Question about a transmitter circuit

[boylesg]

I understand the tank circuit with the adjustable air capacitor to tune its oscillation frequency.

I understand that the audio frequency BC547 just gives the tank circuit and audio frequency nudge every now to keep is oscillating as MHz frequency.

However:

You can tune the first tank circuit but how does that effect the second tank circuit to which the antenna is attached?

Does this mean that this transmitter transmits at a fixed frequency set by the second tank circuit values?

What then is the point of being able to tune the first tank circuit? Why couldn't just leave out the adjustable capacitor?

I can see that the MHz oscillation of the first tank circuit can bypass the first audio frequency BC547 via that 10pF capacitor. But then the oscillation either goes into the base of the next audio frequency BC547 and/or to Vcc of the power source.

Does the frequency of the first tank circuit influence the frequency of the second tank circuit via the 100K resistor and Vcc? And therefore both tank circuits are tunable via the adjustable capacitor on the first tank circuit?

 

[godfreyl]

The first BC547 is an RF oscillator, with it's frequency of oscillation determined by the adjustable tank circuit. The second BC547 is an amplifier to increase the power of the RF signal.

The tuning arrangement is a bit odd. For maximum output power, the second tank circuit should be tuned to the same frequency as the oscillator, but it isn't adjustable. I guess you can adjust the oscillator frequency for maximum output power, but then you can't choose any frequency you want.

If I was building that, I'd put trimmers on both tank circuits. The first one to set the frequency, and the second one to adjust for maximum output power.

 

[boylesg]

The first BC547 is an RF oscillator, with it's frequency of oscillation determined by the adjustable tank circuit. The second BC547 is an amplifier to increase the power of the RF signal.

The tuning arrangement is a bit odd. For maximum output power, the second tank circuit should be tuned to the same frequency as the oscillator, but it isn't adjustable. I guess you can adjust the oscillator frequency for maximum output power, but then you can't choose any frequency you want.

If I was building that, I'd put trimmers on both tank circuits. The first one to set the frequency, and the second one to adjust for maximum output power.

I am still confused by it though.

The BC547s have a max band width of the order of several hundred kHz.

The tank circuits oscillate at 100MHz or so and they are only connected to each other via another BC547.

So how is power from the first one amplified by the second circuit?

 

[godfreyl]

The BC547s have a max band width of the order of several hundred kHz.

The transition frequency is about 320MHz, so they have useful current gain at 100MHz.

The amplifier has another useful function besides increasing power - it provides isolation between the oscillator and the antenna, so changes in antenna load don't affect the oscillator.

With transmitters that have the antenna connected directly to the oscillator, moving your hand near the antenna changes the frequency.

 

[boylesg]

The transition frequency is about 320MHz, so they have useful current gain at 100MHz.

The amplifier has another useful function besides increasing power - it provides isolation between the oscillator and the antenna, so changes in antenna load don't affect the oscillator.

With transmitters that have the antenna connected directly to the oscillator, moving your hand near the antenna changes the frequency.

Sorry my mistake, BC547 goes up to 300MHz rather than kHz.

I suppose because the human body has capacitance.

OK so the audio modulated oscillation in the first stage gets amplified and transmitted as RF in the second stage.

But the second stage is itself a tank circuit oscillator so puting my hand near it would similarly alter its frequency. But from what you have said previously that would reduce the power since it would be slightly de-tuned from the first tank circuit. But I presume the signal itself would remain at the same frequency since that is being generated in the first stage. Correct?

 

[godfreyl]

But the second stage is itself a tank circuit oscillator...

No it's not an oscillator, it's a tuned amplifier.

The first BC547 has a 10pF capacitor from collector to base. That provides feedback and makes it oscillate. The second BC547 has no feedback so it doesn't oscillate, it just amplifies.

p.s. Yes, it's the capacitance between your hand and the antenna that changes the tuning.

 

[boylesg]

No it's not an oscillator, it's a tuned amplifier.

The first BC547 has a 10pF capacitor from collector to base. That provides feedback and makes it oscillate. The second BC547 has no feedback so it doesn't oscillate, it just amplifies.

p.s. Yes, it's the capacitance between your hand and the antenna that changes the tuning.

https://en.wikipedia.org/wiki/LC_circuit

I might be getting confused between resonance and auto-oscillation here. So the second tank circuit must resonate when the second BC547 is on. But even so you must still be able to effect the frequency with your hand

 

[Audioguru]

The LC tank circuit of the second tank is parallel resonant which produces a fairly high impedance. But the antenna load is a fairly low impedance that widens the resonant peak to cover the entire FM broadcast band without re-tuning.

 

[boylesg]

The LC tank circuit of the second tank is parallel resonant which produces a fairly high impedance. But the antenna load is a fairly low impedance that widens the resonant peak to cover the entire FM broadcast band without re-tuning.

Oh OK, I hadn't really occurred to me that the antenna itself was contributing to the resonance equation of the second tank circuit.

If the antenna was either longer or shorter than specified then I suppose that would influence the band of the transmitter to some extent, or perhaps just the power of the transmitter due to it not being quite in resonance with the first tank circuit?

 

[Audioguru]

If the antenna is too long or too short then the output power is reduced. it does not affect the frequency.