circuits and/or filters.LC tuned circuits provide initial selectivity.Filters 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 and its sidebands but narrow enough to eliminate signals on adjacent frequencies.
79-1: Basic Principles of Signal Reproduction Figure 9-1: Selectivity curve of a tuned circuit.
89-1: Basic Principles of Signal Reproduction Selectivity: Shape FactorThe sides of a tuned circuit response curve are known as skirts.The steepness of the skirts, or the skirt selectivity, of a receiver is expressed as the shape factor, the ratio of the 60-dB down bandwidth to the 6-dB down bandwidth.The lower the shape factor, the steeper the skirts and the better the selectivity.
99-1: Basic Principles of Signal Reproduction SensitivityA communication receiver’s sensitivity, or ability to pick up weak signals, is 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.
109-1: Basic Principles of Signal Reproduction SensitivityAnother factor that affects the sensitivity of a receiver is the signal-to-noise (S/N) ratio (SNR).One method of expressing the sensitivity of a receiver is to establish the minimum discernible signal (MDS).The MDS is the input signal level that is approximately equal to the average internally generated noise value.This noise value is called the noise floor of the receiver.MDS is the amount of signal that would produce the same audio power output as the noise floor signal.
119-1: Basic Principles of Signal Reproduction Basic Receiver ConfigurationThe 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.
129-1: Basic Principles of Signal Reproduction Figure 9-4: The simplest receiver—a crystal set.
139-1: Basic Principles of Signal Reproduction Tuned Radio Frequency (TRF) ReceiverIn 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.
149-1: Basic Principles of Signal Reproduction Figure 9-5: Tuned radio-frequency (TRF) receiver.
159-1: Basic Principles of Signal Reproduction Tuned Radio Frequency (TRF) ReceiverMany RF amplifiers use multiple tuned circuits.Whenever resonant LC circuits tuned to the same frequency are cascaded, overall selectivity is improved.The greater the number of tuned stages cascaded, the narrower the bandwidth and the steeper the skirts.The main problem with TRF receivers is tracking the tuned circuits.In a receiver, the tuned circuits must be made variable so that they can be set to the frequency of the desired signal.Another problem with TRF receivers is that selectivity varies with frequency.
169-2: Superheterodyne Receivers 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 signal.
179-2: Superheterodyne Receivers Figure 9-8: Block diagram of a superheterodyne receiver.
189-2: Superheterodyne Receivers RF AmplifierThe 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 are sometimes called 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.
199-2: Superheterodyne Receivers Mixers and Local OscillatorsThe 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.
209-2: Superheterodyne Receivers IF AmplifiersThe 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 usually lower than the input frequency, IF amplifiers are easier to design and good selectivity is easier to obtain.
219-2: Superheterodyne Receivers DemodulatorsThe highly amplified IF signal is finally applied to the demodulator, 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.
229-2: Superheterodyne Receivers Automatic Gain ControlThe output of a demodulator 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.
239-2: Superheterodyne Receivers Automatic Gain ControlThe amplitude of the RF signal at the antenna of a receiver can range from a fraction of a microvolt to thousands of microvolts; this wide signal range is known as the dynamic range.Typically, receivers are designed with very high gain so that weak signals can be reliably received.However, applying a very high-amplitude signal to a receiver causes the circuits to be overdriven, producing distortion and reducing intelligibility.With AGC, the overall gain of the receiver is automatically adjusted depending on the input signal level.
249-3: 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.
259-3: Frequency Conversion Mixing PrinciplesFrequency conversion is a form of amplitude modulation carried out by a mixer circuit or converter.The function performed by the mixer is called heterodyning.
269-3: Frequency Conversion Mixing PrinciplesMixers accept two inputs: The signal to be translated to another frequency is applied to one input, and the sine wave from a local oscillator is applied to the other input.Like an amplitude modulator, a mixer essentially performs a mathematical multiplication of its two input signals.The oscillator is the carrier, and the signal to be translated is the modulating signal.The output contains not only the carrier signal but also sidebands formed when the local oscillator and input signal are mixed.
279-3: Frequency Conversion Figure 9-9: Concept of a mixer.
289-3: Frequency Conversion Mixer and Converter Circuits: Diode MixerThe primary characteristic of mixer circuits is nonlinearity.Any device or circuit whose output does not vary linearly with the input can be used as a mixer.One of the most widely used types of mixer is the simple diode modulator.
299-3: Frequency Conversion Mixer and Converter Circuits: Diode MixerThe input signal 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 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.
309-3: Frequency Conversion Figure 9-10: A simple diode mixer.
319-3: Frequency Conversion Mixer and Converter CircuitsSingly balanced mixer: A popular mixer circuit using two diodes.Doubly balanced mixer: This version of the diode balanced modulator is probably the single best mixer available, especially for VHF, UHF, and microwave frequencies.FET Mixers: FETs make good mixers because they provide gain, have low noise, and offer a nearly perfect square-low response.
329-3: Frequency Conversion Mixer and Converter Circuits: IC MixerThe NE602, a typical IC mixer, is also known as a Gilbert transconductance cell or Gilbert cell.It consists of a double balanced mixer circuit made up of two cross-connected differential amplifiers.Mixer and Converter Circuits: Image Reject MixerAn image reject mixer is a special type of mixer used in designs in which images cannot be tolerated.It uses Gilbert cell mixers in a configuration like that used in a phasing-type SSB generator.
339-3: Frequency Conversion Figure 9-15: NE602 IC mixer. (a) Block diagram and pinout. (b) Simplified schematic.
349-3: Frequency Conversion Local Oscillator and Frequency SynthesizersThe 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.
359-3: Frequency Conversion Local Oscillator and Frequency Synthesizers: LC OscillatorA local oscillator is sometimes referred to as a variable-frequency oscillator, or VFO.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.
369-3: Frequency Conversion Figure 9-17: A VFO for receiver local oscillator service.
379-3: Frequency Conversion Local Oscillator and Frequency Synthesizers: Frequency SynthesizerMost 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.
389-3: Frequency Conversion Figure 9-18: A frequency synthesizer used as a receiver local oscillator.
399-4: 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 with high gain.
409-4: Intermediate Frequency and Images At low frequencies, image interference is possible. An image is an RF signal two times the IF above or below the incoming frequency.At higher frequencies, circuit layouts must take into account stray inductances and capacitances.At higher frequencies, there is a need for shielding.
419-4: Intermediate Frequency and Images Figure 9-19: Relationship of the signal and image frequencies.
429-4: Intermediate Frequency and Images Figure 9-20: Signal, local oscillator, and image frequencies in a superheterodyne.
439-4: Intermediate Frequency and Images Solving the Image ProblemTo reduce image interference, high-Q tuned circuits should be used ahead of the mixer or RF amplifier.The IF is made as high as possible for effective elimination of the image problem, yet low enough to prevent design problems.In most receivers the IF varies in proportion to the frequencies that must be covered.
449-4: Intermediate Frequency and Images Figure 9-21: A low IF compared to the signal frequency with low-Q tuned circuits causes images to pass and interfere.
459-4: Intermediate Frequency and Images Dual-Conversion ReceiversAnother way to obtain selectivity while eliminating the image problem is to use a dual-conversion superheterodyne receiver.A typical receiver uses two mixers and local oscillators, so it has two IFs.The first mixer converts the incoming signal to a high intermediate frequency to eliminate the images.The second mixer converts that IF down to a much lower frequency, where good selectivity is easier to obtain.
469-4: Intermediate Frequency and Images Figure 9-22: A dual-conversion superheterodyne.
479-4: Intermediate Frequency and Images Direct Conversion ReceiversA 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).
489-4: Intermediate Frequency and Images Figure 9-23: A direct-conversion (zero-IF) receiver.
499-4: Intermediate Frequency and Images Direct Conversion ReceiversAdvantages:No separate IF filter is needed.No separate detector circuit is needed.In transceivers that use half duplex and in which the transmitter and receiver are on the same frequency, only one PLL frequency synthesizer voltage-controlled oscillator is needed.There is no image problem.
509-4: Intermediate Frequency and Images Direct Conversion ReceiversDisadvantages:In designs with no RF amplifier (LNA), the LO signal can leak through the mixer to the antenna and radiate.An undesired dc offset can develop in the output.The ZIF receiver can be used only with CW, AM, SSB, or DSB. It cannot recognize phase or frequency variations.
519-4: Intermediate Frequency and Images Figure 9-24: A direct conversion receiver for FM, FSK, PSK, and digital modulation.
529-4: Intermediate Frequency and Images Direct Conversion ReceiversTo demodulate FM and PM modulations in a zero-IF receiver, two mixers and filters are needed.There must be a 90° phase shift between the LO signals to produce I and Q signals for the DSP demodulation.
539-4: Intermediate Frequency and Images Software-Defined RadioA software-defined radio (SDR) is a receiver in which most of the functions are performed by a digital signal processor (DSP).The benefits of SDRs are improved performance and flexibility.The receiver characteristics (type of modulation, selectivity, etc.) can be easily changed by running a different program.
549-5: NoiseNoise is an electronic signal that gets added to a radio or information signal as it is transmitted from one place to another.It is not the same as interference from other information signals.
559-5: NoiseNoise is the static you hear in the speaker when you tune any AM or FM receiver to any position between stations. It is also the “snow” or “confetti” that is visible on a TV screen.The noise level in a system is proportional to temperature and bandwidth, the amount of current flowing in a component, the gain of the circuit, and the resistance of the circuit.
569-5: Noise Signal-to-Noise Ratio The signal-to-noise (S/N) ratio indicates the relative strengths of the signal and the noise in a communication system.The stronger the signal and the weaker the noise, the higher the S/N ratio.The S/N ratio is a power ratio.
579-5: Noise External Noise External noise comes from sources over which we have little or no control, such as:Industrial sourcesmotors, generators, manufactured equipmentAtmospheric sourcesThe naturally occurring electrical disturbances in the earth’s atmosphere; atmospheric noise is also called static.SpaceThe sun radiates a wide range of signals in a broad noise spectrum.
589-5: Noise Internal Noise Electronic components in a receiver such as resistors, diodes, and transistors are major sources of internal noise. Types of internal noise include:Thermal noiseSemiconductor noiseIntermodulation distortion
599-5: Noise Expressing Noise Levels The noise quality of a receiver can be expressed 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 (TN) is used.SINAD is the composite signal plus noise and distortion divided by noise and distortion contributed by the receiver.
609-5: Noise Noise in Cascaded Stages Noise has its greatest effect at the input to a receiver because that is the point at which the signal level is lowest.The noise performance of a receiver is determined in the first stage of the receiver, usually an RF amplifier or mixer.
629-6: Typical Receiver Circuits RF Input AmplifierThe 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.
639-6: Typical Receiver Circuits Figure 9-30: A typical RF amplifier used in receiver front ends.
649-6: Typical Receiver Circuits IF AmplifierMost 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.
659-6: Typical Receiver Circuits Figure 9-33: A two-stage IF amplifier using double-tuned transformer coupling for selectivity.
669-6: Typical Receiver Circuits Traditional IF Amplifier Circuits: Coupled Circuit SelectivityChanging the amount of coupling between the primary and secondary windings allows the desired amount of bandwidth to be obtained. At some particular degree of coupling, known as critical coupling, the output reaches a peak value.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.
679-6: Typical Receiver Circuits Traditional IF Amplifier Circuits: Coupled Circuit SelectivityMost modern receivers do not use LC tuned filters but instead use crystal, ceramic, mechanical, SAW or DSP filters.
689-6: Typical Receiver Circuits Automatic Gain Control CircuitsReceiver 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 can be achieved by using a potentiometer in RF and IF stages.Receivers include volume controls in audio circuits.AGC circuits are more effective in handling large signals and give the receiver a very wide dynamic range.
699-6: Typical Receiver Circuits Automatic Gain Control Circuits: Controlling Circuit GainThe 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 called 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.
709-6: Typical Receiver Circuits Figure 9-37: An IF differential amplifier with AGC.
719-6: Typical Receiver Circuits Squelch CircuitA 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.
729-6: Typical Receiver Circuits SSB and Continuous-Wave ReceptionCommunication 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.
739-6: Typical Receiver Circuits Figure 9-42: The use of a BFO.
749-6: Typical Receiver Circuits Integrated Circuits (ICs) in ReceiversIn new designs, virtually all receiver circuits are ICs.A complete receiver usually consists of three or four ICs, plus coils, transformers, capacitors, and filters.Most modern receivers are contained on a single IC.
759-6: Typical Receiver Circuits Integrated Circuits (ICs) in ReceiversIC receivers are typically broken down into three major sections:The tuner, with RF amplifier, mixer, and local oscillatorThe IF section, with amplifiers, demodulator, and AGC and muting circuitsThe audio power amplifier.The second and third sections are entirely implemented with ICs. The tuner may or may not be, for often the LNA is separate.
769-7: Receivers and Transceivers VHF Aircraft Communication CircuitA 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.
779-7: Receivers and Transceivers Figure 9-44 The aviation receiver—a superheterodyne unit built around four ICs—is designed to receive AM signals in the 118- to 135-MHz frequency range. (Popular Electronics, January 1991, Gernsback Publications, Inc.)
789-7: Receivers and Transceivers Single-IC FM ReceiverThe Motorola MC3363 FM receiver IC chip contains all receiver circuits except for the audio power amplifier (a separate chip).It is designed to operate at frequencies up to about 200 MHzIt is widely used in cordless telephones, paging receivers, and other portable applications.This dual-conversion receiver contains two mixers, two local oscillators, a limiter, a quadrature detector, and squelch circuits.The first local oscillator has a built-in varactor that allows it to be controlled by an external frequency synthesizer.
799-7: Receivers and Transceivers Figure 9-45: The Motorola MC3363 dual-conversion receiver IC.
809-7: Receivers and Transceivers 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.
819-7: Receivers and Transceivers Figure 9-47: An SSB transceiver showing circuit sharing.