direct conversion receiver Question
 

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

direct conversion receiver Question
 

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

direct conversion receiver Question
 

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.

direct conversion receiver Question
 

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

direct conversion receiver Question
 

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 ?

direct conversion receiver Question
 

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

direct conversion receiver Question
 

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!

direct conversion receiver Question
 

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

direct conversion receiver Question
 

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

direct conversion receiver Question
 

"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

direct conversion receiver Question
 

Hello Mike,

If you Google on "Synchronous AM Detector" and "Exalted Carrier Reception"
you'll find lots of info

This is good place to start....
http://www.radio-electronics.com/inf...t/sync_det.php


Regards ............ Zim

direct conversion receiver Question
 

Hey OM:

In the ole NTSC days gone bye, that's how they detected the video
signal in the last of the ole NTSC TV's, took the SAW filtered IF and
squeezed the video out by converting the RF right to video baseband,
with the carrier as an LO OSC all inside one little tiny IC

I just never new what happened to the frequencies that were below ZERO
from the mixer.

73 OM
de n8zu


On Jul 11, 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

--
MikeK

direct conversion receiver Question
 

--
MikeK
"Graeme Zimmer" wrote in message
...
Hello Mike,

If you Google on "Synchronous AM Detector" and "Exalted Carrier Reception"
you'll find lots of info

This is good place to start....
http://www.radio-electronics.com/inf...t/sync_det.php


Regards ............ Zim
Thanks Graeme,
I'm thinking about building the mixer that was in QST Feb., 1993, an
article by
Jacob Makhinson. Titled, "A High-Dynamic-Range MF/HF Receiver Front End"
The mixer uses a quad FET IC, (Si8901 from Calolgic corp.)
The only online reference that I can find is here; See pdf pages 26 (text)
27 (schematic)
http://files.bluecrow.net/Amateur%20...ok_2007/11.pdf
The front end is designed in three modules, preamp, mixer, and post amp.
Anyway I'm thinking about building the mixer and playing with it in the
AMBCB.
Even thoughts about building an I/Q receiver.
Mike
(Note; I do more thinking than building ;-)

direct conversion receiver Question
 

In article ,
amdx wrote:

Thanks Graeme,
I'm thinking about building the mixer that was in QST Feb., 1993, an
article by
Jacob Makhinson. Titled, "A High-Dynamic-Range MF/HF Receiver Front End"
The mixer uses a quad FET IC, (Si8901 from Calolgic corp.)
The only online reference that I can find is here; See pdf pages 26 (text)
27 (schematic)
http://files.bluecrow.net/Amateur%20...ok_2007/11.pdf

Google around for "H-mode mixer" and "Tayloe mixer" if you want more
information. http://www.webalice.it/romano.cartoc...mode_links.htm
will get you started with *many* links to investigate.

There has been a lot of interest in this sort of switching mixer
topology in recent years... quite a few articles in QEX for example.
Many designs have come out, some of which are based on general-
purpose multi-FET parts such as you suggest, and others which are
based on CMOS "fast bus switching" ICs, such as the FST3125 and
similar. This seems to be a very good topology for handing signals
having a very wide dynamic range... I believe that Elecraft uses a
design of this sort in their K3 transceiver.

The front end is designed in three modules, preamp, mixer, and post amp.
Anyway I'm thinking about building the mixer and playing with it in the
AMBCB.
Even thoughts about building an I/Q receiver.

Yeah, I've been toying with that sort of idea too... making a
"binaural" direct-conversion receiver. There was a binaural design
published in QST a year or so ago, which uses the NE602 or similar
Gilbert-cell mixers... building one which uses H-mode switching mixers
and a couple of low-noise audio amplifying chains could be fun.

--
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!

direct conversion receiver Question
 

In article ,
"amdx" wrote:

--
MikeK
"Graeme Zimmer" wrote in message
...
Hello Mike,

If you Google on "Synchronous AM Detector" and "Exalted Carrier Reception"
you'll find lots of info

This is good place to start....
http://www.radio-electronics.com/inf...t/sync_det.php


Regards ............ Zim
Thanks Graeme,
I'm thinking about building the mixer that was in QST Feb., 1993, an
article by
Jacob Makhinson. Titled, "A High-Dynamic-Range MF/HF Receiver Front End"
The mixer uses a quad FET IC, (Si8901 from Calolgic corp.)
The only online reference that I can find is here; See pdf pages 26 (text)
27 (schematic)
http://files.bluecrow.net/Amateur%20...ok_2007/11.pdf
The front end is designed in three modules, preamp, mixer, and post amp.
Anyway I'm thinking about building the mixer and playing with it in the
AMBCB.
Even thoughts about building an I/Q receiver.
Mike
(Note; I do more thinking than building ;-)

You should look at the Northern Radio Model N570/571 Marine SSB Radios,
that were designed and built back in the Early 70's. They used at rather
unique design with a Double Balanced Mixer at the Receive Antenna Port,
that moved the IF up to a low VHF biDirectional Amp and Filter, and then
back down to a 10.7 Mhz IF with another Double Balanced Mixer, and on to
the Crystal SSB Filter, and Demodulator. The Double Balanced Mixers were
driven by an LO System that canceled out any Frequency Drift, by using
opposite Mixing Images.

direct conversion receiver Question
 

Hi again Mike,

I'm thinking about building the mixer that was in QST Feb., 1993

You might have a look at my old DSP receiver at:
http://members.wideband.net.au/gzimmer/


Regards ............. Zim

direct conversion receiver Question
 

Graeme Zimmer wrote:

You might have a look at my old DSP receiver at:
http://members.wideband.net.au/gzimmer/

I have built several Softrock kits and have no inclination to use any
other receiver as a result. It's a receiver from heaven.

Looking at yours and the simple block diagram I am curious about how
this differs from quadrature processing.

Can you provide a few words of explanation.

Jack K9act

direct conversion receiver Question
 

Hi Jack,

Looking at yours and the simple block diagram I am curious about how this
differs from quadrature processing.

I'm not sure I understand your question.

As the diagrams show, the VFO is divided by four to produce 90deg quadrature
LO signals which drive two mixers.The two audio channels pass through two
BPF filters, one of which has a Gilbert transform to give 90 deg delay. It's
the standard Phasing Method..

If you're asking about Binaural reception, it's the same as above, but less
the Hilbert transform so there's no sideband suppression..

Admittedly that page is somewhat condensed (simplified) over the original.

Regards .......... Zim ........ VK3GJZ

direct conversion receiver Question
 

What about SSB and CW? Would syncro conversion detect just the upper
sideband?

No. You can select either sideband.
Just change the sign when you combine the I and Q signals.

this is a fairly simple intro.
http://www.nxp.com/acrobat_download2...tes/AN1981.pdf


................ Zim

direct conversion receiver Question
 

raypsi wrote:
Hey OM

This thread begs the questions:
What about SSB and CW? Would syncro conversion detect just the upper
sideband?

It is not possible to use synchronous detection with SSB or CW because
there is no carrier in the signal. So the circuit that recovers the
carrier will work on the modulation and the result is unusable.
(i.e. no audible modulation with SSB, and at best a 0Hz tone with CW)

Because the lower sideband would be below ZERO hertz?
If you use a BFO how do you get below ZERO hertz?
Doesn't the Fourier transformation of the mixers' LO and rf input in
a syncro conversion produce frequencies below ZERO?

There are no frequencies below zero. When you are using only a single
mixer, both sidebands will produce frequencies above zero. And there
will be no suppression of the "unwanted" sideband.
(the synchronous AM detector is a dual-sideband detector)

direct conversion receiver Question
 

Hey OM:

I beg to differ there are frequencies below ZERO hertz you are just
not aware of them.

Simply put the frequencies add and subtract, towit below ZERO. When
mixing frequencies they add and subtract there is no doubt about it.
So it must be for all cases.

If I take 2 frequencies and mix them I get the Sum and the Difference
of those frequencies plus the original frequencies. NO?

Tell me this is wrong?

Because that is what you have just told me?

73 OM
de n8zu


There are no frequencies below zero. *When you are using only a single
mixer, both sidebands will produce frequencies above zero. *And there
will be no suppression of the "unwanted" sideband.
(the synchronous AM detector is a dual-sideband detector)

direct conversion receiver Question
 

the synchronous AM detector is a dual-sideband detector

Some are, some aren't.

My Sony ICF-SW7600GR lets you select sidebands while in Synchro mode,
as does my homebrew DSP RX.

Selecting sideband in AM mode is most useful.

..................... Zim

direct conversion receiver Question
 

raypsi wrote:
Hey OM:

I beg to differ there are frequencies below ZERO hertz you are just
not aware of them.

Because the "frequencies below zero" will come out of a single mixer
as equal frequencies above zero. Only the phase is different. So
when you use two mixers and quadrature signals, it is possible to
select a sideband. But with a single mixer and synchronous detection,
this is not possible.

direct conversion receiver Question
 

Graeme Zimmer wrote:
the synchronous AM detector is a dual-sideband detector

Some are, some aren't.

My Sony ICF-SW7600GR lets you select sidebands while in Synchro mode,
as does my homebrew DSP RX.

Selecting sideband in AM mode is most useful.

Ok it can be done but not with the circuit described in this thread
and on the referenced site.

You need two mixers and 90deg phase shifts to do it.

direct conversion receiver Question
 

Hey OM:

The ole TV systems that used NTSC type signals, the video is Vestigial
Sideband.
So if what you say is TRUE then I never ever would have seen anything
on an NTSC tv but the low frequencies that are present in NTSC signal
part of the lower sideband.
All the now defunked, latest NTSC tv's used synchro' detection on the
Vistigial Sidband video.

I made a living repairing those old NTSC tv's that had synchro'
detection of the Vestigial Sideband video.

73 OM
de n8zu





On Jul 19, 4:00 am, Rob wrote:
But with a single mixer and synchronous detection,
this is not possible.

direct conversion receiver Question
 

raypsi wrote:
Hey OM:

The ole TV systems that used NTSC type signals, the video is Vestigial
Sideband.
So if what you say is TRUE then I never ever would have seen anything
on an NTSC tv but the low frequencies that are present in NTSC signal
part of the lower sideband.
All the now defunked, latest NTSC tv's used synchro' detection on the
Vistigial Sidband video.

In those TVs, the selection of the sideband is not done by the detector
but by an IF filter that passes only the frequencies of interest.

I did not say the synchronous detector cannot demodulate a single sideband,
I said it cannot select the sideband. So if you pass both sidebands
to the detector, it will demodulate both of them. Unless you use a
quadrature approach.

(and it cannot demodulate SSB because there is no carrier. in TV VSB
there is a carrier present that can be used to control the synchronous
detector)