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Can Anybody explain How this Transmitter Works?


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Hi
This schematic is related to PT2262 IC DATSHEET which is an encoder used in transmitters.
I have seen this schematic used in many RF APPs. As You see it is operating in UHF Band.
Few Questions:

1- What's the Modulation Of the transmitter?Why?
2- How to calculate The operating frequency?
3- How to build the inductor which I have signed in the schematic?
4- What's the maximum data rate in this circuit and similar circuits depends on?

ThanX in advance
Shahriar

post-1483-14279142273761_thumb.jpg

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Guest Alun

The PT2262 is a standerd remote control IC.

The transistor, capacitor and inductor form an RF hartly oscillator that's modulated (both FM and AM) by the encoder IC. This ocsillator looks like it's designed for UHF and could probably be tuned between 300MHz and 600MHz. This circuit won't work unless you build it on a small PCB, the inductor you've circled could be just a PCB trace.

I'll have a search for you to see if I can find any calculations for PCB trace inductors.

The formula for the frequency is:

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Dea Alun
As you know each transmitter have two important parts. Oscillator and modulator which mix the RF wave
with your Data (data can be Pulse or sound wave or etc.) In most circuit the oscillator section, also plays the modulator roll too. (Like 4watt FM transmitter in the Project section). The PT2262 do not play the roll of a Modulator. It just creat a 11 bit pulse and gives it to the transmitter. now how this Tx modulate the 11bit Pulse. does it act like a FM Tx?
When you buy a Tx module, for example TLP434, It has a maximum baud rate. What this Baude rate depends on?

ThanX in advance
Shahriar

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Guest Alun

The oscilator is both frequency and amplitude modulated, as the signal level increases the transistor will turn on more and more current will flow leading to a higher output level, the parasitic capacitances within the transistor will also change (decrease I think) this will increase the frequency very slightly.

The due to this circuits high frequency bandwidth will be quite high you should be able to transmit a good few hundered kHz or even more, I don't know how this translates into baud as I haven't done digital electronics for quite a long time.

What do you want to do with this circuit?

How to you plan to construct the reciever for the data?

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Dear Alun

Here two schematics for Receiver. Figure 1 is from PT2272 datasheet. whivh is a decoder.
Th second schematic is from a kit in my area that is goos for students. both are very similar to each other. in figure two the input section is a common-base amplifier instead of that dual gate Mosfet.
but so many things are strange to me.
1- what is the value of The inductor i marked it. (there is no explantion in datasheet)
2- as you guest the first opamp is a amplifier (because of the feedback) and the second is a comaprator to convert analogue signal to pulses. the strage thing is why both inputs are connected to each other.
3- I guess the demodulator is AM dedicated to your explanations. Do you agree?

ThanX in advance
Shahriar

post-1483-14279142279743_thumb.jpg

post-1483-14279142280079_thumb.jpg

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Hi Shariar.


1- what is the value of The inductor i marked it. (there is no explantion in datasheet)

Either it has windings capacitance that makes it self-resonant at the RF frequency, or it resonates with the 33pF coupling capacitor.

2- as you guest the first opamp is a amplifier (because of the feedback) and the second is a comaprator to convert analogue signal to pulses. the strage thing is why both inputs are connected to each other.

The inputs on the comparator do not connect together. The non-inverting input is pulled high a little by the 10M resistor, probably to ensure that the comparator's output is low during no signal input. The non-inverting input is also delayed with the series resistor and the capacitor to ground, probably to make the comparator switch as fast as it can.
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Guest Alun

Sorry, while I have done a crappy course in RF communications as a module in my HNC, radio isn't my forte.

But here is my limited understanding of the circiuts:

Circuit 1
Q1 is an RF amplifier, Q2 is a tuned amplifier

Circuit 2
T1 is a simple RF amplifier, and T2 is a regenerative receiver, that demodulates the signal.

The comparators in both cases square up the demodulated signal. I would go for circuit 2 personally as it's the one I understand the most ;D and it's pretty similar to a circuit I would've designed anyway, just a bit more complex. ;D

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As Alun stated, Q1 is an RF amplifier for the antenna, Q2 is the tuned cct.
The first coil is mainly used as an antenna impedance match that also shorts to ground the lower frequence range. Hence it must have an inductance greater or equal to that of the other coil of the tuned cct.

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