[help] Designing am transmitter

Discussion in 'Homework Help' started by cck, Jul 11, 2014.

Jul 11, 2014
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2. BeerBelly New Member

Dec 16, 2013
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Q1 works as the oscillator
Q2 as a buffer amp
Q3 as the power amp
.
Analog signal modulates/mixes at the power amp

cck likes this.
3. Papabravo Expert

Feb 24, 2006
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2,187
Oscillator makes the carrier. Digital Input on Q2 or analog input on Q3 adjusts the amplitude of the carrier.

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Jul 11, 2014
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5. Brownout Well-Known Member

Jan 10, 2012
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The analog input changes the gain of Q3, so the output is a gain-modulated version of the oscillator input. There is some math that shows this, but I don't have time to post it. Just remember, the transistor has an exponential gain, so the higher the input, the more gain you get out. That's why the analog input changes the gain.

6. cck Thread Starter New Member

Jul 11, 2014
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ok i wil try to find it. can you tell me that which equation i use?

Can i get it from Vdd*10k/20k - 10k//10k* Ib - Vbe - Ie* 1k = 0

7. Brownout Well-Known Member

Jan 10, 2012
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You get it from expanding the exponential equation for a transistor using Taylor series

e^x = ... + x^2 + (higher order terms): This isn't exactly it, look it up.

Use the x^2 term:

(sina + sinb)^2 = sine^2 + 2sinasinb + sinb^2 now use the middle term

2sinasinb = 2sinω(a)T*sinω(b)T, where ω(a) is the carrier frequency and ω(b) is the audio frequency.

So, by using the exponential transfer function, cleaver biasing and filtering of unwanted signal products, you have effected a multiplication of the carrier and audio signals.

This is the basic idea. There is a lot more to it, but maybe this gets you started.

8. cck Thread Starter New Member

Jul 11, 2014
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i will search taylor series and i think i can handle with it but which equation you talk about?

Ic = Is * ( e^Vbe/Vt - 1) from it?

9. Brownout Well-Known Member

Jan 10, 2012
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yup. .The site you linked had a better development than what I wrote. You should read and understand that, but you can use my simplified math to maybe get the general idea.

10. cck Thread Starter New Member

Jul 11, 2014
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Last question guys,

How can i model xtal in spice for simulation?

11. Brownout Well-Known Member

Jan 10, 2012
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Sure. A crystal is electrically an LC pi network. Extracting the parameters might be a challenge though.

Last edited: Jul 15, 2014
12. cck Thread Starter New Member

Jul 11, 2014
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i tried but i couldn't handle with it. Can you help me little more?

13. Brownout Well-Known Member

Jan 10, 2012
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Couldn't handle what?

Jul 11, 2014
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Jul 11, 2014
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help me guys

Jan 10, 2012
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