Amplitude Modulation Reception (Chapter 5)

AM Demodulation

The reverse process of AM modulation.

RF Section

The first stage of the receiver of which primary functions are detecting, band limiting, and amplifying the received.

Mixer / Converter

This section down-converts the received RF frequencies to intermediate frequencies (IFs).

IF Section

This section primary functions are amplification and selectivity.

AM Detector

This section demodulates the AM wave and converts it to the original information signal.

Audio Section

This section amplifies the recovered information. Comprises several cascaded audio amplifiers and one or more speakers.

Selectivity

A receiver parameter that is used to measure the ability of the receiver to accept a given band of frequencies and reject all others.

Shape Factor

The ratio of the bandwidth 60dB below maximum signal level and bandwidth 3dB below maximum signal level.

Thermal Noise

The most prevalent form of noise and is directly proportional to bandwidth.

Bandwidth Improvement

Noise reduction ratio achieved by reducing the Bandwidth.

Noise Figure Improvement

The corresponding reduction in the noise figure due to the reduction in bandwidth expressed mathematically in dB.

Sensitivity

The ________ of a receiver is the minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal. Also known as receiver threshold.

Dynamic Range

Defined as the difference in decibels between the minimum input level necessary to discern a signal and the input level that will overdrive the receiver and produce distortion.

1-dB Compression Point

Defined as the output power when the RF amplifier response is 1 dB less than the ideal linear-gain response.

Fidelity

A measure of the ability of a communication system to produce, at the output of the receiver, an exact replica of the original source information.

Distortion

Any frequency, phase, or amplitude variations that are present in the demodulated waveform that were not in the original information signal.

Absolute Phase Shift

The total phase shift encountered by a signal and can generally be tolerated as long as all frequencies undergo the same amount of phase delay.

Differential Phase Shift

Occurs when different frequencies undergo different phase shifts and ay have a detrimental effect on a complex waveform.

Insertion Loss ( IL )

Defined as 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.

Equivalent Noise Temperature

A hypothetical value that cannot be directly measured. A parameter that is used in low-noise, sophisticated radio receivers rather than noise figure.

Coherent / Synchronous Receiver

The frequencies generated in the receiver and used for demodulation are synchronized to oscillator frequencies generated in the transmitter.

Noncoherent / Asynchronous Receiver

Either no frequencies are generated in the receiver or the frequencies used for demodulation are completely independent from the transmitter's carrier frequency.

Tuned Radio Frequency

One of the earliest types of AM receivers and are probably the simplest designed radio receivers available today.

Skin Effect

A phenomenon at radio frequencies where current flow is limited to the outermost area of a conductor.

Stagger Tuning

A technique where TRF receiver's instability can be reduced somewhat by tuning each amplifier to a slightly different frequency, slightly above or below the desired center frequency.

Heterodyne

Means to mix two frequencies together in a nonlinear device or to translate one frequency to another using nonlinear mixing.

Preselector

A broad -tuned bandpass filter with an adjustable center frequency that is tuned to desired carrier frequency.

455 kHz

The most common intermediate frequency used in AM broadcast-band receivers is ________.

IF Section

Consists of a series of IF amplifiers and bandpass filters and is often called IF strip.

Intermediate Frequency

Refer to frequencies that are used within a transmitter or receiver that fall somewhere between the radio frequencies and the original source information frequencies.

Gang Tuning

Means that the two adjustments are mechanically tied together so that a single adjustment will change the center frequency of the preselector and, at the same time, change the local oscillator frequency.

High-side Injection / High-beat Injection

When the local oscillator is tuned above the RF it is?

Low-side Injection / Low-beat Injection

When the local oscillator is tuned below the RF it is?

Sideband Inversion

The side frequencies undergo a sideband reversal during the heterodyning process called?

Tracking

The ability of the local oscillator in a receiver to oscillate above or below the selected radio frequency carrier by an amount equal to the intermediate frequency throughout the entire radio frequency band.

Tracking Error

The difference between the actual oscillator frequency and the desired frequency.

Image Frequency

Any frequency other than the selected radio frequency carrier that, if allowed to enter a receiver and mix with the local oscillator, will produce a crossproduct frequency that is equal to the intermediate frequency.

Image-frequency Rejection Ratio

A numerical measure of the ability of a preselector to reject the image frequency.

Double Spotting

Occurs when a receiver picks up the same station at two nearby points on the receiver tuning dial.

RF Amplifier

A high-gain, low-noise, tuned amplifier that, when used, is the first active stage encountered by the received signal.

Low-noise Amplifier ( LNA )

High-performance microwave receivers require a ________ as the input stage of the RF section to optimize their noise figure.

MEsa Semiconductor FET Semiconductor FET ( MESFET )

A FET with a metal-semiconductor junction at the gate of the device, called a Schottky barrier.

NE / SA5200

A wideband, unconditionally stable, low-power, dualgain linear integrated-circuit RF amplifier manufactured by Signetics Corporation.

Mixer / Converter Stage

This section purpose is to down-convert the incoming radio frequencies to intermediate frequencies proportional to bandwidth.

Conversion Gain

The difference between the level of the IF output with an RF input signal to the level of the IF output with an IF input signal.

Self-excited Mixer

A configuration where the mixer excites itself by feeding energy back to the local oscillator tank circuit to sustain oscillations noise figure.

NE / SA602A

A low-power VHF monolithic double-balanced mixer with input amplifier, on-board oscillator, and voltage regulator.

Intermediate Frequency ( IF ) Amplifier

Are relatively high-gain amplifiers that are very similar to RF amplifiers, except that IF amplifiers operate over a relatively narrow, fixed frequency band.

Inductive or Transformer Coupling

The most common technique used for coupling where the voltage that is applied to the primary windings of a transformer is transferred to the secondary windings.

Inductance

Ability of a coil to induce a voltage within its windings.

Mutual Inductance

Ability of one coil to induce a voltage in another coil.

Coefficient of Coupling

The ratio of the secondary flux to the primary flux.

Flux Linkage

The transfer of flux from the primary to the secondary windings and is directly proportional to the coefficient of coupling.

Critical Coupling

The point where the reflected resistance is equal to the primary resistance an d the Q of the primary tank circuit is halved and the bandwidth doubled.

Double Peaking

Is caused by the reactive element of the reflected impedance being significant enough to change the resonant frequency of the primary tuned circuit.

Optimum Coupling

The coefficient of coupling approximately 50% greater than the critical value yields a good compromise between flat response and steep skirts.

IF Cans

IF transformers come as specially designed tuned circuits in groundable metal packages called _______.

CA3028A

A differential cascoded amplifier designed for use in communications and industrial equipment as an IF or RF amplifier at frequencies from dc to 120 MHz.

AM Detector

The function of this circuit is to demodulate the AM signal and recover or reproduce the original source information.

Peak Detector

A simple noncoherent AM demodulator using a diode. Also called as diode, shape, or envelope detector.

Rectifier Distortion

A distortion in the detection process where the RC time constant is too short, the output waveform resembles a half-wave rectified signal.

Diagonal Clipping

A distortion in the detection process where the RC time constant is too long, the slope of the output waveform cannot follow the trailing slope of the envelope.

Automatic Gain Control ( AGC )

A circuit that compensates for minor variations in the received RF signal.

Delayed AGC

It prevents the AGC feedback voltage from reaching the RF or IF amplifiers until the RF level exceeds a predetermined magnitude.

Forward AGC

Is similar to conventional AGC except that the receive signal is monitored closer to the front end of the receiver and the correction voltage is fed forward to the IF amplifiers.

Squelch Circuit

Its purpose is to quiet a receiver in the absence of a received signal.

Limiters / Clippers

Are used to remove sporadic, high-amplitude noise transients of short duration, such as impulse noise in the audio section of a receiver.

Signal-to-Notched Noise Ratio

A method of measuring signal strength relative to noise strength where an RF carrier modulated 30% by a 1-kHz tone is applied to the input of the receiver.

LM1820

A National Semiconductor Corporation linear integrated circuit AM radio chip that has an onboard RF amplifier, mixer, local oscillator, and IF amplifier stages. An LIC audio amplifier, such as the LM386, and a speaker are necessary to complete a functional receiver.

PLL Receivers

This receivers would need only two external components: a volume control and a station tuning control.

Net Receiver Gain

The ratio of the demodulated signal level at the output of the receiver (audio) to the RF signal level at the input to the receiver, or the difference between the audio signal level in dBm and the RF signal level in dBm.

System Gain

Includes all the gains and losses incurred by a signal as it propagates from the transmitter output stage to the output of the detector in the receiver and includes antenna gain and transmission line and propagation losses.