In article ,
Scott wrote:

In a direct conversion receiver, does the osc. run at
exactly the same frequency as the received signal?

Assuming the answer is yes;
Is it possible to have a high Q LC front end at the output from the
antenna,
split the signal and feed one output to the rf input of the mixer and feed
the other splitter output to an amplifier and use the amplified signal to
drive the osc input of the mixer?

Well, what you'd be getting here (I think) is a high-tech version of a
crystal radio.

Remember that a mixer (diode-ring or Gilbert cell or etc.) is
essentially a multiplier... it multiplies the "RF" signal with the
"LO" signal, thus creating additional signals at the sum and
difference frequencies. In a simple DC receiver for sideband, you tune
the LO to the nominal carrier frequency of the incoming signal, and
the "difference" frequencies which come out of the mixer are thus the
audio sidebands, shifted down to baseband.

If you multiply the signal by itself (i.e. splitting it and putting it
into two ports of a mixer) you'll create base-band "difference"
components which match the original sidebands (if there's some carrier
present in the signal) plus *all* of the sum-and-differences of the
original audio components. In short, you'll get the audio, plus a lot
of harmonic and intermodulation distortion.

That's just what you get when you feed AM into a simple diode
detector, with no LO (i.e. a crystal radio). The square-law response
of the diode has the effect of multiplying every RF component in the
incoming signal with every other, thus recreating the original audio
plus lots of HD and IMD.

If the incoming signal is fully-suppressed-carrier SSB, you won't have
any carrier present to act as a frequency reference or to result in
the recovery of the original signal content via multiplication. All
you'll have is every sideband in the signal, multiplied by every other
sideband... an amazing mishmash of intermodulation, without the original
baseband signal.

If you're just interested in AM signals, then you could filter
the incoming signal, split it, amplify one half, and run this
through a *very* narrow-band filter (think "oscillator and PLL")
and use this as the LO input to the mixer. In effect, what
you'd be doing is creating an LO signal which is a copy of the
original signal's carrier, with the sidebands stripped away by
the ultra-narrow filter / PLL. This is a pretty basic way of
doing AM reception.

This won't work with SSB, though, as there's no carrier to lock
onto.






--
Dave Platt AE6EO
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