<s>
In	O
radio	O
,	O
a	O
detector	B-Algorithm
is	O
a	O
device	O
or	O
circuit	O
that	O
extracts	O
information	O
from	O
a	O
modulated	B-Application
radio	O
frequency	O
current	O
or	O
voltage	O
.	O
</s>
<s>
Therefore	O
,	O
early	O
radio	O
receivers	B-Device
did	O
not	O
have	O
to	O
demodulate	B-Device
the	O
radio	O
signal	O
,	O
but	O
just	O
distinguish	O
between	O
the	O
presence	O
or	O
absence	O
of	O
a	O
radio	O
signal	O
,	O
to	O
reproduce	O
the	O
Morse	O
code	O
"	O
dots	O
"	O
and	O
"	O
dashes	O
"	O
.	O
</s>
<s>
The	O
device	O
that	O
performed	O
this	O
function	O
in	O
the	O
receiver	B-Device
circuit	O
was	O
called	O
a	O
detector	B-Algorithm
.	O
</s>
<s>
A	O
variety	O
of	O
different	O
detector	B-Algorithm
devices	O
,	O
such	O
as	O
the	O
coherer	O
,	O
electrolytic	O
detector	B-Algorithm
,	O
magnetic	O
detector	B-Algorithm
and	O
the	O
crystal	O
detector	B-Algorithm
,	O
were	O
used	O
during	O
the	O
wireless	O
telegraphy	O
era	O
until	O
superseded	O
by	O
vacuum	O
tube	O
technology	O
.	O
</s>
<s>
After	O
the	O
invention	O
of	O
amplitude	B-Device
modulation	I-Device
(	O
AM	B-Device
)	O
enabled	O
the	O
development	O
of	O
AM	B-Device
radiotelephony	O
,	O
the	O
transmission	O
of	O
sound	O
(	O
audio	O
)	O
,	O
during	O
World	O
War	O
1	O
,	O
the	O
term	O
evolved	O
to	O
mean	O
a	O
demodulator	B-Device
,	O
(	O
usually	O
a	O
vacuum	O
tube	O
)	O
which	O
extracted	O
the	O
audio	O
signal	O
from	O
the	O
radio	O
frequency	O
carrier	O
wave	O
.	O
</s>
<s>
This	O
is	O
its	O
current	O
meaning	O
,	O
although	O
modern	O
detectors	B-Algorithm
usually	O
consist	O
of	O
semiconductor	O
diodes	O
,	O
transistors	B-Application
,	O
or	O
integrated	O
circuits	O
.	O
</s>
<s>
In	O
a	O
superheterodyne	O
receiver	B-Device
the	O
term	O
is	O
also	O
sometimes	O
used	O
to	O
refer	O
to	O
the	O
mixer	O
,	O
the	O
tube	O
or	O
transistor	B-Application
which	O
converts	O
the	O
incoming	O
radio	O
frequency	O
signal	O
to	O
the	O
intermediate	O
frequency	O
.	O
</s>
<s>
The	O
mixer	O
is	O
called	O
the	O
first	O
detector	B-Algorithm
,	O
while	O
the	O
demodulator	B-Device
that	O
extracts	O
the	O
audio	O
signal	O
from	O
the	O
intermediate	O
frequency	O
is	O
called	O
the	O
second	O
detector	B-Algorithm
.	O
</s>
<s>
In	O
microwave	O
and	O
millimeter	O
wave	O
technology	O
the	O
terms	O
detector	B-Algorithm
and	O
crystal	O
detector	B-Algorithm
refer	O
to	O
waveguide	O
or	O
coaxial	O
transmission	O
line	O
components	O
,	O
used	O
for	O
power	O
or	O
SWR	O
measurement	O
,	O
that	O
typically	O
incorporate	O
point	O
contact	O
diodes	O
or	O
surface	O
barrier	O
Schottky	O
diodes	O
.	O
</s>
<s>
One	O
major	O
technique	O
is	O
known	O
as	O
envelope	B-Algorithm
detection	I-Algorithm
.	O
</s>
<s>
The	O
simplest	O
form	O
of	O
envelope	B-Algorithm
detector	I-Algorithm
is	O
the	O
diode	O
detector	B-Algorithm
that	O
consists	O
of	O
a	O
diode	O
connected	O
between	O
the	O
input	O
and	O
output	O
of	O
the	O
circuit	O
,	O
with	O
a	O
resistor	O
and	O
capacitor	O
in	O
parallel	O
from	O
the	O
output	O
of	O
the	O
circuit	O
to	O
the	O
ground	O
to	O
form	O
a	O
low	B-Algorithm
pass	I-Algorithm
filter	I-Algorithm
.	O
</s>
<s>
An	O
early	O
form	O
of	O
envelope	B-Algorithm
detector	I-Algorithm
was	O
the	O
crystal	O
detector	B-Algorithm
,	O
which	O
was	O
used	O
in	O
the	O
crystal	O
set	O
radio	O
receiver	B-Device
.	O
</s>
<s>
The	O
limited	O
frequency	O
response	O
of	O
the	O
headset	O
eliminates	O
the	O
RF	O
component	O
,	O
making	O
the	O
low	B-Algorithm
pass	I-Algorithm
filter	I-Algorithm
unnecessary	O
.	O
</s>
<s>
More	O
sophisticated	O
envelope	B-Algorithm
detectors	I-Algorithm
include	O
the	O
grid-leak	O
detector	B-Algorithm
,	O
the	O
plate	O
detector	B-Algorithm
,	O
the	O
infinite-impedance	O
detector	B-Algorithm
,	O
transistor	B-Application
equivalents	O
of	O
them	O
and	O
precision	O
rectifiers	B-Algorithm
using	O
operational	O
amplifiers	O
.	O
</s>
<s>
A	O
product	O
detector	B-Algorithm
is	O
a	O
type	O
of	O
demodulator	B-Device
used	O
for	O
AM	B-Device
and	O
SSB	B-Device
signals	O
,	O
where	O
the	O
original	O
carrier	O
signal	O
is	O
removed	O
by	O
multiplying	O
the	O
received	O
signal	O
with	O
a	O
signal	O
at	O
the	O
carrier	O
frequency	O
(	O
or	O
near	O
to	O
it	O
)	O
.	O
</s>
<s>
Rather	O
than	O
converting	O
the	O
envelope	B-Algorithm
of	O
the	O
signal	O
into	O
the	O
decoded	O
waveform	O
by	O
rectification	O
as	O
an	O
envelope	B-Algorithm
detector	I-Algorithm
would	O
,	O
the	O
product	O
detector	B-Algorithm
takes	O
the	O
product	O
of	O
the	O
modulated	B-Application
signal	O
and	O
a	O
local	O
oscillator	O
,	O
hence	O
the	O
name	O
.	O
</s>
<s>
By	O
heterodyning	B-Algorithm
,	O
the	O
received	O
signal	O
is	O
mixed	O
(	O
in	O
some	O
type	O
of	O
nonlinear	O
device	O
)	O
with	O
a	O
signal	O
from	O
the	O
local	O
oscillator	O
,	O
to	O
give	O
sum	O
and	O
difference	O
frequencies	O
to	O
the	O
signals	O
being	O
mixed	O
,	O
just	O
as	O
a	O
first	O
mixer	O
stage	O
in	O
a	O
superhet	O
would	O
produce	O
an	O
intermediate	O
frequency	O
;	O
the	O
beat	O
frequency	O
in	O
this	O
case	O
,	O
the	O
low	O
frequency	O
modulating	B-Application
signal	I-Application
is	O
recovered	O
and	O
the	O
unwanted	O
high	O
frequencies	O
filtered	B-Algorithm
out	I-Algorithm
from	O
the	O
output	O
of	O
the	O
product	O
detector	B-Algorithm
.	O
</s>
<s>
Because	O
the	O
sidebands	O
of	O
an	O
amplitude-modulated	O
signal	O
contain	O
all	O
the	O
information	O
in	O
the	O
carrier	O
displaced	O
from	O
the	O
center	O
by	O
a	O
function	O
of	O
their	O
frequency	O
,	O
a	O
product	O
detector	B-Algorithm
simply	O
mixes	O
the	O
sidebands	O
down	O
into	O
the	O
audible	O
range	O
so	O
that	O
the	O
original	O
audio	O
may	O
be	O
heard	O
.	O
</s>
<s>
Product	O
detector	B-Algorithm
circuits	O
are	O
essentially	O
ring	O
modulators	B-Application
or	O
synchronous	O
detectors	B-Algorithm
and	O
closely	O
related	O
to	O
some	O
phase-sensitive	O
detector	B-Algorithm
circuits	O
.	O
</s>
<s>
They	O
can	O
be	O
implemented	O
using	O
something	O
as	O
simple	O
as	O
ring	O
of	O
diodes	O
or	O
a	O
single	O
dual-gate	O
Field	O
Effect	O
Transistor	B-Application
to	O
anything	O
as	O
sophisticated	O
as	O
an	O
Integrated	O
Circuit	O
containing	O
a	O
Gilbert	O
cell	O
.	O
</s>
<s>
Product	O
detectors	B-Algorithm
are	O
typically	O
preferred	O
to	O
envelope	B-Algorithm
detectors	I-Algorithm
by	O
shortwave	O
listeners	O
and	O
radio	O
amateurs	O
as	O
they	O
permit	O
the	O
reception	O
of	O
both	O
AM	B-Device
and	O
SSB	B-Device
signals	O
.	O
</s>
<s>
They	O
may	O
also	O
demodulate	B-Device
CW	O
transmissions	O
if	O
the	O
beat	O
frequency	O
oscillator	O
is	O
tuned	O
slightly	O
above	O
or	O
below	O
the	O
carrier	O
.	O
</s>
<s>
AM	B-Device
detectors	B-Algorithm
cannot	O
demodulate	B-Device
FM	B-Application
and	O
PM	B-Device
signals	O
because	O
both	O
have	O
a	O
constant	O
amplitude	O
.	O
</s>
<s>
However	O
an	O
AM	B-Device
radio	O
may	O
detect	O
the	O
sound	O
of	O
an	O
FM	B-Application
broadcast	O
by	O
the	O
phenomenon	O
of	O
slope	O
detection	O
which	O
occurs	O
when	O
the	O
radio	O
is	O
tuned	O
slightly	O
above	O
or	O
below	O
the	O
nominal	O
broadcast	O
frequency	O
.	O
</s>
<s>
Frequency	O
variation	O
on	O
one	O
sloping	O
side	O
of	O
the	O
radio	B-Device
tuning	I-Device
curve	O
gives	O
the	O
amplified	O
signal	O
a	O
corresponding	O
local	O
amplitude	O
variation	O
,	O
to	O
which	O
the	O
AM	B-Device
detector	B-Algorithm
is	O
sensitive	O
.	O
</s>
<s>
Slope	O
detection	O
gives	O
inferior	O
distortion	O
and	O
noise	O
rejection	O
compared	O
to	O
the	O
following	O
dedicated	O
FM	B-Application
detectors	B-Algorithm
that	O
are	O
normally	O
used	O
.	O
</s>
<s>
A	O
phase	O
detector	B-Algorithm
is	O
a	O
nonlinear	O
device	O
whose	O
output	O
represents	O
the	O
phase	O
difference	O
between	O
the	O
two	O
oscillating	O
input	O
signals	O
.	O
</s>
<s>
It	O
has	O
two	O
inputs	O
and	O
one	O
output	O
:	O
a	O
reference	O
signal	O
is	O
applied	O
to	O
one	O
input	O
and	O
the	O
phase	O
or	O
frequency	B-Application
modulated	I-Application
signal	O
is	O
applied	O
to	O
the	O
other	O
.	O
</s>
<s>
In	O
phase	B-Device
demodulation	I-Device
the	O
information	O
is	O
contained	O
in	O
the	O
amount	O
and	O
rate	O
of	O
phase	O
shift	O
in	O
the	O
carrier	O
wave	O
.	O
</s>
<s>
The	O
Foster	B-Device
–	I-Device
Seeley	I-Device
discriminator	I-Device
is	O
a	O
widely	O
used	O
FM	B-Application
detector	B-Algorithm
.	O
</s>
<s>
The	O
detector	B-Algorithm
consists	O
of	O
a	O
special	O
center-tapped	O
transformer	B-Algorithm
feeding	O
two	O
diodes	O
in	O
a	O
full	O
wave	O
DC	O
rectifier	B-Algorithm
circuit	O
.	O
</s>
<s>
When	O
the	O
input	O
transformer	B-Algorithm
is	O
tuned	O
to	O
the	O
signal	O
frequency	O
,	O
the	O
output	O
of	O
the	O
discriminator	O
is	O
zero	O
.	O
</s>
<s>
When	O
there	O
is	O
no	O
deviation	O
of	O
the	O
carrier	O
,	O
both	O
halves	O
of	O
the	O
center	O
tapped	O
transformer	B-Algorithm
are	O
balanced	O
.	O
</s>
<s>
As	O
the	O
FM	B-Application
signal	O
swings	O
in	O
frequency	O
above	O
and	O
below	O
the	O
carrier	O
frequency	O
,	O
the	O
balance	O
between	O
the	O
two	O
halves	O
of	O
the	O
center-tapped	O
secondary	O
is	O
destroyed	O
and	O
there	O
is	O
an	O
output	O
voltage	O
proportional	O
to	O
the	O
frequency	O
deviation	O
.	O
</s>
<s>
The	O
ratio	O
detector	B-Algorithm
is	O
a	O
variant	O
of	O
the	O
Foster	B-Device
–	I-Device
Seeley	I-Device
discriminator	I-Device
,	O
but	O
one	O
diode	O
conducts	O
in	O
an	O
opposite	O
direction	O
,	O
and	O
using	O
a	O
tertiary	O
winding	O
in	O
the	O
preceding	O
transformer	B-Algorithm
.	O
</s>
<s>
The	O
output	O
across	O
the	O
diodes	O
is	O
connected	O
to	O
a	O
large	O
value	O
capacitor	O
,	O
which	O
eliminates	O
AM	B-Device
noise	O
in	O
the	O
ratio	O
detector	B-Algorithm
output	O
.	O
</s>
<s>
The	O
ratio	O
detector	B-Algorithm
has	O
the	O
advantage	O
over	O
the	O
Foster	B-Device
–	I-Device
Seeley	I-Device
discriminator	I-Device
that	O
it	O
will	O
not	O
respond	O
to	O
AM	B-Device
signals	I-Device
,	O
thus	O
potentially	O
saving	O
a	O
limiter	O
stage	O
;	O
however	O
the	O
output	O
is	O
only	O
50%	O
of	O
the	O
output	O
of	O
a	O
discriminator	O
for	O
the	O
same	O
input	O
signal	O
.	O
</s>
<s>
The	O
ratio	O
detector	B-Algorithm
has	O
wider	O
bandwidth	O
but	O
more	O
distortion	O
than	O
the	O
Foster	B-Device
–	I-Device
Seeley	I-Device
discriminator	I-Device
.	O
</s>
<s>
In	O
quadrature	B-Algorithm
detectors	I-Algorithm
,	O
the	O
received	O
FM	B-Application
signal	O
is	O
split	O
into	O
two	O
signals	O
.	O
</s>
<s>
This	O
phase-shifted	O
signal	O
is	O
then	O
applied	O
to	O
an	O
LC	O
circuit	O
,	O
which	O
is	O
resonant	O
at	O
the	O
FM	B-Application
signal	O
's	O
unmodulated	O
,	O
"	O
center	O
,	O
"	O
or	O
"	O
carrier	O
"	O
frequency	O
.	O
</s>
<s>
If	O
the	O
received	O
FM	B-Application
signal	O
's	O
frequency	O
equals	O
the	O
center	O
frequency	O
,	O
then	O
the	O
two	O
signals	O
will	O
have	O
a	O
90-degree	O
phase	O
difference	O
and	O
they	O
are	O
said	O
to	O
be	O
in	O
"	O
phase	O
quadrature	O
"	O
—	O
hence	O
the	O
name	O
of	O
this	O
method	O
.	O
</s>
<s>
The	O
two	O
signals	O
are	O
then	O
multiplied	O
together	O
in	O
an	O
analog	O
or	O
digital	O
device	O
,	O
which	O
serves	O
as	O
a	O
phase	O
detector	B-Algorithm
;	O
that	O
is	O
,	O
a	O
device	O
whose	O
output	O
is	O
proportional	O
to	O
the	O
phase	O
difference	O
between	O
two	O
signals	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
an	O
unmodulated	O
FM	B-Application
signal	O
,	O
the	O
phase	O
detector	B-Algorithm
's	O
output	O
is	O
—	O
after	O
the	O
output	O
has	O
been	O
filtered	B-Architecture
;	O
that	O
is	O
,	O
averaged	O
over	O
time	O
—	O
constant	O
;	O
namely	O
,	O
zero	O
.	O
</s>
<s>
However	O
,	O
if	O
the	O
received	O
FM	B-Application
signal	O
has	O
been	O
modulated	B-Application
,	O
then	O
its	O
frequency	O
will	O
vary	O
from	O
the	O
center	O
frequency	O
.	O
</s>
<s>
Now	O
the	O
output	O
from	O
the	O
phase	O
detector	B-Algorithm
will	O
differ	O
from	O
zero	O
,	O
and	O
in	O
this	O
way	O
,	O
one	O
recovers	O
the	O
original	O
signal	O
that	O
was	O
used	O
to	O
modulate	O
the	O
FM	B-Application
carrier	O
.	O
</s>
<s>
The	O
detection	O
process	O
described	O
above	O
can	O
also	O
be	O
accomplished	O
by	O
combining	O
,	O
in	O
an	O
exclusive-OR	O
(	O
XOR	O
)	O
logic	O
gate	O
,	O
the	O
limited	O
original	O
FM	B-Application
signal	O
and	O
either	O
a	O
copy	O
of	O
that	O
signal	O
passed	O
through	O
a	O
network	O
which	O
imposes	O
a	O
phase	O
shift	O
that	O
varies	O
with	O
frequency	O
,	O
e.g.	O
</s>
<s>
Due	O
to	O
the	O
varying	O
phase	O
difference	O
between	O
the	O
two	O
inputs	O
,	O
a	O
pulse-width	B-Algorithm
modulated	I-Algorithm
(	O
PWM	O
)	O
signal	O
is	O
produced	O
.	O
</s>
<s>
When	O
a	O
low-pass	B-Algorithm
filter	I-Algorithm
is	O
applied	O
to	O
those	O
pulses	O
,	O
the	O
filter	O
's	O
output	O
rises	O
as	O
the	O
pulses	O
grow	O
longer	O
and	O
its	O
output	O
falls	O
as	O
the	O
pulses	O
grow	O
shorter	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
one	O
recovers	O
the	O
original	O
signal	O
that	O
was	O
used	O
to	O
modulate	O
the	O
FM	B-Application
carrier	O
.	O
</s>
<s>
When	O
a	O
phase-shifted	O
version	O
of	O
the	O
original	O
signal	O
is	O
used	O
,	O
the	O
result	O
is	O
a	O
frequency	O
demodulation	B-Device
,	O
as	O
the	O
frequency	O
difference	O
between	O
the	O
inputs	O
of	O
the	O
XOR	O
gate	O
remains	O
zero	O
and	O
thus	O
does	O
not	O
affect	O
their	O
phase	O
relationship	O
.	O
</s>
<s>
With	O
a	O
fixed-frequency	O
carrier	O
,	O
the	O
result	O
is	O
a	O
phase	B-Device
demodulation	I-Device
,	O
which	O
,	O
in	O
this	O
case	O
is	O
an	O
integral	O
of	O
the	O
original	O
modulating	B-Application
signal	I-Application
.	O
</s>
<s>
Less	O
common	O
,	O
specialized	O
,	O
or	O
obsolescent	O
types	O
of	O
detectors	B-Algorithm
include	O
:	O
</s>
<s>
The	O
phase-locked	B-Algorithm
loop	O
detector	B-Algorithm
requires	O
no	O
frequency-selective	O
LC	O
network	O
to	O
accomplish	O
demodulation	B-Device
.	O
</s>
<s>
In	O
this	O
system	O
,	O
a	O
voltage	O
controlled	O
oscillator	O
(	O
VCO	O
)	O
is	O
phase	O
locked	O
by	O
a	O
feedback	O
loop	O
,	O
which	O
forces	O
the	O
VCO	O
to	O
follow	O
the	O
frequency	O
variations	O
of	O
the	O
incoming	O
FM	B-Application
signal	O
.	O
</s>
<s>
The	O
low-frequency	O
error	O
voltage	O
that	O
forces	O
the	O
VCO	O
's	O
frequency	O
to	O
track	O
the	O
frequency	O
of	O
the	O
modulated	B-Application
FM	B-Application
signal	O
is	O
the	O
demodulated	O
audio	O
output	O
.	O
</s>
<s>
The	O
phase-locked	B-Algorithm
loop	O
detector	B-Algorithm
should	O
not	O
be	O
confused	O
with	O
the	O
phase-locked	B-Algorithm
loop	O
frequency	O
synthesizer	O
,	O
which	O
is	O
often	O
used	O
in	O
digitally-tuned	O
AM	B-Device
and	O
FM	B-Application
radios	O
to	O
generate	O
the	O
local	O
oscillator	O
frequency	O
.	O
</s>
