## INTRODUCTION
It would seem the practice these days to denigrate
the AM service on the broadcast band. In fact,the medium wave
AM service can probably cover a greater service area with better average fidelity
than any other broadcast system. This is particularly valuable in Australia where
large sparsely populated areas must be covered. ## INDEX## AM TUNER## REGENERATIVE RECEIVER## AM TRANSMITTER## AM and FM TRANSMISSION## HISTORY## AM FUNDAMENTALS## FM FUNDAMENTALS## LOOP ANTENNAS## AM BROADCASTING
## TUNERThe Tuner is all solid state and looks like this:- ## GENERAL DESCRIPTION:-
The maximum bandwidth of the tuner is +(-)19KHz. ## TUNER TRANSIENT RESPONSE## BANDWIDTH = +(-)19 KHz.
In the 19KHz. bandwidth position the slope of
the amplitude response flanks is not very steep, so that the
transient response is essentially monotonic with a rise time of
about 22uS. ## BANDWIDTH = 8KHz.-- Sharp Cutoff
In this position two sharp cutoff ceramic filters are added in series
with the normal IF. The sharp cutoff causes ringing in the transient response.
The assymetry in the passband produces a significant quadrature component.
## REGENERATIVE RECEIVER
AM allows the simplest demodulation of all modulation systems.
This was one of the reasons for its initial adoption in broadcasting.
It is instructive to compare the performance of an early regenerative
receiver with the tuner. top ## AM TRANSMITTER
## GENERAL DESCRIPTION:-The transmitter is all vacuum tube and divided into two sections:-
(a) A low power exciter driving: ## DISTORTION:-
The classical method of examining distortion in AM transmitters and
receivers is the trapezoidal pattern. ## FREQUENCY RESPONSE:-
The trapezoidal pattern provides a quick and easy method of evaluating
the amplitude and phase response of an AM transmitter. ## TRANSIENT RESPONSE:-
Since the transient response is uniquely related to the steady state
response, it provides a quick method of evaluating the dynamic behaviour of
a transmitter. top ## DISTORTION in AM and FM TRANSMISSION
## OPENING REMARKS:-When an AM signal is transmitted through a linear network of finite
bandwidth non-linear distortion may occur to the modulation content.
R.A Fessenden, a Canadian from Knowlton, Quebec, Canada invented and
patented AM in 1901.[ US Pat.753,863 ] One of his early ideas to produce
a high power AM transmitter without the use of vacuum tubes (invented 1906)
was to frequency modulate an arc transmitter with a condenser microphone
and turn this into AM by detuning the antenna system. He therefore was
aware of FM but did not patent it. This was done by C.D.Eheret of Chicago
in 1902. top
An AM signal is usually defined as:-
If product demodulation is used in AM transmission, the demodulated output can be
described as the sum of the solutions of two linear differential equations for transmission
through top
In FM we deal with In a signal defined by the equation:- Virtually all the properties of FM can be understood
from the study of a simple
THE FM COMPONENT OF TWO ADDED SINUSOIDS IS A NON-LINEAR FUNCTION. If the first sinusoid is assigned to the FM carrier, then the other may be
due to transients in a transmission network or an interfering signal caused by
multipath transmission. All will produce distortion in the FM modulation content.
In practice we have to design the bandwidth and phase response of
the FM transmission network to reduce the level of the induced transients - and hence the
distortion - to an acceptable level.
The graph shown opposite plots the instantaneous frequency for two
ratios of sine wave:- B/A = 1.32 and B/A = 1.05.
If we plot the above equation for B/A ratios between 0.116
and 8.62 we get the plot shown on the right. In any transmission system we can decrease the noise
by decreasing the sensitivity of the receiver while still keeping
a constant signal output.
In a sense, the benign nature of AM is its own worst enemy. Note that all these desirable characteristics apply
only to pure AM. Any attempt to multiplex with any other form of modulation,
say FM for stereo, can introduce horrendous distortion. top ## LOOP ANTENNA
In AM reception on the broadcast band, loop aerials can reject
unwanted signals in two ways:- ## General Solution of Field Surrounding Oscillating Current Element
Consider a current element I ## Discussion of General SolutionWe now examine the terms one by one:-
[A] The third term of the form: - ωsin(ωt
(1) Since this term varies as 1/r and the other terms as
1/r
[B] The second term of the form: cos(ωt
(1) The magnetic field H is simply that predicted by Ampere's law, but with a time delay.
[C] The third term of the form: sin(ωt
(1) The electric field is that predicted for a charge doublet of
moment M = (Idl/ω). ## Charge Conservation in Maxwell's Equations
To show that Maxwell's Equations have charge conservation inbuilt. top ## AM BROADCASTING
This site is a reaction, in part, to the modern
tendency to denigrate the AM service.
Although programs with a 9KHz cutoff can still sound remarkably bright,
it should be possible to operate the AM service with full bandwidth, at
least during the day. top back |