Improving low power AM transmitter circuit

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Bitrex 14 years ago

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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.

Version 4 SHEET 1 880 952 WIRE 448 -96 448 -128 WIRE 304 -64 304 -128 WIRE 448 0 448 -16 WIRE 464 0 448 0 WIRE 576 0 528 0 WIRE 592 0 576 0 WIRE 704 0 656 0 WIRE 752 0 704 0 WIRE 448 16 448 0 WIRE 304 64 304 16 WIRE 304 64 112 64 WIRE 384 64 304 64 WIRE 576 80 576 64 WIRE 144 96 144 -128 WIRE 304 96 304 64 WIRE 656 96 656 64 WIRE 704 96 704 80 WIRE 496 112 448 112 WIRE 576 112 560 112 WIRE -208 144 -272 144 WIRE -144 144 -208 144 WIRE -112 144 -112 80 WIRE -112 144 -144 144 WIRE -64 144 -112 144 WIRE 48 144 48 64 WIRE 48 144 16 144 WIRE 80 144 48 144 WIRE -112 192 -112 144 WIRE 48 192 48 144 WIRE 752 256 752 0 WIRE -272 272 -272 208 WIRE -208 272 -208 208 WIRE 48 272 48 256 WIRE -144 320 -144 144 WIRE 368 320 368 144 WIRE 368 320 -144 320 WIRE -480 400 -480 368 WIRE -224 400 -224 368 WIRE -48 400 -48 368 WIRE 544 432 544 352 WIRE -112 480 -224 480 WIRE 672 480 672 432 WIRE 672 480 608 480 WIRE 752 480 752 320 WIRE 752 480 672 480 WIRE -624 528 -720 528 WIRE -480 528 -480 480 WIRE -480 528 -560 528 WIRE -448 528 -480 528 WIRE -352 528 -368 528 WIRE -288 528 -352 528 WIRE -112 528 -112 480 WIRE 672 528 672 480 WIRE 544 544 544 528 WIRE 544 544 384 544 WIRE -480 560 -480 528 WIRE -352 576 -352 528 WIRE 96 576 -48 576 WIRE -224 592 -224 576 WIRE -176 592 -224 592 WIRE -48 592 -48 576 WIRE 224 592 224 192 WIRE 672 624 672 608 WIRE 96 640 96 576 WIRE 96 640 16 640 WIRE 160 640 96 640 WIRE -224 688 -224 672 WIRE -224 688 -288 688 WIRE -288 720 -288 688 WIRE -224 720 -224 688 WIRE -720 832 -720 608 WIRE -480 832 -480 640 WIRE -352 832 -352 640 WIRE -288 832 -288 784 WIRE -224 832 -224 800 WIRE -176 880 -176 592 WIRE 0 880 -176 880 WIRE 192 880 80 880 WIRE 384 880 384 544 WIRE 384 880 256 880 FLAG -112 272 0 FLAG -496 240 0 FLAG -496 160 Vcc FLAG -112 0 Vcc FLAG -272 272 0 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 FLAG 448 -128 Vcc FLAG 448 192 0 FLAG 656 96 0 FLAG 576 112 Out FLAG -224 832 0 FLAG -224 368 Vcc FLAG -288 832 0 FLAG 576 80 0 FLAG 704 96 0 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 SYMATTR Value BC547C SYMATTR InstName Q1 SYMBOL npn 368 96 M0 WINDOW 3 56 50 Left 0 SYMATTR Value BC547C SYMATTR InstName Q2 SYMBOL res 288 -80 R0 SYMATTR InstName R2 SYMATTR Value 2.2k SYMBOL ind 32 128 R90 WINDOW 0 5 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName L1 SYMATTR Value 220µH SYMBOL res -128 176 R0 SYMATTR InstName R4 SYMATTR Value 47k SYMBOL voltage -496 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 9 SYMBOL res -128 -16 R0 SYMATTR InstName R3 SYMATTR Value 47k SYMBOL polcap -288 144 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 47µ SYMATTR InstName C2 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=6.3 Irms=65m 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 1µF SYMBOL cap 32 192 R0 SYMATTR InstName C1 SYMATTR Value 100p SYMBOL cap -224 144 R0 SYMATTR InstName C4 SYMATTR Value 0.1µF SYMBOL npn 160 592 R0 SYMATTR InstName Q4 SYMATTR Value BC547C SYMBOL npn 16 592 M0 SYMATTR InstName Q5 SYMATTR Value BC547C SYMBOL res 208 672 R0 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL res -64 672 R0 SYMATTR InstName R6 SYMATTR Value 1k SYMBOL pnp -112 576 M180 SYMATTR InstName Q6 SYMATTR Value BC557C SYMBOL res -64 384 R0 SYMATTR InstName R7 SYMATTR Value 2.2k SYMBOL res -496 384 R0 SYMATTR InstName R8 SYMATTR Value 120k SYMBOL res -496 544 R0 SYMATTR InstName R9 SYMATTR Value 22k SYMBOL cap -560 512 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C5 SYMATTR Value 0.1µF SYMBOL voltage -720 512 R0 WINDOW 3 -83 162 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value SINE(0 0.06 10000) SYMATTR InstName V2 SYMBOL npn 384 16 R0 SYMATTR InstName Q3 SYMATTR Value BC547C SYMBOL res 432 96 R0 SYMATTR InstName R1 SYMATTR Value 5.6k SYMBOL res 432 -112 R0 SYMATTR InstName R11 SYMATTR Value 5.6k SYMBOL cap 528 -16 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C6 SYMATTR Value 220p SYMBOL cap 640 0 R0 WINDOW 3 -44 54 Left 0 SYMATTR Value 680p SYMATTR InstName C7 SYMBOL diode 592 16 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 88 37 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL npn -288 480 R0 SYMATTR InstName Q7 SYMATTR Value BC547C SYMBOL res -240 704 R0 WINDOW 0 -42 79 Left 0 WINDOW 3 -38 109 Left 0 SYMATTR InstName R12 SYMATTR Value 470 SYMBOL res -240 384 R0 SYMATTR InstName R13 SYMATTR Value 2.2k SYMBOL res 320 176 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R14 SYMATTR Value 220 SYMBOL res 240 176 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R15 SYMATTR Value 220 SYMBOL cap 560 96 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C9 SYMATTR Value 22p SYMBOL res -240 576 R0 SYMATTR InstName R16 SYMATTR Value 50 SYMBOL polcap -304 720 R0 WINDOW 0 -8 -27 Left 0 WINDOW 3 -11 -46 Left 0 SYMATTR InstName C10 SYMATTR Value 100µF SYMBOL diode 592 64 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL res 688 -16 R0 SYMATTR InstName R17 SYMATTR Value 47k SYMBOL npn 608 432 M0 SYMATTR InstName Q8 SYMATTR Value BC547C SYMBOL res 528 528 R0 SYMATTR InstName R10 SYMATTR Value 5.6k SYMBOL cap 736 256 R0 SYMATTR InstName C8 SYMATTR Value 0.1µF SYMBOL res 656 336 R0 SYMATTR InstName R18 SYMATTR Value 33k SYMBOL res 656 512 R0 SYMATTR InstName R19 SYMATTR Value 47k SYMBOL res 96 864 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R20 SYMATTR Value 220 SYMBOL cap 256 864 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C11 SYMATTR Value 2.2µF SYMBOL res -352 512 R90 WINDOW 0 0 56 VBottom 0 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

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Glenn Gundlach 14 years ago

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

G=B2

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Jan Panteltje 14 years ago

On a sunny day (Tue, 26 Jul 2011 23:08:10 -0400) it happened Bitrex wrote in :

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

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).

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Tim Wescott 14 years ago

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

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Jan Panteltje 14 years ago

On a sunny day (Wed, 27 Jul 2011 11:37:23 -0500) it happened Tim Wescott wrote in :

He idiot, who are you replying to?

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John S 14 years ago

and the audio level.

I like it!

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Bitrex 14 years ago

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.

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Jan Panteltje 14 years ago

On a sunny day (Wed, 27 Jul 2011 15:20:24 -0500) it happened John S wrote in :

and the audio level.

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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Improving low power AM transmitter circuit

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