MAc wrote:
To clarify - the idea is to build very simple receiver - Not "play with
quadrature detector". AD607 look simply and small

Andy writes:

While I agree with everything MAC wrote, I think a point is being
missed.
A Quadrature detector is normally used only for FM demodulation. It
accomplishes this by limiting the signal and applying it to one port
of a 90deg detector. If the limited signal is shifted 90 degrees
(narrow
band), the quad detector will detect FM... If the limited signal is
shifted
ZERO degrees, the quad detector will detect AM, and in a much more
linear way than a diode type detector......

With CW, there would be no signal to zero beat with, and no audio
note would be formed.... Exactly the same with SSB....all the output
would be "DC"...

Mac's explanation is correct for an IQ detector... In this case a
local
signal EQUAL to the suppressed carrier of the SSB signal has to
be supplied..... I don't know how a local carrier can be synchronized
to
a SSB carrier that, if done really well, doesn't exist......

In Homodyne receivers, it is easy, since the transmit signal can be

the LO signal..... Otherwise, it has to be "guessed" at..... With
really
really really stable local oscillators, the guess can be really close,
and
the difference will creat an error that is so small it doesn't
matter... But,
since you don't know the incident phase of the received signal, you
have to do both I and Q and shift the IF by 90 degrees and sum (or
difference, depending whether you want USB or LSB) the outputs of
the two channels...... For SSB detection, the RF port is NOT shifted,
the LO port is both 0 and 90 degrees, and the IF port ( I and Q) are
shifted 90 and combined........

It is a hell of a lot easier to "approximate" the LO, and shift it
manually
until Donald Duck starts sounding human.... In older SSB rigs, this was

called a "clarifier" control..... It isn't necessary in many modern
systems
with really really really good TCXOs and stuff....

Just my two cents worth.... If anyone disagrees, I'd be happy to
learn from them where my explanation is wrong..... but I warn you,
it ain't far wrong.... cause it worked for me for many many years
in designing receivers for both commercial and military products
....:)))))

Andy in Eureka, W4OAH ( retired comm/radar engineer and ham for 45
years )