This question is about pros and cons of making an RF transmitter (eg in the FM band) in the classical way (eg. with a "transistor based" oscillator circuit, like this one which is very classic and simple ) or, instead, using a VCO module like this one.

About the first solution, making a transmitter with a transistor and a bunch of components, I know that most of the times, could be quite hard, the tuning of the transmitter: we have to make a precise coil, if we put our hand near the circuit the frequency will drift (also if we change the antenna length or position), and if the circuit is very basic, I guess that will also transmit some harmonics. Obviously I know that an oscillator transmitter could be more stable with some "trick": would be better make short connections (eg to avoid parasitic capacitance), we can put the circuit in a metal case (and connecting the ground to the metal case), provide a stable power supply, avoid the heating of the transistor and so on.

But I'm asking myself if starting with a VCO module would be more simple from the beginning. Taking again as example the POS-150 VCO Module I see that if I provide 7V to the V-TUNE pin the module will be tuned to 103.66 Mhz. So the main questions are here: a VCO module will assure more stability o will suffer of the same trouble of a classic transistor transmitter? Since the module is inside a metal case I guess that at least will not suffer of the frequency drifting if we put our hand near the module: but about the emiting power? Will also need a power amplifier on the output? The POS-150 claims an output power of 9.5dBm. And just for starting to transmit a frequency, is enough just connecting the V+, ground connections, providing a clean VTUNE and connecting any kind of antenna? (eg a simple piece of wire).

Can someone explain me if a VCO module has some advantages and the correct ways to wiring it as transmitter? (will also need some external components, eg using some capacitors as filter, maybe on the power supply?)

Many thanks.


It will take considerable skill to make a transistor oscillator as good as Mini-Circuits can.

As a bonus, it can be frequency modulated by simply adding a few mv of signal onto the tuning voltage. The "Tuning Sensitivity" spec suggests you get close to the max 75kHz deviation for the FM band with only 10mV.

Obligatory notice : make sure that whatever you do applies with the rules for transmitting signals on your chosen band in your jurisdiction...

  • \$\begingroup\$ However I live in Italy and here we can quietly use these low-power transmitters, also in FM band: I can see that the POS-150 has a total power of 9.5dBm: at which max distance this transmitter can transmit RF? \$\endgroup\$ – Mister D Aug 23 '15 at 15:57

Most of the time a well-built VCO module will assure more stability than self-designed transistor VCO. These modules have been designed by very experienced engineers and it will be hard to build a better circuit. Also the VCO module is shielded which also helps. But these modules are still using transistor-based oscillators internally. They will still have some drift with temperature and some variations from device to device. As you can see from the datasheet, frequency at 7 V will change from 103.66 Mhz at -55 °C to 101.92 Mhz at 85 °C. So 10 °C will change the frequency by around 0.1 Mhz. Your voltage source will also not be exactly 7.0V, even a good quality voltage reference will have something like ~30 mV offset.

If you want really good stability you should use a VCO module and also go for a PLL based circuit. Here is POS-150+PLL based circuit: http://radioinitiation.chez-alice.fr/english/synthe3/synthe3.html

  • \$\begingroup\$ Thank you. If I understand correctly, you told me that: ok: the most of these VCO modules aren't totally perfect, free from some little trouble of use (eg temperature related). I guess that build a transmitter using a bunch of discrete components, as Brian Drummond said, will require some skills and a good assembling (otherwise there will be a list of troubles which I described in my question); instead, using a VCO to build a transmitter, at least will require less tricks during the assembling and is most "error proof": there is the risk that a "classic" transmitter is more unstable, right? \$\endgroup\$ – Mister D Aug 23 '15 at 19:22
  • \$\begingroup\$ I'm not completely sure that I get the question correctly, but yes using a VCO module will be more error proof than trying to build a transmitter without such a module. But even a transmitter with a VCO module but without a PLL can have some frequency drift issues and might require retuning the receiver from time to time because the frequency of the transmitted drifted. Also take care that not just V-TUNE is clean but VCC should be very clean as well. \$\endgroup\$ – Jan Lucas Aug 23 '15 at 20:20
  • \$\begingroup\$ You get the question correctly, and your reply is very clear! \$\endgroup\$ – Mister D Aug 24 '15 at 0:05

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