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?

Simple block diagram of proposed idea;
http://i395.photobucket.com/albums/p...conversion.jpg

--
MikeK

amdx 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?

Simple block diagram of proposed idea;
http://i395.photobucket.com/albums/p...conversion.jpg


I don't think so. You would be using the exact same frequencies as both
the RF and LO signals, resulting in an I.F. of (difference) zero
(DC--You can't hear DC.)
) or (sum) 2XRF (Can't hear that either).

N0EDV

On 07/11/2010 11:50 AM, amdx 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?

Simple block diagram of proposed idea;
http://i395.photobucket.com/albums/p...conversion.jpg

NO, for the same reason you can't fake the BFO at the final detector the
same way. You need to (re)inject a carrier to receive a CW or SSB
signal, and the receiver must supply this carrier. A direct conversion
receiver is just a tunable IF, detector, and BFO without a superhet
front end. There ARE synchronous AM detectors that do what you propose,
but this works because the incoming carrier is already there.

On 7/11/2010 10:50 AM, amdx 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?

Simple block diagram of proposed idea;
http://i395.photobucket.com/albums/p...conversion.jpg


Probably not.
You would like to boost the input with a broadband amp, then select the
signal of interest with a local oscillator.

Using the signal itself to select the same signal from dozens of others
would need processing a carrier to provide an LO.
Interesting!

Brian W

On Sun, 11 Jul 2010 10:50:58 -0500, "amdx" 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?

Are you trying to reinvent the regenerative receiver ?

amdx wrote:

...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? ...

the idea is not new (a few decades ago it was called "carrier
regeneration", and has been extensively used in synchronous demodulation
of AM and DSB, VSB and SSB with partially suppressed carrier), but you
need at least a residual carrier.

In the case of SSB, although carrier suppression is far from perfect,
filtering and amplifying the residual carrier up to an usable level is
not impossible, but definitely far from easy...

Speaking about CW, recovering the carrier is very easy (a simple PLL
with a very slow time constant would do the trick), but the next step
should be displacing said recovered carrier of a useful amount (say,
600-800 Hz) to give a beat note, and this is not so easily done.

Regards,
--
73 es 51 de i3hev, op. mario

Non è Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non è uno che ascolta la radio"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia

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
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Sun, 11 Jul 2010 19:25:06 -0700, (Dave Platt)
wrote:


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 is how some early video-IF chips performed the video detection
(instead of simple envelope detection). There was a high-Q resonance
circuit tuned to the nominal video-IF, which was excited by the visual
carrier. Due to the high Q, the resonator was "ringing" for a few
cycles, even if there was no carrier present for a few cycles.

This filtered carrier was then used by the product detector
(multiplier) to extract the video and intercarrier sound.

In countries using negative video modulation (sync 100 %, black 80-90
% and white about 10 %), the intercarrier sound system could be used.
After the video detector, the sound FM signal was at 4.5, 5.5, 6.0 or
6.5 MHz depending on system and was then processed with an ordinary FM
strip tuned to that frequency.

For proper operation, this required that both the visual as well as
audio carrier frequency was always present in order to produce the
frequency difference signal for the 4.5-6.5 MHz audio subcarrier.

For various reasons (bad filter group delay, overmodulated white
levels, especially subtitles), the video level dropped to 0 %, the
simple envelope detector did not work and hence the intercarrier audio
signal could not be formed and there was various forms "picture in
audio" buzz.

The ringing in high-Q section at the product detector made it possible
to "ride through" the short video signal interruption, intercarrier
audio-IF was produced constantly and thus the buzz in the audio was
reduced.

Paul OH3LWR

On Sun, 11 Jul 2010, Paul Keinanen wrote:

On Sun, 11 Jul 2010 10:50:58 -0500, "amdx" 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?

Are you trying to reinvent the regenerative receiver ?


No, because regeneration is about feeding the output of a stage back into
the input, so there is near infinite amplification.

Michael VE2BVW

"amdx" wrote in message
...
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?

Simple block diagram of proposed idea;
http://i395.photobucket.com/albums/p...conversion.jpg

--
MikeK
Hi all,
I started my post writing about AMBCB and an example frequency of 1430 khz.
Then I rewrote it and skipped all that.
So from what I have gleened from the responses, I would need to feed the
1430 khz
into the IF mixer input to get the baseband audio.
I don't know what the advantage of this would be but the local osc would be
as
stable as the radio station. And no need to build an oscilator.
Any other feedback now that I'm talking AM BCB radio?
Mike