Improving low power AM transmitter circuit

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I've attached the LTSpice file of a little low-power AM transmitter
circuit I'm working on, basically to just transmit a signal from an MP3
player to an AM radio across a room.  However, while I'd like the
circuit to be relatively simple, I'd also like the audio to be as high
quality as possible.

Some issues with the current design:

The single transistor audio stage doesn't provide much gain for
corrective audio frequency feedback to work.

There is no mechanism to stabilize the RF output against amplitude
variations.

The negative-resistance oscillator is not very spectrally pure - I could
probably change the output circuit to use a tuned circuit, but perhaps
there is additionally another oscillator topology which is better suited
for this application.

Thanks for any advice.


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SYMATTR InstName R19
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SYMBOL cap 256 864 R90
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SYMATTR InstName C11
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SYMBOL res -352 512 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R21
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SYMBOL cap -368 576 R0
WINDOW 3 -55 69 Left 0
SYMATTR InstName C12
SYMATTR Value 680p
TEXT -536 64 Left 0 !.tran 0.1

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Content-Type: text/plain;
 name="AMTransmitter2.asc"
Content-Transfer-Encoding: 8bit
Content-Disposition: attachment;
 filename="AMTransmitter2.asc"

Version 4
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WIRE 448 -96 448 -128
WIRE 304 -64 304 -128
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WIRE 592 0 576 0
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WIRE 304 64 304 16
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WIRE 0 880 -176 880
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WIRE 384 880 384 544
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FLAG -496 160 Vcc
FLAG -112 0 Vcc
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FLAG 144 -128 Vcc
FLAG 304 -128 Vcc
FLAG 48 272 0
FLAG -208 272 0
FLAG -48 768 0
FLAG 224 768 0
FLAG -480 832 0
FLAG -480 368 Vcc
FLAG -48 368 Vcc
FLAG -720 832 0
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FLAG 448 192 0
FLAG 656 96 0
FLAG 576 112 Out
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FLAG -224 368 Vcc
FLAG -288 832 0
FLAG 576 80 0
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FLAG 544 352 Vcc
FLAG 544 624 0
FLAG 672 624 0
FLAG 672 352 Vcc
FLAG -352 832 0
SYMBOL npn 80 96 R0
WINDOW 3 55 48 Left 0
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SYMATTR InstName Q1
SYMBOL npn 368 96 M0
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SYMATTR InstName Q2
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SYMATTR InstName R2
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WINDOW 0 5 56 VBottom 0
WINDOW 3 32 56 VTop 0
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SYMBOL res -128 176 R0
SYMATTR InstName R4
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SYMATTR InstName V1
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SYMATTR InstName R3
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SYMBOL polcap -288 144 R0
WINDOW 3 24 64 Left 0
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SYMATTR SpiceLine V=6.3 Irms65%m Rser=2.1 Lser=0 mfg="Nichicon" pn="UPR0J470MAH"
type="Al electrolytic"
SYMBOL cap 112 48 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C3
SYMATTR Value 1F
SYMBOL cap 32 192 R0
SYMATTR InstName C1
SYMATTR Value 100p
SYMBOL cap -224 144 R0
SYMATTR InstName C4
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SYMATTR InstName Q4
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SYMATTR InstName R17
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SYMATTR InstName Q8
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SYMATTR InstName R10
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SYMBOL cap 736 256 R0
SYMATTR InstName C8
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SYMATTR InstName R18
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SYMBOL res 656 512 R0
SYMATTR InstName R19
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SYMBOL res 96 864 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R20
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SYMBOL cap 256 864 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C11
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WINDOW 3 32 56 VTop 0
SYMATTR InstName R21
SYMATTR Value 22k
SYMBOL cap -368 576 R0
WINDOW 3 -55 69 Left 0
SYMATTR InstName C12
SYMATTR Value 680p
TEXT -536 64 Left 0 !.tran 0.1

--------------020709020805080301080506--

Re: Improving low power AM transmitter circuit

If you're interested in quality, why aren't you considering FM where
you can have wider bandwidth and stereo?

GB2%

Re: Improving low power AM transmitter circuit
On a sunny day (Tue, 26 Jul 2011 23:08:10 -0400) it happened Bitrex

Quoted text here. Click to load it

Then why use AM? You wil lose stereo and bandwidth.


Quoted text here. Click to load it

VERY complicated.
Last time I did a quick AM modulator I used a CMOS switch:
                 +5
                 |
                 R1M      1MHz 5V  square wave (555 timer ??????)  
                 |         |  controls switch                     \/
                 |---------0\                                      | antenna
audio in  --||---|           \------||---- AM out --- bandpass  ---
           100n  |         0                              
                 R         | 1/3 74HC4053
                 |1M       |
                ///       ///


Just joking about the 555, any thing will do, as long as it is a bit stable,
For example a 1 MHz crystal with CMOS gate, all digital.
Or a higher freq crystal with a CMOS gate oscillator and CMOS divider.

The way this works is that the output of the CMOS switch switches between 0V and
the audio level.
The audio changes between min 0V and max +5V,  causein AM out.
Without audio the output is a 2.5V square wave.

Bandpass can just be a simple LC filter, or PI filter (harmonics).


Re: Improving low power AM transmitter circuit

Quoted text here. Click to load it

* Without notes as to what the pieces are, it's an uphill battle to
figure out your schematic.

* Transistors aren't really good devices for AM output stages -- you
can't collector modulate them fully; you have to modulate the base as
well as the collector.  

* Transistors aren't really good devices for AM output stages -- as the
modulation dips, the collector-base capacitance changes, which changes
how the transistor loads the preceding stage.  This will cause some
spurious phase shift at best, or spurious FM if it loads the oscillator.

* Use a diode clipper (or mixer), modulate that, then apply the modulated
signal to a linear amp.  Or just straight to your antenna, since it's low
power.

* Why not FM?  Quick, easy, published circuits, 100.1MHz is more manly
than 500kHz, ...

* I'd address the audio chain as if I were driving a small speaker of the
same power requirements of the stage I was modulating.  That'd be more
than one transistor, for sure.  Instead of discretes I'd use an op-amp
known to make a good audio device (or an LM353 with one of the common
biasing dodges).  I'd only build it out of discretes if I were on a
personal mission to learn some obsolete technology.

* If I were on a personal mission to learn some obsolete technology, I'd
use tubes.  Octal-base metal tubes, because that's what was in the
electronics that I got for free as a kid, and I still have a visceral
positive response to them now, decades later.

* Tubes make much better final RF stages than transistors.  Class C
triode stages can be modulated all the way down to nothing if they're
biased and neutralized right, class C screen grid tube stages can be
modulated all the way down to nothing if the screen is correctly
modulated (and sometimes that can be done with a passive choke to the
screen), or a class C screen tube stage can be _screen_ modulated with
very low audio power input, at the cost of quite some plate efficiency.  
Options abound.  And real radios warm the room, not just the heart.

--
www.wescottdesign.com

Re: Improving low power AM transmitter circuit
On a sunny day (Wed, 27 Jul 2011 11:37:23 -0500) it happened Tim Wescott

Quoted text here. Click to load it

He idiot, who are you replying to?
<snip crap>

Re: Improving low power AM transmitter circuit
Quoted text here. Click to load it

Thank you for your suggestions - I have an FM design that I'm working on
concurrently.  Using an op-amp based audio stage is definitely on my
list of improvements to work on.

Re: Improving low power AM transmitter circuit
Quoted text here. Click to load it
and the audio level.
Quoted text here. Click to load it

I like it!

Re: Improving low power AM transmitter circuit
On a sunny day (Wed, 27 Jul 2011 15:20:24 -0500) it happened John S

Quoted text here. Click to load it
and the audio level.
Quoted text here. Click to load it

Please note that you need to drive this from a low impedance from the RF point
of view,
else little comes out.
So if driven from some audio opamp, put a say 1 nF cap to ground from the input.
That should not attenuate high audio frequencies,

Those switches are cool, I once did a quadrature modulator for PAL color
all with switches and CMOS gates.
If you want the above circuit for a wide tunable frequency range,
perhaps a 74HC4046 as driver would do.
The other thing is that maybe you want to drive the switch  with a very
symmetrical
square wave, so from the output of a flip flop (4013?),
else you get a number of sidebands that perhaps are more difficult to filter out.




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