Help understanding FM transmitter

[pepewaw]

I'm trying to understand a fm transmitter (attached as an image), which I built and works fine. However, there are a couple of issues I'm trying to figure out (all of them regarding the 2nd part, the first part I figured it's a preenfasis filter):


1. I'm assuming it's a Colpitts oscillator, where DV1 acts as a variable capacitor (you cant see on the schematic, but it's actually a varicap) dependant on the signal inputed through R6. Is this assumption correct? (or is it some other kind of oscillator? maybe a Hartley?)

2. What's the use of R6? (82K) Why cant I just connect it directly to the varicap?

3. Which is the correct output of the antenna, A or B? and what's the use of the other output?

 

[betwixt]

There is RF at the varicap. R6 is to isolate it from the output of the pre-amp stage. Without it, the preamp would load the signal and it probably wouldn't oscillate. Note that the varicap is reverse biased so it is not conducting DC and therefore the audio voltage dropped across R6 is minimal.

One side of the output signal is grounded by a capacitor so if you were to use a single wire, use the other output. If you are center feeding a dipole, use both outputs.

This oscillator is not frequency stabilized and it has no isolation between the antenna and oscillator tuned circuit so expect it to drift in frequency and be very sensitive to nearby objects.

Brian.

 

[BigBoss]

Load is connected in parallel to tank circuit..??
Oh.. brilliant!!

 

[pepewaw]

Would a common collector effectively isolate the antenna from the tunning circuit and prevent (or reduce) the sensitivity to nearby objects?

 

[betwixt]

Yes it would.

Couple the output through a small capacitor to the base of the transistor and take the output from the emitter. If you keep the bias resistor values fairly high (>10K) it will help to reduce the loading effects. I doubt a single stage will completely isolate the de-tuning but it will help enormously.

Brian.

 

[pepewaw]

Thanks again, gonna try it.

 

[pepewaw]

betwixt, I still quite don't get the use of R6:

Below is an image of the Colpitts oscillator which I believe most closely matches the one implementer in the transmitter


My question is how R6 (82K) resembles a ground there? (as in the Colpitts oscillator in the image)?



And what's the use of C11?

 

[betwixt]

It isn't a Colpitts oscillator, it's a Hartley oscillator. The two are very similar but Colpitts uses a center-tapped tuning capapcitor and a Hartley uses a center-tapped inductor.

The tuned circuit in your design uses an inductor with a center tap at which the DC power to the transistor is connected. The choke is to filter noise from the supply line to stop it modulating the frequency and the capacitor to ground is to ensure the center tap is 'ground' as far as signal are concerned.

The tuning capacitor across the inductor is made in two parts. The first part is the single fixed capacitor C9, the second is the varicap in series with a fixed capacitor C7. To the tuned circuit it looks like one capacitor although in reality it is three, two fixed and one being the varicap.

One side of the varicap is always at supply voltage by virtue of being connected through the tuning inductor and choke, the other side is held a lower voltage by the bias resistors on the amplifier stage. The varicap is always reversed biased, in other words always behaving like a capacitor and never conducting DC. The amount by which it is reverse biased depends on the DC and on the modulation signal, together they set the center frequency and provide frequency modulation. Because there is oscillation voltage on the varicap, R6 is necessary to prevent the output of the amplifier absorbing some or all of it which might stop the oscillator running altogether. Because no DC current is flowing through it (varicap isn't conducting) the value is relatively unimportant. The value chosen is a compromise between loading the oscillation if it's too low and the high frequencies in the modulation being attenuated if it's too high.

Brian.

 

[pepewaw]

Thanks one more time!