6 FSK WLAN diode detector

>> The last part of our circuit
is the diode detector.

We have two diode detectors right here.

One that's designed for 2.4 gigahertz

and the other that's
designed for 2.6 gigahertz.

The way the diode detector works.

I'm just going to show you
the little circuit right here of these.

The way the diode detector
works is the AC signal comes

out of either one of the band-pass filters.

Remember, we don't know
which one's going to

have the signal. One or the other will.

But AC signal comes in right here,

goes through a diode and then
is integrated by a capacitor.

The original signal that comes in is

the black sine wave that
you see right here.

That is VAC.

Maybe, it's at 2.4 gigahertz in this case.

The diode only allows
current to go one way.

So it only allows the positive part
of our signal to get through.

The red line right here
is the signal when it

goes to the diode and that's
called halfway rectified.

Rectified means that it's
turned right side up.

In this case, only half of
the wave is passed through.

Then, when it goes through this capacitor,

the capacitor integrates
or averages that signal.

So the dashed line is the DC voltage

that's going to come out
of that capacitor VDC.

Let's see where those are
in our diode circuit.

So right here's my diode circuit

VAC comes out of the band-pass
filter at this point,

let's say it's 2.4 gigahertz.

It comes into my system and
there's the diode right there.

You can't see but it's
a little triangle thing like so.

But there's the diode and that is

half-wave rectified signal.
Here's the capacitor.

It's just like the capacitors we had on

the amplifier and it goes down to ground.

The ground is underneath.

Here goes down to ground and
then right there is going to be

the DC signal that comes
out if it is 2.4 gigahertz.

That signal then can go
into a comparator and be

converted to a zero or a one
to be used by the computer.

On this side, we have the same thing and
it's designed for 2.6 gigahertz where

the AC signal comes in because the diode is

integrated across the capacitor
and comes out as a DC value.

Now, there are some other little parts
of this circuit too.

These have short circuited
stubs and that impedance

matches these 50 Ohm lines to
the diodes that are not 50 Ohm.

There's 100 Ohm line here and
an open-circuited stub that match

the impedance of this particular diode
to this particular transmission line.

Another interesting thing
that you can see right here

is because we had a little trouble
matching this particular one.

We actually put two stubs on just to
help with the impedance matching.

So, that's a picture of
our frequency shift keyed system.

Let's look again to see where the
inductors and capacitors are.

We've got our antenna right here.

The transmission line is seen
as a series of resistors,

inductors, capacitors, and conductors.

We have capacitors right here
to block the DC signal from

the AC and an inductor to keep
the AC out of our DC power supply.

We have a surface mount
resistor right there in

our 3dB coupler and then
we have a series of

band-pass filters that
are made up of a set of

effective resistors,
capacitors and inductors.

Then when we get over to
the diode detector circuit,

a capacitor is used again in order to
integrate my halfway rectified signal.

So we've used a lot of capacitors
and inductors in the circuit.

I just thought you might like to see it.