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Superheterodyne receiver
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(December 2008)

A 5-tubes superhet receiver made in Japan about 1955.In electronics,
the superheterodyne receiver (also known as the supersonic heterodyne
receiver, or by the abbreviated form superhet) is a receiver which
uses the principle of frequency mixing or heterodyning to convert the
received signal to a lower (sometimes higher) "intermediate"
frequency, which can be more conveniently processed than the original
carrier frequency. Virtually all modern radio and TV receivers use the
Superheterodyne principle.

Contents [hide]
1 History
2 Overview
3 Design and its evolution
4 Drawbacks
4.1 High-side and low-side injection
4.2 Image Frequency (fimage)
4.3 Local oscillator radiation
4.4 Local oscillator sideband noise
5 See also
6 References
7 Footnotes
8 External links



[edit] History

Two section variable capacitor, used in superhet receiverThe word
heterodyne is derived from the Greek roots hetero- "different", and -
dyne "power". The original heterodyne technique was pioneered by
Canadian inventor-engineer Reginald Fessenden but was not pursued far
because local oscillators were not very stable at the time.[1]

Later, the superheterodyne (superhet) principle was conceived in 1918
by Edwin Armstrong during World War I, as a means of overcoming the
deficiencies of early vacuum triodes used as high-frequency amplifiers
in radio direction finding (RDF) equipment. Unlike simple radio
communication, which only needs to make transmitted signals audible,
RDF requires actual measurements of received signal strength, which
necessitates linear amplification of the actual carrier wave.

In a triode RF amplifier, if both the plate and grid are connected to
resonant circuits tuned to the same frequency, stray capacitive
coupling between the grid and the plate will cause the amplifier to go
into oscillation if the stage gain is much more than unity. In early
designs, dozens (in some cases over 100) low-gain triode stages had to
be connected in cascade to make workable equipment, which drew
enormous amounts of power in operation and required a team of
maintenance engineers. The strategic value was so high, however, that
the British Admiralty felt the high cost was justified.

Armstrong had realized that if RDF could be operated at a higher
frequency, it would allow detection of enemy shipping much more
effectively, but at the time, no practical "short wave" amplifier
existed, (defined then as any frequency above 500 kHz) due to the
limitations of triodes of the day.

A "heterodyne" refers a beat or "difference" frequency produced when
two or more radio frequency carrier waves are fed to a detector. The
term was originally coined by Canadian Engineer Reginald Fessenden
describing his proposed method of making Morse Code transmissions from
an Alexanderson alternator type transmitter audible. With the Spark
gap transmitters then in wide use, the Morse Code signal consisted of
short bursts of a heavily modulated carrier wave which could be
clearly heard as a series of short chirps or buzzes in the receiver's
headphones.

The signal from an Alexanderson Alternator on the other hand, did not
have any such inherent modulation and Morse Code from one of those
would only be heard as a series of clicks or thumps. Fessenden's idea
was to run two Alexanderson Alternators, one producing a carrier
frequency 3kHz higher than the other. In the receiver's detector the
two carriers would beat together to produce a 3kHz tone and so in the
headphones the morse signals would then be heard as a series of 3kHz
beeps. For this he coined the term "heterodyne" meaning "Generated by
a Difference" (in frequency).

Later, when vacuum triodes became available, the same result could be
achieved more conveniently by incorporating a "local oscillator" in
the receiver, which became known as a "Beat Frequency Oscillator" or
BFO. As the BFO frequency was varied, the pitch of the heterodyne
could be heard to vary with it. If the frequences were too far apart
the heterodyne became supersonic and hence no longert audible.

It had been noticed some time before that if a regenerative receiver
was allowed to go into oscillation, other receivers nearby would
suddenly start picking up stations on frequencies different from those
that the stations were actually transmitted on. Armstrong (and others)
eventually deduced that this was caused by a "supersonic heterodyne"
between the station's carrier frequency and the oscillator frequency.
Thus, for example, if a station was transmitting on 300 kHz and the
oscillating receiver was set to 400 kHz, the station would be heard
not only at the original 300 kHz, but also at 100 kHz and 700 kHz.

Armstrong realized that this was a potential solution to the "short
wave" amplification problem, since the beat frequency still retained
its original moduation, but on a lower carrier frequency. To monitor a
frequency of 1500 kHz for example, he could set up an oscillator to
say, 1560 kHz, which would produce a heterodyne of 60kHz, a frequency
that could then be much more conveniently amplified by the triodes of
the day. He termed this the "Intermediate Frequency" often abbreviated
to "IF"

Early Superheterodyne receivers actually used IFs as low as 20 kHz,
often based around the self-resonance of iron-cored transformers. This
made them extremely susceptible to image frequency interference, but
at the time, the main objective was sensitivity rather than
selectivity. Using this technique, a small number triodes could be
made to do work that formerly required dozens or even hundreds.

1920s commercial IF transformers actually look very similar to 1920s
audio interstage coupling transformers, and were wired up in an almost
identical manner. By the mid-1930s superhets were using much higher
intermediate frequencies, (typically around 440-470kHz), using tuned
coils very similar in construction to the aerial and oscillator coils.
However the term "Intermediate Frequency Transformer" or "IFT" still
persists to this day.

Modern receivers typically use a mixture of Ceramic Filters and/or Saw
Resonators as well as traditional tuned-inductor IF transformers

Armstrong was able to put his ideas into practice quite quickly, and
the technique was rapidly adopted by the military. However, it was
less popular when commercial radio broadcasting began in the 1920s.
There were many factors involved,but the main issues were the need for
an extra tube for the oscillator, the generally higher cost of the
receiver, and the level of technical skill required to operate it. For
early domestic radios, Tuned RFs ("TRF"), also called the Neutrodyne,
were much more popular because they were cheaper, easier for a non-
technical owner to use, and less costly to operate. Armstrong
eventually sold his superheterodyne patent to Westinghouse, who then
sold it to RCA, the latter monopolizing the market for superheterodyne
receivers until 1930.[2]

By the 1930s, improvements in vacuum tube technology rapidly eroded
the TRF receiver's cost advantages, and the explosion in the number of
broadcasting stations created a demand for cheaper, higher-performance
receivers.

First, the development of practical indirectly-heated-cathode tubes
allowed the mixer and oscillator functions to be combined in a single
Pentode tube, in the so-called Autodyne mixer. This was rapidly
followed by the introduction of low-cost multi-element tubes
specifically designed for superheterodyne operation. These allowed the
use of much higher Intermediate Frequencies (typically around
440-470kHz) which eliminated the problem of image frequency
interference. By the mid-30s, for commercial receiver production the
TRF technique was obsolete.

The superheterodyne principle was eventually taken up for virtually
all commercial radio and TV designs.


[edit] Overview
The superhet receiver consists of three principle parts, the local
oscillator, a mixer that mixes the local oscillator's signal with the
received signal, and a tuned amplifier.

Reception starts with an antenna signal, optionally amplified,
including the frequency the user wishes to tune, fd. The local
oscillator is tuned to produce a frequency close to fd, fLO. The
received signal is mixed with the local oscillator's, producing four
frequencies in the output; the original signal, the original fLO, and
the two new frequencies fd+fLO and fd-fLO. The output signal also
generally contains a number of undesirable mixtures as well. (These
are 3rd- and higher-order intermodulation products. If the mixing were
performed as a pure, ideal multiplication, the original fd and fLO
would also not appear; in practice they do appear because mixing is
done by a nonlinear process that only approximates true ideal
multiplication.)

The amplifier portion of the system is tuned to be highly selective at
a single frequency, fIF. By changing fLO, the resulting fd-fLO (or fd
+fLO) signal can be tuned to the amplifier's fIF. In typical amplitude
modulation ("AM radio" in the U.S., or MW) receivers, that frequency
is 455 kHz; for FM receivers, it is usually 10.7 MHz; for television,
45 MHz. Other signals from the mixed output of the heterodyne are
filtered out by the amplifier.


[edit] Design and its evolution
The diagram below shows the basic elements of a single conversion
superhet receiver. The essential elements of a local oscillator and a
mixer followed by a fixed-tuned filter and IF amplifier are common to
all superhet circuits. Cost-optimized designs may use one active
device for both local oscillator and mixer--this is sometimes called a
"converter" stage. One such example is the pentagrid converter.


The advantage to this method is that most of the radio's signal path
has to be sensitive to only a narrow range of frequencies. Only the
front end (the part before the frequency converter stage) needs to be
sensitive to a wide frequency range. For example, the front end might
need to be sensitive to 1-30 MHz, while the rest of the radio might
need to be sensitive only to 455 kHz, a typical IF. Only one or two
tuned stages need to be adjusted to track over the tuning range of the
receiver; all the intermediate-frequency stages operate at a fixed
frequency which need not be adjusted.

To overcome obstacles such as image response, multiple IF stages are
used, and in some case multiple stages with two IFs of different
values. For example, the front end might be sensitive to 1-30 MHz, the
first half of the radio to 5 MHz, and the last half to 50 kHz. Two
frequency converters would be used, and the radio would be a "Double
Conversion Super Heterodyne"--a common example is a television receiver
where the audio information is obtained from a second stage of
intermediate frequency conversion. Occasionally special-purpose
receivers will use an intermediate frequency much higher than the
signal, in order to obtain very high image rejection.

Superheterodyne receivers have superior characteristics to simpler
receiver types in frequency stability and selectivity. They offer much
better stability than Tuned radio frequency receivers (TRF) because a
tuneable oscillator is more easily stabilized than a tuneable
amplifier, especially with modern frequency synthesizer technology. IF
filters can give much narrower passbands at the same Q factor than an
equivalent RF filter. A fixed IF also allows the use of a crystal
filter when exceptionally high selectivity is necessary. Regenerative
and super-regenerative receivers offer better sensitivity than a TRF
receiver, but suffer from stability and selectivity problems.

In the case of modern television receivers, no other technique was
able to produce the precise bandpass characteristic needed for
vestigial sideband reception, first used with the original NTSC system
introduced in 1941. This originally involved a complex collection of
tuneable inductors which needed careful adjustment, but since the
early 1980s these have been replaced with precision electromechanical
surface acoustic wave (SAW) filters. Fabricated by precision laser
milling techniques, SAW filters are much cheaper to produce, can be
made to extremely close tolerances, and are extremely stable in
operation.

Microprocessor technology allows replacing the superheterodyne
receiver design by a software defined radio architecture, where the IF
processing after the initial IF filter is implemented in software.
This technique is already in use in certain designs, such as very low
cost FM radios incorporated into mobile phones where the necessary
microprocessor is already present in the system.

Radio transmitters may also use a mixer stage to produce an output
frequency, working more or less as the reverse of a superheterodyne
receiver.


[edit] Drawbacks
Drawbacks to the superheterodyne receiver include interference from
signal frequencies close to the intermediate frequency. To prevent
this, IF frequencies are generally controlled by regulatory
authorities, and this is the reason most receivers use common IFs.
Examples are 455 kHz for AM radio, 10.7 MHz for FM, and 38.9 MHz
(Europe) 45 MHz (US) for television.

(For AM radio, a variety of IFs have been used, but most of the
Western World settled on 455kHz, in large part because of the almost
universal transition to Japanese-made ceramic resonators which used
the US standard of 455kHz. In more recent digitally tuned receivers,
this was changed to 450kHz as this figure simplifies the design of the
synthesizer circuitry).

Additionally, in urban environments with many strong signals, the
signals from multiple transmitters may combine in the mixer stage to
interfere with the desired signal.


[edit] High-side and low-side injection
The amount that a signal is down-shifted by the local oscillator
depends on whether its frequency f is higher or lower than fLO. That
is because its new frequency is |f - fLO| in either case. Therefore,
there are potentially two signals that could both shift to the same
fIF one at f = fLO + fIF and another at f = fLO - fIF. One or the
other of those signals, called the image frequency, has to be filtered
out prior to the mixer to avoid aliasing. When the upper one is
filtered out, it is called high-side injection, because fLO is above
the frequency of the received signal. The other case is called low-
side injection. High-side injection also reverses the order of a
signal's frequency components. Whether or not that actually changes
the signal depends on whether it has spectral symmetry or not. The
reversal can be undone later in the receiver, if necessary.


[edit] Image Frequency (fimage)
One major disadvantage to the superheterodyne receiver is the problem
of image frequency. In heterodyne receivers, an image frequency is an
undesired input frequency equal to the station frequency plus twice
the intermediate frequency. The image frequency results in two
stations being received at the same time, thus producing interference.
Image frequencies can be eliminated by sufficient attenuation on the
incoming signal by the RF amplifier filter of the superheterodyne
receiver.


Early Autodyne receivers typically used IFs of only 150kHz or so, as
it was difficult to maintain reliable oscillation if higher
frequencies were used. As a consequence, most Autodyne receivers
needed quite elaborate antenna tuning networks, often involving double-
tuned coils, to avoid image interference. Later superhets used tubes
especialy designed for oscillator/mixer use, which were able work
reliably with much higher IFs, reducing th eproblem of image
interference and so allowing simpler and cheaper aerial tuning
circuitry.





[edit] Local oscillator radiation
It is difficult to keep stray radiation from the local oscillator
below the level that a nearby receiver can detect. This means that
there can be mutual interference in the operation of two or more
superheterodyne receivers in close proximity. In espionage, oscillator
radiation gives a means to detect a covert receiver and its operating
frequency.

Further information: Electromagnetic compatibility

[edit] Local oscillator sideband noise
Local oscillators typically generate a single frequency signal that
has negligible amplitude modulation but some random phase modulation.
Either of these impurities spreads some of the signal's energy into
sideband frequencies. That causes a corresponding widening of the
receiver's frequency response, which would defeat the aim to make a
very narrow bandwidth receiver such as to receive low-rate digital
signals. Care needs to be taken to minimize osicllator phase noise,
usually by ensuring that the oscillator never enters a non-linear
mode.

"Greegor"

What specifically are your complaints with this Wiki ?


** What a ****ing stupid thing to do - post an entire bloody 180KB Wiki
!!!

Even worse, the post is nor directed to anyone.

Greegor = MORON !!



...... Phil

Joerg wrote:

John Larkin wrote:

On Mon, 20 Apr 2009 12:36:12 -0700, Joerg
wrote:

Tim Shoppa wrote:

A terminology question I suppose about the derivation of the term
"Superheterodyne" more than anything else:

Does the "Super" actually mean anything? Is there a Subheterodyne?

Traditionally superhets mix a higher radio frequency down to a lower
IF frequency, but certainly in the past few decades radios with IF's
above the RF frequency have become very common in broadband
applications, and those are still called superhets, not subhets :-).

Google turns up a couple hits on subheterodyne but other than one that
might mean "IF higher in frequency than RF" I don't recognize what
they mean..

I suspect that "Super" was more a marketing term than anything
else :-).

If you want to file a new patent for old stuff you could try
subheterodyne and it just might sail through :-)

Oh wait, call it hyperheterodyne, has more glitz. Just like the
supermercados in Spain.



I meant hypermercados :-)


I think we're going to be doing a superhet receiver soon. Maybe we'll
do it in an FPGA!


How'd you do the preamp in there?

Bit shift?

"Robert Baer" wrote in message net...
Joerg wrote:

John Larkin wrote:

On Mon, 20 Apr 2009 12:36:12 -0700, Joerg
wrote:

Tim Shoppa wrote:

A terminology question I suppose about the derivation of the term
"Superheterodyne" more than anything else:

Does the "Super" actually mean anything? Is there a Subheterodyne?

Traditionally superhets mix a higher radio frequency down to a lower
IF frequency, but certainly in the past few decades radios with IF's
above the RF frequency have become very common in broadband
applications, and those are still called superhets, not subhets :-).

Google turns up a couple hits on subheterodyne but other than one that
might mean "IF higher in frequency than RF" I don't recognize what
they mean..

I suspect that "Super" was more a marketing term than anything
else :-).

If you want to file a new patent for old stuff you could try subheterodyne and it just might sail through :-)

Oh wait, call it hyperheterodyne, has more glitz. Just like the supermercados in Spain.



I meant hypermercados :-)


I think we're going to be doing a superhet receiver soon. Maybe we'll
do it in an FPGA!


How'd you do the preamp in there?

Bit shift?

Yes, but 2x zero is still zero...

M

On Apr 21, 1:05*am, "Phil Allison" wrote:
"AF6AY"

Everyone ought to realize that "Wikipedia" data can be written by
ANYONE

** As are NG posts.

The difference being that Wikis are full of checkable references and are
subject to on-going correction.

The best Wikipedia articles are often filled with good checkable
references, but other times it sounds like they were written in a
foreign language and translated into English and have few (if any)
good references.

Just because a Wikipedia entry isn't well-written or sounds awkward
doesn't mean it's wrong, but I will often reject what I don't like in
the poorly written ones.

Somewhere there's a bunch of people who spend their time correcting
and improving Wikipedia entries, and I think overall they are doing a
good job, but that doesn't mean the result is always devoted to my
interests. Just like anything else in this world, it's got workers and
it's got managers and they aren't always devoting their attention to
the little corners of arcania that I live in.

It's not that the Encyclopedia Britannica is perfect either. I can
open it up to the very few subjects that I happen to be expert on and
find over-simplifications and a lack of cites to what I consider to be
the best references. That doesn't mean it's out-and-out wrong, just
that it's an Encyclopedia, and by definition they can't do anything
but touch on the surface of all the interesting stuff in the world.

Of course in academia I got real used to opening a journal and instead
of reading the articles, to go straight to the references and see if
they are quoting one of my articles :-). Breadth vs specialization,
can't pick them both.

Tim.

"Tim Shoppa"
"Phil Allison"

Everyone ought to realize that "Wikipedia" data can be written by
ANYONE

** As are NG posts.

The difference being that Wikis are full of checkable references and are
subject to on-going correction.

The best Wikipedia articles are often filled with good checkable
references, but other times it sounds like they were written in a
foreign language and translated into English

** Only indicates your lack of comprehension.


Just because a Wikipedia entry isn't well-written or sounds awkward

** You are irrationally obsessed with style over content.

Mostly likely because you cannot comprehend the content.


Somewhere there's a bunch of people who spend their time correcting
and improving Wikipedia entries, and I think overall they are doing a
good job, but that doesn't mean the result is always devoted to my
interests.

** What a revolting, pompous little narcissist you are - Tim.


Just like anything else in this world, it's got workers and
it's got managers and they aren't always devoting their attention to
the little corners of arcania that I live in.


** I was much too kind earlier ....


It's not that the Encyclopedia Britannica is perfect either. I can
open it up to the very few subjects that I happen to be expert on and
find over-simplifications and a lack of cites to what I consider to be
the best references.

** Mere narcissism has just turned into full blown ego-mania.


Of course in academia I got real used to opening a journal and instead
of reading the articles, to go straight to the references and see if
they are quoting one of my articles :-).


** Wot a nauseating computer geek puke.



....... Phil

On Apr 21, 2:02*am, Greegor wrote:
What specifically are your complaints with this Wiki ?

http://en.wikipedia.org/wiki/Superheterodyne_receiver

A 180 Kbyte article about a technology developed in the 1910's, yet
the oldest reference is to a textbook from 1996 aimed at freshman or
sophomore EE students of the 90's. More original references would have
gone a long way, especially to the patents and journals of the 1910's
and 1920's.

Don't get me wrong, it's a kinda nice textbook that they reference, as
I realize that by the 90's many EE programs had been so entirely taken
over by VLSI and CAD techniques and the particular textbook fills a
very important niche in education. It isn't the textbook that I
learned about radio from but I see how it fits the modern times well.
(I prefer Terman or Clarke&Hess but those guys weren't around in 1918
either.).

A smaller point, is that the language sounds a lot like it was written
in a language other than English and then translated. In principle
this isn't fundamentally bad, it's just that a lot of the terminology
used sounds very awkward. I think that's fine if they reference 80
year old patents using the same language, but they don't.

Tim.

On Apr 21, 8:49*am, "Phil Allison" wrote:
** *Only indicates your lack of comprehension.
** You are irrationally obsessed with style over content.
** What a revolting, pompous little narcissist you are *- *Tim.
** *I was much too kind earlier ....
** *Mere narcissism has just turned into full blown ego-mania.
** *Wot a nauseating computer geek puke.

There's a thin line between ignorance and arrogance, Phil. I have
erased that line.

Tim.

In rec.radio.amateur.homebrew Phil Allison wrote:

"AF6AY"


Everyone ought to realize that "Wikipedia" data can be written by
ANYONE

** As are NG posts.

The difference being that Wikis are full of checkable references and are
subject to on-going correction.


The word prefix 'super' generally refers to something 'better' than
the word without that prefix.


** So this radio ham clot has no idea what the origin of the term is really
is ( although it has been posted) and is making the classic ****wit
BLUNDER of trying to de-cipher the meaning from the word alone.



Think also SUPERman.

** And supercilious.


73, Len AF6AY
ex-ARRL member (for good reason)


** Lunatics like Len are not welcome as members ?

Killfile, Phil. Phil, killfile.

--
Mike Andrews, W5EGO

Tired old sysadmin

On Apr 20, 12:50*pm, Tim Shoppa wrote:
A terminology question I suppose about the derivation of the term
"Superheterodyne" more than anything else:

Does the "Super" actually mean anything? Is there a Subheterodyne?

Traditionally superhets mix a higher radio frequency down to a lower
IF frequency, but certainly in the past few decades radios with IF's
above the RF frequency have become very common in broadband
applications, and those are still called superhets, not subhets :-).

Google turns up a couple hits on subheterodyne but other than one that
might mean "IF higher in frequency than RF" I don't recognize what
they mean..

I suspect that "Super" was more a marketing term than anything
else :-).

Tim N3QE

Remember that his previous receiver invention, the regenerative
detector, would produce an audible heterodyne when used in the
oscillating-detector mode (for reception of CW, not AM signals). So
it would make sense for him to think of his new principle as producing
a supersonic heterodyne (IIRC around 50kHz or so).

And then there's the super regenerative detector, a regenerative
detector which is driven into and out of oscillation at (typically) a
supersonic frequency.

Mike