RF Transmitter Questions

Thread Starter

tejasdj12

Joined Jul 11, 2010
20
I just saw this Simple RF Transmitter circuit on CircuitsToday



Here are some of the things I didn't understand:
1. Why is Capacitor C3 (2.2nF) connected to Ground? Wouldn't the input AC signal go straight to the ground as capacitor acts as short for AC (not exactly). Is there a need of that Capacitor?
2. On what parameters does the value of Capacitor C6 (10pF) depend? Can I just take any other value?
3. It's mentioned that the circuit is for FM Transmitter. But I don't see any block that can perform Frequency Modulation to that signal. Or should I apply an FM signal to Audio IN instead?
4. If I use any Digital Signal (Square Wave) will the output at Antenna be a Square Wave or a Sine wave? If Sine wave, is it the Tank circuit that produces it? If it is a Square wave, will it still propagate through the Antenna into the surrounding?
5. If my Square wave is of, say, 1KHz, then if I want an output of 30MHz at antenna, should I adjust my Square wave or the Tank Circuit values?

I'm a beginner in Analog Electronics and have basic knowledge of Modulation. But these stuff are pretty hard to understand.
All I wanna make is a Transmitter that can Transmit digital data. The Frequency can be anything, but I would even prefer the standard values like 27MHz
 

BillO

Joined Nov 24, 2008
990
1) Capacitor C3, in conjunction with R3 and R4 acts as a low-pass (shunt) filter. It keeps higher frequencies from getting to Q2. My guess is that it is there to prevent unwanted oscillation.

2) C6 helps tune/balance the output to the transmit frequency. adjusting it correctly will make the transmitter more efficient.

3) The output frequency depends on L1, R5 and C5 and even less on C6. Placing a signal on the base of Q2 acts to change the resistance in that circuit and hence change the frequency.

4) It will be a sine wave with square wave frequency modulation. The square wave itself will not be transmitted, only the modulated carrier. The carrier is generated by the oscillator circuit which is made up of Q2, L1, R5 and C5.

5) To change the carrier frequency you'd need to change one or more of L1 or R5.

Edit: Changing C6 and C7 would also have some effect on oscillator frequency. This seems to be designed to work at a frequency closer 100mHz.
 
Last edited:

Thread Starter

tejasdj12

Joined Jul 11, 2010
20
1) Capacitor C3, in conjunction with R3 and R4 acts as a low-pass (shunt) filter. It keeps higher frequencies from getting to Q2. My guess is that it is there to prevent unwanted oscillation.
Got that

2) C6 helps tune/balance the output to the transmit frequency. adjusting it correctly will make the transmitter more efficient.
I asked one of my professors, and he said that it is called as Neutralizing Capacitor. It neutralizes what? Any guesses?

3) The output frequency depends on L1, R5 and C5 and even less on C6. Placing a signal on the base of Q2 acts to change the resistance in that circuit and hence change the frequency.
What C4 is used for then? Also, R5 is emitter negative feedback resistor. It gets bypassed by C7 anyways. So does it really affect the frequency?

4) It will be a sine wave with square wave frequency modulation. The square wave itself will not be transmitted, only the modulated carrier. The carrier is generated by the oscillator circuit which is made up of Q2, L1, R5 and C5.
You mean FSK?

5) To change the carrier frequency you'd need to change one or more of L1 or R5.
Even varying C5 will do the job, won't it? :cool:

Some more questions:
1. Which type of oscillator circuit is this? Hartley or Colpitts?
2. If it is an oscillator, where is the positive feedback?
3. Any book that deals with designing such circuits? Or any way to determine the values for each of the components in this circuit? I'm familiar with small signal amplifier designing.
 

BillO

Joined Nov 24, 2008
990
2) Neutralize? Hmmm... neutralize any unbalance caused by c5 and c6 being different I guess.

3) C4 and C8 are just across the supply voltage. My guess is they are there for stability. C7/R5 tune at 159 mHz, and that's pretty close to the operating frequency, so I'm going to have to say it will still have an effect. maybe not a huge one, but an effect any way. If you wanted to eliminate it, C7 should be around 1nF or larger.

4) You could call it FSK.

5) Yes, but to make huge changes you'll need to change both L1 and C5

New questions...(and this can't go on forever...)

1) No idea
2) The bias of the transistor changes as the voltage on the collector varies.
3) Look for a book on simple radio transmitters.
 

Dodgydave

Joined Jun 22, 2012
10,027
If you bothered to read the write up, it says C5,L1 alter the freq, c6 sets the gain, to increase frequency decrease the capacitance of C5, or L1 turns, the opposite will decrease the freq. You will need a frequency counter to test the result.
 

Shagas

Joined May 13, 2013
804
You will probably need a pre-amp stage if you are planning to use a bare microphone as an input to this circuit.

Ok my turn :
I see that Q1 only acts as the carrier wave modulator and q2 paired with the inductor and the trimcap actually makes the carrier wave (the trim cap and inductor form a tank circuit ) . BUT where is the feedback to the base of q2?? how do the oscillations sustain?

I know very little RF magic
 

t_n_k

Joined Mar 6, 2009
5,455
You will probably need a pre-amp stage if you are planning to use a bare microphone as an input to this circuit.

Ok my turn :
I see that Q1 only acts as the carrier wave modulator and q2 paired with the inductor and the trimcap actually makes the carrier wave (the trim cap and inductor form a tank circuit ) . BUT where is the feedback to the base of q2?? how do the oscillations sustain?

I know very little RF magic
Google common base Colpitts oscillator - which is what Q2 appears to be configured as.
 

t_n_k

Joined Mar 6, 2009
5,455
Oh I see , it's common base configuration
That's correct.

Feedback is applied to the emitter via the voltage divider formed by C6 & C7.

The actual oscillating frequency is related to the parallel combination of C5 with the series equivalent of C6 & C7. Not C5 alone. In fact one must consider the BJT internal capacitances as well for a complete analysis.

There are many documents available on the web giving detailed analysis of the BJT Common Base Colpitts.
 
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