Brian wrote:

The picture of "noise" tells us nothing - other than 10khz is down 30db
from carrier. Were it relative to something - that would tell us much.
As it is - it just shows an unknown spectrum of ???? Put a reference in
there (or a known weighted/gated noise source such as specified by NRSC
- like a BruelKjar or equiv.) THEN you can make some solid deductions.

That spectrum was taken during a quiet piano passage with background
noise. The piano, played softly, had little treble, so the spectrum
above about 3 kHz is the product of the program noise spectrum, and
the spectral response of the station, which includes playback
electronics, processor, transmitter, and antenna. The dominant
spectral feature of the station's frequency response is the processor
preemphasis. If the noise spectrum is flat, what you see in the screen
shot is the preemphasis curve. Its absolute level reflects the level
of the background noise, which isn't relevant. But the shape is. The
curve shown is typical of the spectral response you'd expect to see
for a preemphasized AM transmitter. The key point is that it stops
suddenly at 10 kHz, not somewhere below. All of the spectra I've shown
do the same. (The spectra of the two Mexican signals stop at 8 kHz.)

Here's a final screen shot: http://n2.net/k6sti/am1210.jpg . This is
nearby station at 1210 kHz that was broadcasting a live announcer from
a local studio at the time I recorded the spectrum. The carrier is at
the left edge of the screen, the center of the screen is 1220 kHz, and
the horizontal scale is 2 kHz/div. This image shows the upper sideband
in some detail.

Would not the use of pink noise through a low pass filter
and used as the carrier signal modulation be a better way to see the
frequency contour on an analyser, why noise + piano?



If you were designing a high-performance AM receiver, what IF passband
would you use to fully recover the modulation from this signal?

Brian

There are 5 divisions where there seems to be a signal, so to get
the 10 kHz of AF BW involved so that the 10 kHz response was 1 dB down at
10kHz,
about 30 kHz of IF BW would be required, ie, 15 kHz each side of the IF centre
F

This would be somewhat difficult using normal high Q 455 kHz IFTs.

I think one might have a much better chance if one used 2 MHz IFTs,
perhaps 3 of them, and settled for -3dB at 10kHz each side of 2MHz centre F.
Then an emphasis RC filter could boost the 10 kHz back up a bit.


In 1982 in the Australian magazine Electronics Australia, there was an
elaborate AM tuner kit
offered for sale for aud $250 back then which is about equal to usd $1,400
now.
It had 10 different coils types including 3 well damped 455 kHz IFTs, RF
coils, and 9 Khz whistle filter,
5 j-fets, 6 opamps, one ceramic filter, and 3 signal transistors, a 3 gang
variable tuning cap,
and lots of diodes, and R&C bits, and that doesn't include the +15v PS.

The set had non tuned RF input coil feeding 1st RF LC, then fet RF amp,
2nd RF LC, then two untuned balanced transformers and a two fet PP balanced F
converter
feeding IFT1, a fet IF amp, IF2, a 2nd IF fet amp, then IFT3.
The oscillator has a three winding coil, and 3 bjts.
The AF detector is a CA3016 with shunt FB to linearise the detection.
AVC is via TL071, 741, and UAA180m is used to drive
sigal leds and tuning meter.
Output audio is filtered by two TL072 and
with a passive bridged T filter.

I doubt that any of the coil components would be findable today.

The final AF response was - 3 dB at 10 kHz on the "wide" bw setting.

A tube kit to do the same thing today would cost at least the same shirtload
of money,
probably more.

Imagine trying to build any tubed radio today in small batch numbers, and in
doing so
include for an extra 3 tubes to achieve the low thd and wide BW
of the 1982 EA circuit.
It would make the cost greater than a tube power amp.

My paper files have around 20 different circuits for AM tuners including
a fairly simple synchrodyne ( or direct conversion ) two tube sets which
use a 6EJ7 for an RF amp, and followed by a 6BE6 synchronous detector.

I tried building one, but audio output was low, stability was difficult,
and and a superhet proved far better.

Then there were several if not many synchrodyne and some homodyne designs in
Wireless World
over the years, but these were all chip based, except the early
D.G.Tucker circuit of 1947.

Mr Noring wants some miraculously simple cheap design solution to drop out of
the sky.
He should pray to the God of Triodes, perhaps He will send a schematic
directly.

But then perhaps He won't, but there is much information on
AM reception out there in the old publications which mainly lay slowly rotting

in university basement achives if they havn't all been chucked out years ago.
I spent quite some time reading all I could and my copied paper files consist
of a couple of hundred sheets.

Why the heck would I ever want to re-invent the wheel with AM?

Better to consider the wisdom of the past before deciding on something novel.

The tubed tuners are in the minority in my files.

The commercialisation of the complex synchronous receiver types
was exceedingly limited, because when such receivers were concieved,
nearly everyone was farewelling AM for serious listening,
and going to FM.
But AM was good for the cricket, football, news, talkback and pop trash.
Rarely if ever was there any Bethoven.
And nearly everyone started using cheap japanese SS portables
in 1965. There was 3kHz of BW, if you were lucky.

I think one can get ceramic filters with 20 kHz of BW, -3 dB,
Murata part number CFU455E2 offers -6 dB at +/- 12.5 kHz.
The attenuation 10 kHz away from the -3 dB point is 90 dB.
These are usually low impedance input devices, maybe 1 kohm, so they need to
be driven with an untuned IF transformer with a low impedance secondary, or
cathode follower.
Don't apply DC to any of the pins on ceramic filters.

Patrick Turner.