synchronous AM detection
leconte thierry
I've read that some receiver have a synchronous AM detection mode.
I don't know what it is at all and I'm looking for any information about
this subject. Please email to lec...@irisa.fr or answer on the news.
Thanks in advance
Thierry.
John Covington WN4BBJ
Synchronous detection is a method of demodulating AM transmissions that
involves "subtracting" the carrier from the signal instead of using
rectification as a simple diode detector does. It offers excellent
fidelity.
(I'll try not to get TOO technical here, but here goes)
Most AM radios (including SW sets) don't use synchronous detection, they
use a diode and a capacitor instead. The frequency response of a diode
detector is limited because of the impedance of the capacitor used to smooth
the rectified signal drops at higher frequencies, making it useless
above 5000 Hz. This is necessary because using a capacitor with an
higher impedance at those frequencies does not do a sufficient job of
smoothing the signal. In other words, the value of capacitor used is a
tradeoff between overall fidelity and frequency response. This is why
AM radios sound so bad even though the sound quality of the transmitted
signal is frequently excellent.
A synchronous detector is capable of much greater frequency response
because it does not modify the information part of the signal at all. It
simply removes the carrier and leaves the information alone. It is
called a synchronous detector because you must have a subtraction signal
that is the same frequency as the carrier (i.e. they must be
synchronized). This is normally done after an IF stage so this is not a
problem.
There are synchronous detectors available on integrated circuits so it
is not real expensive to implement; however most receiver manufacturers
are not using them. I have heard the difference and I think it is well
worth paying a little extra for synchronous detection; it sounds MUCH
better.
If the sound quality of broadcasts is important to you then make sure
you get a radio with a synchronous detector. You won't be sorry.
73 de John WN4BBJ
John Covington WN4BBJ E-mail: cs7...@unccvax.UUCP
P.O. Box 217122 MCI Mail: JCOVINGTON 342-6957
Charlotte, NC 28221-7122 Packet Radio: WN4BBJ @ KK4L
(704) 537-7653 "Give it back, Eric, it's not yours" (the ICOM)
Lenny Saaf
>> etc......
>> Leconte Thierry
> Synchronous detection is a method of demodulating AM transmissions
> that involves "subtracting" the carrier from the signal instead of
> using rectification as a simple diode detector does.
> etc......
> WN4BBJ
When I read this I felt I should throw in my (radically different) two
cents worth:
The constant fading of a shortwave signal is said to be "selective."
That is, it affects the different frequencies within the bandwidth of
the signal in different amounts. If the carrier signal strength is
severely reduced by selective fading and the sidebands are not
correspondingly reduced by the fading, the result is that, for a short
period of time, the carrier strength is not sufficient to provide
enough demodulation of the sidebands, resulting in a burst of signal
that sounds more like SSB than AM.
Synchronous AM detection superimposes a carrier onto the carrier of
the received signal. This internally-generated carrier is
"phase-locked" to the received signal carrier and it can remain at the
proper frequency even if the signal carrier fades out for a short
period of time. The strong internal carrier insures that the
sidebands are always sufficiently demodulated.
I believe the Sony 2010 is the only radio (less than $5000) with this
feature.
--
* Len Saaf, The Institute of Optics, University of Rochester, Rochester, NY *
* Internet: sa...@joker.optics.rochester.edu Bitnet: SAAF@UOROPT Radio: NV2Z *
* Internet (last resort): saaf%joker.optics....@vm.cc.rochester.edu *
Chris Thomas
> feature.
Actually, any communications receiver that has an SSB mode
does synchronous detection -- and you have the choice of
which sideband, upper or lower, to detect. Usually, there
is less QRM on one side than on the other. My Kenwood R-5000
does just fine. Note that you need to have an IF filter
narrow enough to remove the carrier. If you leave your
receiver in AM mode (usually wide) and turn on the BFO, you
don't get the fading reduction.
The advantage of the Sony 2010 is that it *automatically* locks
onto (zero beats) the carrier. This is a touchy tuning adjustment
otherwise. The 2010 is a neat box.
Ray Berry
> The advantage of synchronous demodulation is that 1) it is trivial
> to design one which receives either the upper or the lower sideband only and
could you please elaborate on this a bit? i.e., briefly describe
the (trivial) circuitry required?
> 2) it is more free from quadrature distortion due to carrier phase
> rotation (particularly a problem in skywave...
why is this (mathematically)?
My concept of sync demodulation is to regenerate a local version of the
carrier (via phaselock?), limit it, and multiply the <if> by it. This may
be naive. I would enjoy learning of other techniques, especially which
support statement (1) above.
--
Ray Berry kb7ht uucp: ...ole!ray CIS: 73407,3152 /* rent this space */
Seattle Silicon Corp. 3075 112th Ave NE. Bellevue WA 98004 (206) 828 4422
York David Anthony @ WKTD, Wilmington, NC
> In article <13...@irisa.irisa.fr>, lec...@irisa.irisa.fr (leconte thierry) writes:
> > I am a SWL and I'm interested in AM reception of broadcasts on the h.f bands.
> > I've read that some receiver have a synchronous AM detection mode.
> Synchronous detection is a method of demodulating AM transmissions that
> involves "subtracting" the carrier from the signal instead of using
> rectification as a simple diode detector does. It offers excellent
> fidelity.
> Most AM radios (including SW sets) don't use synchronous detection, they
> use a diode and a capacitor instead. The frequency response of a diode
> detector is limited because of the impedance of the capacitor used to smooth
> the rectified signal drops at higher frequencies, making it useless
> above 5000 Hz. This is necessary because using a capacitor with an
> higher impedance at those frequencies does not do a sufficient job of
> smoothing the signal. In other words, the value of capacitor used is a
Don't blame the envelope detector for AM frequency response
woes, because an AM envelope detector is inherently a high fidelity
device. Even at the less-commonly used IF bandwidth of 262.5 kHz, it
is **trivial** to design an envelope detector with reasonably flat
frequency response to 10 kHz, which is the upper limit for NRSC-1
equipped standard broadcasting stations in the US.
The lack of bandwidth in an AM receiver is completely due
to excessive selectivity in the IF amplifier stage. This selectivity
is provided to 1) save money (fewer stages of IF gain required) and
2) to eliminate first adjacent channel interference. "Modern" AM
receiver design use ceramic resonators, which necessarily have a
high "Q" and poor bandwidth.
The detector used in **ALL** modulation monitors (with
possibly the exception of the Delta Electronics splatter meter)
is envelope detection. The Belar AMM-3 is flat to 30 kHz. Many
colour television sets use envelope detection under much more severe
carrier - to - baseband conditions than a simple AM radio.
Of course, this all means diddly squat to a SWL, who is
basically listening to islands of signal in oceans of interference.
The advantage of synchronous demodulation is that 1) it
is trivial to design one which receives either the upper or the
lower sideband only and 2) it is more free from quadrature distortion
due to carrier phase rotation (particularly a problem in skywave
reception). The synchronous demodulator also does not suffer
from severe distortion under carrier-only amplitude distortion
(selective fading, again principally a skywave condition);
this is also a benefit for listeners in the "null fields" of
a BC directional station.
The disadvantage of a synchronous demodulator is
cost. In the presence of heavy QRM, synchronous detectors
sometimes "act funny" and reduce communications effectivness,
though the theoretical advantages of sync are undisputed.
This is probably more an implementation issue rather than
an inherent problem.
Finally, you still have to have an integrating
type carrier detector to implement AGC. You get this DC
control voltage at no extra charge with an envelope detector.
'Nuff said.
York David Antony
BPH-880505OT (WRPL) Wadesboro, NC
Walter Underwood
I believe the Sony 2010 is the only radio (less than $5000) with this
feature.
Add the Grundig Satellit 500 and 650 receivers, at about $500 and $1000.
wunder
John Covington WN4BBJ
>
> > Synchronous detection is a method of demodulating AM transmissions that
> > involves "subtracting" the carrier from the signal instead of using
> > rectification as a simple diode detector does. It offers excellent
> > fidelity.
> >
>
>
> I have never heard an explanation of just what syncronous detection is, but
> I have made what I believe is a good guess. Unfortunately, your non-technical
> explanation leaves me no more or less informed as to how well my understanding
> of it really is.
>
> In particular, what do you mean by "subtracting"? Could you read through my
Phil,
Not knowing the depth of technical knowledge of the original poster, I
posted a simplistic answer highlighting the benefits of synchronous
detection rather than explaining enough for someone to build one.
To explain synchronous detection further I will publish an article on
the net in a few days explaining it in more detail. Hopefully this will
give everybody a better idea of what it really is than my simplistic
answer was intended to give.
ph...@ux1.cso.uiuc.edu
> Synchronous detection is a method of demodulating AM transmissions that
> involves "subtracting" the carrier from the signal instead of using
> rectification as a simple diode detector does. It offers excellent
> fidelity.
>
> (I'll try not to get TOO technical here, but here goes)
Sometimes being "not ... TOO technical" just doesn't help in understanding.
I have never heard an explanation of just what syncronous detection is, but
I have made what I believe is a good guess. Unfortunately, your non-technical
explanation leaves me no more or less informed as to how well my understanding
of it really is.
In particular, what do you mean by "subtracting"? Could you read through my
explanation and let me know if this is what you mean by syncronous detection
(if I am not too technical)?
Clearly, the mere removal of a carrier from the AM signal in the IF stage is
not what you meant, because that would leave you with two sets of sidebands
around the IF frequency.
However, syncronous detection as I see it is little more than a sophisticated
version of SSB reception. Here is my slightly-more-technical explanation:
A very narrow filter at the exact center of the IF bandpass strips off a copy
of the carrier. Another circuit reproduces that carrier (by any of many
possible methods) in a pure form. This carrier is then used as a "BFO",
but instead of simply being injected into the signal, it is modulated with
it using a balanced mixer. This results in signal products at 2*IF as well
as BASEBAND audio. A smooth rolloff low-pass filter can then strip off the
2*IF and any leaking IF signals, and leave an accurate audio reproduction.
This scheme can also be used to detect 2 other modes related to AM. The
first is simply AM with a reduced carrier level. On an ordinary "envelope
stripper" the sound quality would be terrible in this mode. The syncronized
carrier would still be stronger than the sidebands it is intermodulating with
so it can detect just the sidebands quite well. It is only necessary to have
enough carrier to syncronize with. The second mode is one where the original
carrier is shifted by 90 degrees. This trick has the curious effect of
reducing the envelope peaks of the AM signal, even though the carrier is at
full strength. This is because a carrier at 90 degrees off will demodulate
one sideband at 180 degrees from the other sideband (one +90, the other -90).
Some envelope will still exist and will represent intermodulation directly
between the sidebands, and will be at 2*F where F is the audio frequency.
The syncronous detector would have to have a -90 degrees phase shift to adjust
for the original phase error in the carrier.
A syncronous detector should also be able to detect, with best fidelity,
a SINGLE sideband signal with half (H3E) or reduced (R3E) carrier levels
present (legal on ham bands, too).
73 de ka9wgn
--Phil howard-- <ph...@ux1.cso.uiuc.edu>
Larry Dighera
>I am a SWL and I'm interested in AM reception of broadcasts on the h.f bands.
>I've read that some receiver have a synchronous AM detection mode.
>I don't know what it is at all and I'm looking for any information about
The Sony ICF-2010 portable (~$350) boasts a synchronous AM
detector. Switching it on causes phase distortion & heterodynes,
which have always been the bane of SWLing, to virtually
disappear. After years of trying to decipher the distorted
transmissions on the international broadcast bands, it is, at
last, a pleasure to listen to an intelligible signal.
Synchronous AM detection is a must for pleasurable listening to
the international shout wave broadcasts.
Larry Dighera
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Kok Chen
>Actually, any communications receiver that has an SSB mode
>does synchronous detection -- and you have the choice of
>which sideband, upper or lower, to detect. Usually, there
Not quite. You are just beating the sidebands down to baseband
here. To achieve the full advantage of synchronous demodulation,
which includes a 3 db gain in S/N ratio over asynchronous means,
if I am not wrong, you need to have the injection oscillator be
in phase lock with the received carrier.
There is a McGraw-Hill book called "Communications Receivers" by
Rohde (yes, THE Rohde of Rohde und Schwarz, GmbH) and a second
author whose name I don't recall. I think synchronous demodulation
is described pretty well there, if I remember correctly. If there is
enough interest, I will dig it out at home and get the ISBN.
Warning: Rohde is one of these chaps with many doctorates. If you
believe what you have been reading here about breath in engineering,
you may not want to read his book :-).
---
Kok Chen {decwrl,sun}!apple!kchen
Apple Computer, Inc.
Rick Karlquist
> Rohde (yes, THE Rohde of Rohde und Schwarz, GmbH) and a second
> Kok Chen /|\
|
|
Not THE Rohde of Rohde and Schwarz. Ulrich Rohde is the *son* of
the founder of Rohde and Schwarz. He never even worked there.
Also, I wouldn't take any of Ulrich Rohde writings as gospel.
He has a history of writing articles with just enough information
to make you want to hire him as a consultant, but not enough
information to actually build the circuit being described. He
reminds me of the principle that "a half-truth is more dangerous
than a lie."
David L. Kensiski
about the transmitted signal. What is the bandwidth of an AM broadcast
signal?
--
David L. Kensiski [KB6HCN] Martin Marietta Data Systems
Software Engineer 1770 Tribute Road, upstairs
Phone: (916) 929-8844 Sacramento, CA 95815
UUCP: uunet!mmsac!david INTERNET: mmsac!da...@uunet.uu.net
Dave Turner
The April, 1982 issue of Popular Electronics has a construction article
on a 455kHz add-on synchronous detector:
Build a Synchronous Detector for AM Radio
Dave Hershberger, W9GR
p 61-71
It detects DSB, SSB (USB or LSB), switches to an envelope detector while
tuning and whenever the signal is not usable for synchronous detection.
In the January, 1984 issue of Radio-Electronics is an article about
C-QUAM that describes the Motorola chip MC13020P which has a block
diagram similar to the one shown in the PE article which makes me
suspect that it might be used to make a synchronous detector.
York David Anthony has posted articles on C-QUAM and may be able to
shed some light on the use of this chip for sync detector purposes.
Build this C-QUAM AM stereo Converter
Marty Bergan
p 41-46,102,114
Another chip for sync detection is the Plessey SL624C Multimode detector.
Plessey chips are not listed in most of my catalogs.
--
Dave Turner 415/542-1299 {att,bellcore,sun,ames,decwrl}!pacbell!dmt
York David Anthony @ WKTD, Wilmington, NC
> With all this talk of high fidelity AM reception, I began to wonder
> about the transmitted signal. What is the bandwidth of an AM broadcast
> signal?
Depends on whether or not the broadcast station adheres to the NRSC
standards, which become law next June.
Most current production AM transmitters can easily reach 100% modulation
at less than 1.5 % harmonic distortion, from 30 to 15,000 Hz. A properly adjusted
Ampliphase can equal or better this to 30,000 Hz. The principal limiting
factor in older transmitters is the heavy iron in the modulator (modulation
impedance matching choke and the modulation reactor) and the lack of power supply
reserve.
However, the distortion bit is academic, because of the peak limiting
and compression amplifiers usual to broadcast stations.
Three weekends ago, I hooked up a CD player directly to the input of
KGOL (Humble-South Houston, Texas)'s RCA BTA50-J, and was completely amazed
at the results. The monitor receiver was the station's Sansui AM Stereo tuner,
about 6 miles away.
With the NRSC filter switched in (more about this in a minute) there
was noticeable blunting of the highs, but power consumption does drop and there
is much less adjacent channel splatter.
The law requires a brickwall filter of essentially linear phase which
must be flat to 10 kHz and something like -30 dB at 11 khz (I don't have the
standard here). This is high fidelity as far as medium wave AM is concerned,
because the listener still is IF amplifier limited to 3-4 kHz (-6 dB points).
In general practice, the transmitting plant is almost never the limiting
factor, it's the receiver. Unless you are inside the interference free contour,
though, narrowband reception is usually preferable.
York David Anthony
BPH-880505OT (WRPL) Wadesboro, NC