AM radio receiver - design

[Joe G \(Home\)]

Hi All,

I am looking for a super sensitive / selective AM radio design.

Where I am I get lots of interference ....

Have you seen any good designs on the internet you can suggest.

AM broadcast band 500K - 1.6MHz


PS... I will be able to manually tune to correct frequency.

Regards
Joe

 

[Rich Grise]

I am looking for a super sensitive / selective AM radio design.
Where I am I get lots of interference ....
Have you seen any good designs on the internet you can suggest.
AM broadcast band 500K - 1.6MHz
PS... I will be able to manually tune to correct frequency.

For sensitivity, more antenna, preferably vertically polarized;
possibly an antenna tuner:
http://www.google.com/search?hl=en&q="antenna+tuner"
For selectivity, a Q multiplier:
http://www.google.com/search?hl=en&q="Q+multiplier"+circuit

Hope This Helps!
Rich

 

[Charles]

Hi All,

I am looking for a super sensitive / selective AM radio design.

Where I am I get lots of interference ....

Have you seen any good designs on the internet you can suggest.

AM broadcast band 500K - 1.6MHz


PS... I will be able to manually tune to correct frequency.

http://www.dxing.com/rx/r5000.htm

One then adds a noise canceling (phasing) circuit (with two antennas feeding
it) in front of the above radio (or its equivalent).

And one then regrets the whole experiment.

Conclusion: it can be done but it is a major PITA and the results are iffy.

 

[[email protected]]

Hi All,

I am looking for a super sensitive / selective AM radio design.

Where I am I get lots of interference ....

Have you seen any good designs on the internet you can suggest.

AM broadcast band  500K - 1.6MHz

PS... I will be able to manually tune to correct frequency.

Regards
Joe

The antenna is where you control noise. Google magnetic loop antenna.
There are commercial versions, such as made by Wellbrook. There are
also home brew designs, though generally tuned rather than broadband
like the Wellbrook.

 

[MooseFET]

Hi All,

I am looking for a super sensitive / selective AM radio design.

Where I am I get lots of interference ....

Have you seen any good designs on the internet you can suggest.

AM broadcast band  500K - 1.6MHz

PS... I will be able to manually tune to correct frequency.

If this is for voice, you can get some improvement by band limiting
the audio.

If you make the AM radio a phase lock loop, you can get some
improvement in the interference rejection. The simplest way to do
this is to build a normal AM radio up to the detector stage. At the
detector, use something like a HC4046 to phase lock onto the 455KHz
signals carrier. The filter in the PLL needs to be quite slow and you
have to use the XOR phase detector.

The VCO of the PLL needs to run at at least 2x the IF. You use some
flip flops to make a 90 degree version of the IF for the XOR and a in
part to drive a HC4053.

The 4053 sync demodulates the IF giving you the audio. Noise energy
that happens to land at 90 degrees to the long term average of the
carrier is rejected.

 

[[email protected]]

If you make the AM radio a phase lock loop, you can get some
improvement in the interference rejection.  

What makes you think that?

A PLL detector can give THRESHOLD reduction but that is not the same
as interference rejection.

OP..
For broadcast AM reception, a directional magnetic loop antenna is a
good start as others have said. Where/ what is causing the
interference? Get rid of the lamp dimmers in your house.

Mark

 

[JosephKK]

It's been MANY years since I've tinkered AM radio. But I still have,
in my junk box, 455kHz ceramic resonators for interstage filtering
with bipolar transistors. There also were emitter bypass resonators,
allowing for very tight skirts. I don't know if they still make
these... or, for that matter, if the ones I have are even any good
now... they date to the early '60's ;-)

...Jim Thompson

If they are that old, they are probably still good. They still built
forever back then. Besides, what would the failure mechanism be?

 

[MooseFET]

What makes you think that?

The PLL locks onto the carrier and has a filter with a low bandwidth.
This means that the phase of the VCO won't change due to noise that is
far from the carrier. For purposes of understanding why it works,
assume that the VCO is running exactly in step with the carrier and
that in effect the PLL has zero band width.

Now consider the side bands that get demodulated. A noise component
that can be thought of as resulting in a signal like:

Y = f(t) * cos(wt) + sin(wt)

where:
Y = the signal
f(t) = some random (noise) function with no DC
sin(wt) = the carrier
cos(wt) = 90 degrees to the carrier

is the important one for the argument. This signal will result in
noise from a standard AM demodulation but no noise from the PLL and
sync demodulation one.


Now consider the intelligence on the AM signal. It is a function
like:

Y = f(t) * sin(wt) + sin(wt)

The two methods of doing the AM demodulation give equal values for
this input.

The result is that the PLL based method makes the same signal and less
noise on its output.

 

[[email protected]]

The PLL locks onto the carrier and has a filter with a low bandwidth.
This means that the phase of the VCO won't change due to noise that is
far from the carrier.  For purposes of understanding why it works,
assume that the VCO is running exactly in step with the carrier and
that in effect the PLL has zero band width.

Now consider the side bands that get demodulated.  A noise component
that can be thought of as resulting in a signal like:

  Y = f(t) * cos(wt) + sin(wt)

where:
  Y       = the signal
  f(t)    = some random (noise) function with no DC
  sin(wt) = the carrier
  cos(wt) = 90 degrees to the carrier

is the important one for the argument.  This signal will result in
noise from a standard AM demodulation but no noise from the PLL and
sync demodulation one.

Now consider the intelligence on the AM signal.  It is a function
like:

  Y = f(t) * sin(wt) + sin(wt)

The two methods of doing the AM demodulation give equal values for
this input.

The result is that the PLL based method makes the same signal and less
noise on its output.

Maybe in theory you are right. But anyone with a radio that has
synchronous demod will testify that synch demod doesn't help with a
noise source. It is good for rejecting noise from the adjacent channel
if you passband shift.

The magnetic loop really does the job. The designs that float the loop
above ground are more effective. [The Wellbrook does that.]

 

[TheM]

Take a look at the datasheet for the LM3820 AM Receiver IC
(http://www.datasheetarchive.com/search.php?q=LM3820&searchtype=default&p=1)
The chip is long obsolete, but if you're just wanting a hobby project, this would probably be a good place to start. I have
several of those chips left in my junque bin, so if you're interested, I'll be glad to send you a few.
BCB receivers typically are built with approx. 40 uV sensitivity. In-band noise will pretty much swamp out any gains from higher
sensitivity. You might be able to get better performance by tight IF bandwidth control with ceramic filters, but the noise will
still be there.
A good magnetic loop antenna will probably be more value than higher sensitivity.

TCA440 was a popular choice as well.

Anyone know of a modern variant IC that does AM (linear AGC IF)
decently. All I can see now are FM IF receivers and some completely
integrated radios (FM+AM+PLL etc) that are too complex and
draw lots of current (>100mA).

Mark

 

[MooseFET]

Maybe in theory you are right. But anyone with a radio that has
synchronous demod will testify that synch demod doesn't help with a
noise source.

3dB of improvement may be hard for people to notice but it is none the
less real.

It is good for rejecting noise from the adjacent channel
if you passband shift.

It prevents the carrier on the adjacent channel from multiplying with
anything so it keeps all of its side bands at high frequencies so yes
it does a good job of that.

The magnetic loop really does the job. The designs that float the loop
above ground are more effective. [The Wellbrook does that.]

huh?

 

[amdx]

...

You don't state whether your intereference is...
"QRM" (man-made, i.e. co-channel interference from other
transmitters, or even on-channel interference as a result of long-
range skip, etc. or from a myriad of non-"radio" sources around
the house or neighborhood)
or
"QRN" (natural interference from atmospherics because of a low
signal to noise ratio from trying to detect a very weak signal or
trying to listen during an electrical storm, etc.)

You also don't state whether you are "DX"ing (i.e. attempting
to listen to station(s) outside the designed coverage area.)
"Interference" when trying to listen to a local station is likely
very different than "interference" when trying to listen to a station
hundreds of km away.

The solution for each kind of "interference" is not the same, so it
might be prudent to do some more detailed evaluation and reporting
of exactly what kind of "interference" you are experiencing.

Effective solution(s) may very likely turn out to be antenna-related
(or even mitigation of the source), and have little or nothing to do
with the receiver circuitry itself.
Effective antenna solutions don't necessarily have to be expensive
store-bought gadgets, either. They could be long-wire antennas
or home-made tuned loops, etc. There are many sources of
information about AM receiver antennas that could be turned
up spending a few quality milliseconds with your friend Google.

As you have seen the problem is generally not the receiver, it's the noise
on the band.
Check out this site, Dallas Lankford has been working at this problem for
many years.
Reviewing his many projects will give you an education on the source of
problems and
some ways to minimize them.
I built one of his phasing unit about 13 years ago, had a lot of fun with
it. Several times
I had a situation where I nulled a station, and then by flipping a switch I
could hear another
station on the same frequency. I think he is now 2 maybe 3 generations past
the one I built.
http://www.kongsfjord.no/dl/dl.htm

Mike

 

[[email protected]]

Maybe in theory you are right. But anyone with a radio that has
synchronous demod will testify that synch demod doesn't help with a
noise source.

3dB of improvement may be hard for people to notice but it is none the
less real.

It is good for rejecting noise from the adjacent channel
if you passband shift.

It prevents the carrier on the adjacent channel from multiplying with
anything so it keeps all of its side bands at high frequencies so yes
it does a good job of that.

The magnetic loop really does the job. The designs that float the loop
above ground are more effective. [The Wellbrook does that.]

huh?

Some loops are built over a ground plane, other use a transformer and
float above the ground.

I'm pretty sure I'd hear a 3db reduction in noise with synchro. I'm
not sure why you think the snr would be 3db better. If it is because
you are listening to just one side of the AM signal, you need to
remember that you are getting half the energy as well. Thus the narrow
bandwidth doesn't buy you much unless you managed to avoid the QRM/N.
Synchro helps a bit with fading since that phenomena is due to the
mixing of signals (direct and reflected). [Narrow the BW, less room
for constructive and destructive interference.]

It's really hard to design a decent synchronous AM receiver. Most of
the built-in synch demods are real crap. Even Drake has to rev their
synch a few times. Sherwood sells an external synchro box because
everyone else does a piss poor job. Probably the R8b and AR7030 are
the only receivers out there with passable synchronous demod.

 

[[email protected]]

 As you have seen the problem is generally not the receiver, it's the noise
on the band.
  Check out this site, Dallas Lankford has been working at this problemfor
many years.
Reviewing his many projects will give you an education on the source of
problems and
some ways to minimize them.
I built one of his phasing unit about 13 years ago, had a lot of fun with
it. Several times
I had a situation where I nulled a station, and then by flipping a switchI
could hear another
station on the same frequency. I think he is now 2 maybe 3 generations past
the one I built.
 http://www.kongsfjord.no/dl/dl.htm

                                 Mike

I've seen Lankford's work. A lot of it is dubious at best. I recall he
had a "paper" where adding a lowpass filter in the audio path would
reduce fading.

The phasing schemes work if the QRM/N is local. I have a ANC-4 for
that purpose. MFJ makes one as well. Still, nothing is a good as using
a Wellbrook. There is something about the "large aperture" of the
Wellbrook that reduces the local noise.

http://www.wellbrook.uk.com/ALA100b.html
Having hacked with the older ALA100 (there are a few versions now), I
find a 4x4 loop of copper pipe (easy to build) does the job for AM
BCB. If you want a deep null, you need to give the antenna space. I've
done handheld direction finding with the ALA100 and a 2x2 loop. For
most urban areas, about 4x6 is the practical limit. I find the
attenuator turning on for some local stations with that much aperture.
In the boonies, anything goes I suppose. I have use the ALA100 with an
8ft on a side loop. I could pull in NDBs from 1000 miles away in the
daylight, but only in remote areas.

 

[MooseFET]

3dB of improvement may be hard for people to notice but it is none the
less real.

It prevents the carrier on the adjacent channel from multiplying with
anything so it keeps all of its side bands at high frequencies so yes
it does a good job of that.

The magnetic loop really does the job. The designs that float the loop
above ground are more effective. [The Wellbrook does that.]

huh?

Some loops are built over a ground plane, other use a transformer and
float above the ground.

I'm pretty sure I'd hear a 3db reduction in noise with synchro.

You'd have to do an A/B test quickly. 3dB isn't huge.

I'm
not sure why you think the snr would be 3db better.

Go back and re-read what I said. Look carefully at the noise it
removes. It takes out 1/2 the noise. Since we are talking about
random stuff the difference is 3dB.

If it is because
you are listening to just one side of the AM signal,

No, it is because the sync demod rejects half the noise in the
combined sidebands. The is double sideband with a carrier. It is
standard AM radio not SSB. The 3dB improvement happens only if you
have both side bands and the carrier. Only if you have both sidebands
can you have a signal like:


Y = f(t) * cos(wt) + sin(wt)

That equation can't exist in a SSB world.

 

[amdx]

As you have seen the problem is generally not the receiver, it's the noise
on the band.
Check out this site, Dallas Lankford has been working at this problem for
many years.
Reviewing his many projects will give you an education on the source of
problems and
some ways to minimize them.
I built one of his phasing unit about 13 years ago, had a lot of fun with
it. Several times
I had a situation where I nulled a station, and then by flipping a switch
I
could hear another
station on the same frequency. I think he is now 2 maybe 3 generations
past
the one I built.
http://www.kongsfjord.no/dl/dl.htm

Mike

I've seen Lankford's work. A lot of it is dubious at best. I recall he
had a "paper" where adding a lowpass filter in the audio path would
reduce fading.

Please look through the papers and on the URL I postedm and post any
dubious
material you find. Inquiring minds want to know!

The phasing schemes work if the QRM/N is local. I have a ANC-4 for
that purpose.

Phasing schemes are also helpful to null out unwanted stations on the same
frequency
Mike

 

[[email protected]]

  Please look through the papers and on the URL I postedm and post any
dubious
material you find. Inquiring minds want to know!


 Phasing schemes are also helpful to null out unwanted stations on the same
frequency
                                           Mike

That silly paper about the elliptic filter reducing fading comes to
mind. As far as I know, Lankford writes those papers in the format of
a technical journal, but they are neither published or peer reviewed.

 

[[email protected]]

On Feb 6, 5:46 pm, [email protected] wrote:
If you make the AM radio a phase lock loop, you can get some
improvement in the interference rejection.  
What makes you think that?
The PLL locks onto the carrier and has a filter with a low bandwidth.
This means that the phase of the VCO won't change due to noise that is
far from the carrier.  For purposes of understanding why it works,
assume that the VCO is running exactly in step with the carrier and
that in effect the PLL has zero band width.
Now consider the side bands that get demodulated.  A noise component
that can be thought of as resulting in a signal like:
  Y = f(t) * cos(wt) + sin(wt)
where:
  Y       = the signal
  f(t)    = some random (noise) function with no DC
  sin(wt) = the carrier
  cos(wt) = 90 degrees to the carrier
is the important one for the argument.  This signal will resultin
noise from a standard AM demodulation but no noise from the PLL and
sync demodulation one.
Now consider the intelligence on the AM signal.  It is a function
like:
  Y = f(t) * sin(wt) + sin(wt)
The two methods of doing the AM demodulation give equal values for
this input.
The result is that the PLL based method makes the same signal andless
noise on its output.
Maybe in theory you are right. But anyone with a radio that has
synchronous demod will testify that synch demod doesn't help with a
noise source.
3dB of improvement may be hard for people to notice but it is none the
less real.
It is good for rejecting noise from the adjacent channel
if you passband shift.
It prevents the carrier on the adjacent channel from multiplying with
anything so it keeps all of its side bands at high frequencies so yes
it does a good job of that.
The magnetic loop really does the job. The designs that float the loop
above ground are more effective. [The Wellbrook does that.]
huh?

Some loops are built over a ground plane, other use a transformer and
float above the ground.

I'm pretty sure I'd hear a 3db reduction in noise with synchro.

You'd have to do an A/B test quickly. 3dB isn't huge.

I'm
not sure why you think the snr would be 3db better.

Go back and re-read what I said.  Look carefully at the noise it
removes.  It takes out 1/2 the noise.  Since we are talking about
random stuff the difference is 3dB.

If it is because
you are listening to just one side of the AM signal,

No, it is because the sync demod rejects half the noise in the
combined sidebands.  The is double sideband with a carrier.  It is
standard AM radio not SSB.  The 3dB improvement happens only if you
have both side bands and the carrier.  Only if you have both sidebands
can you have a signal like:

  Y = f(t) * cos(wt) + sin(wt)

That equation can't exist in a SSB world.

First, I need to dig up a communications book since I don't know if
you noise model is legit. But look at this this way. You are
receiving one side band. Half the signal, half the noise. Where did
you gain 3db?

y(t) = (1+f(t))*cos(wt)) + n(t)
where n(t) is the noise seems like a more reasonably model.
..

 

[MooseFET]

On Feb 6, 5:46 pm, [email protected] wrote:
If you make the AM radio a phase lock loop, you can get some
improvement in the interference rejection.  
What makes you think that?
The PLL locks onto the carrier and has a filter with a low bandwidth.
This means that the phase of the VCO won't change due to noise that is
far from the carrier.  For purposes of understanding why it works,
assume that the VCO is running exactly in step with the carrierand
that in effect the PLL has zero band width.
Now consider the side bands that get demodulated.  A noise component
that can be thought of as resulting in a signal like:
  Y = f(t) * cos(wt) + sin(wt)
where:
  Y       = the signal
  f(t)    = some random (noise) function with no DC
  sin(wt) = the carrier
  cos(wt) = 90 degrees to the carrier
is the important one for the argument.  This signal will result in
noise from a standard AM demodulation but no noise from the PLLand
sync demodulation one.
Now consider the intelligence on the AM signal.  It is a function
like:
  Y = f(t) * sin(wt) + sin(wt)
The two methods of doing the AM demodulation give equal values for
this input.
The result is that the PLL based method makes the same signal and less
noise on its output.
Maybe in theory you are right. But anyone with a radio that has
synchronous demod will testify that synch demod doesn't help witha
noise source.
3dB of improvement may be hard for people to notice but it is none the
less real.
It is good for rejecting noise from the adjacent channel
if you passband shift.
It prevents the carrier on the adjacent channel from multiplying with
anything so it keeps all of its side bands at high frequencies so yes
it does a good job of that.
The magnetic loop really does the job. The designs that float theloop
above ground are more effective. [The Wellbrook does that.]
huh?
Some loops are built over a ground plane, other use a transformer and
float above the ground.
I'm pretty sure I'd hear a 3db reduction in noise with synchro.

You'd have to do an A/B test quickly. 3dB isn't huge.

Go back and re-read what I said.  Look carefully at the noise it
removes.  It takes out 1/2 the noise.  Since we are talking about
random stuff the difference is 3dB.

No, it is because the sync demod rejects half the noise in the
combined sidebands.  The is double sideband with a carrier.  It is
standard AM radio not SSB.  The 3dB improvement happens only if you
have both side bands and the carrier.  Only if you have both sidebands
can you have a signal like:

  Y = f(t) * cos(wt) + sin(wt)

That equation can't exist in a SSB world.

First, I need to dig up a communications book since I don't know if
you noise model is legit.

Trust me it is. With noise, you can say that any function is part of
it and then say that the rest of it is everything but that function.
This is a nice thing about talking about random stuff.

 But look at this this way. You are
receiving one side band.

I am talking about a normal AM signal with two side bands and a
carrier. You are changing to talking about something else.

[....]

 

[MooseFET]

On Feb 6, 5:46 pm, [email protected] wrote:
If you make the AM radio a phase lock loop, you can get some
improvement in the interference rejection.
What makes you think that?
The PLL locks onto the carrier and has a filter with a low
bandwidth.
This means that the phase of the VCO won't change due to noise
that is
far from the carrier. For purposes of understanding why it works,
assume that the VCO is running exactly in step with the carrier
and
that in effect the PLL has zero band width.
Now consider the side bands that get demodulated. A noise
component
that can be thought of as resulting in a signal like:
Y = f(t) * cos(wt) + sin(wt)
where:
Y = the signal
f(t) = some random (noise) function with no DC
sin(wt) = the carrier
cos(wt) = 90 degrees to the carrier
is the important one for the argument. This signal will result in
noise from a standard AM demodulation but no noise from the PLL
and
sync demodulation one.
Now consider the intelligence on the AM signal. It is a function
like:
Y = f(t) * sin(wt) + sin(wt)
The two methods of doing the AM demodulation give equal values for
this input.
The result is that the PLL based method makes the same signal and
less
noise on its output.
Maybe in theory you are right. But anyone with a radio that has
synchronous demod will testify that synch demod doesn't help witha
noise source.
3dB of improvement may be hard for people to notice but it is none the
less real.
It is good for rejecting noise from the adjacent channel
if you passband shift.
It prevents the carrier on the adjacent channel from multiplying with
anything so it keeps all of its side bands at high frequencies so yes
it does a good job of that.
The magnetic loop really does the job. The designs that float the
loop
above ground are more effective. [The Wellbrook does that.]
huh?
Some loops are built over a ground plane, other use a transformer and
float above the ground.
I'm pretty sure I'd hear a 3db reduction in noise with synchro.

You'd have to do an A/B test quickly. 3dB isn't huge.

Go back and re-read what I said. Look carefully at the noise it
removes. It takes out 1/2 the noise. Since we are talking about
random stuff the difference is 3dB.

No, it is because the sync demod rejects half the noise in the
combined sidebands. The is double sideband with a carrier. It is
standard AM radio not SSB. The 3dB improvement happens only if you
have both side bands and the carrier. Only if you have both sidebands
can you have a signal like:

Y = f(t) * cos(wt) + sin(wt)

That equation can't exist in a SSB world.

First, I need to dig up a communications book since I don't know if
you noise model is legit.  But look at this this way. You are
receiving one side band. Half the signal, half the noise. Where did
you gain 3db?

y(t) = (1+f(t))*cos(wt)) + n(t)
where n(t) is the noise seems like a more reasonably model.

There's what looks to be a reasonable account of synchronous detection ofAM
atwww.arrl.org/tis/info/pdf/9209qex009.pdf.  Using a quadrature regenerated
carrier would resolve one sideband as noted above, so up to half the signal
power would be unused and such an improvement in SNR seems unlikely.
Another use of quadrature in this way is to detect NBFM.

Unfortunately, there appears to be material on numerous other web sites that
claims SNR improvement, for an in-phase regenerated carrier, because it's
claimed that the detector is insensitive to the phase component of the
noise, which is probably not the full story.

The OP is talking about AM band radio. This is not single side band
stuff. There is a huge difference between the two in terms of sync
demodulation's effect.

In single sideband, the phase of the carrier is modulated.