FM modulation of 433,92 MHz RF

Hi All!

First post on this forum. I have the following question:

I want to FM modulate a 433,92 MHz signal coming out of a SAW based oscillator. I was wondering if it was possible to do this just using a simple single-diode mixer. My idea is to feed the mixer with RF from the oscillator and audio as LO from approx. 0 to 5 kHz and take out the FM modulated signal as the "IF". Do you believe that this will work, or maybe the whole thing will be just messy as the carrier is still there as well as the RF+LO and RF-LO. ...

I am pretty new to RF, so this question might be a bit naive, but I would appreciate any answers that I would get...

Best regards

Anders

A mixer might simply produce AM (modulating the amplitude of the carrier), not FM.
FM needs the FREQUENCY of the carrier modulated, usually by using a voltage-controlled capacitor (varactor) in the RF oscillator.

No it will not work. Firstly, the SAW will try to maintain the frequency, that's why they are usually used on AM (OOK) systems. Having said that, they can be pulled slightly by adding additional capacitance across them. The second reason is the difference in fundamental, fundamental + modulation and fundamaental - modulation is far too close to be able to filter out the ones you don't want. When signals are mixed they should be sufficiently far apart that the by-products can be separated by filtering. With only 10KHz (2 x modulation frequency) difference between the +/- carriers at best and virtually no reduction in the fundamental you have almost no chance of isolating just the one you want.

As the modulation frequency is so low it would probably work if you just pulled the SAW frequency using a variable capacitance diode. The concept is you connect a fixed capacitor (for DC isolation) and a varicap diode in series across the SAW device. The varicap should be reverse biased by a small DC voltage to make sure the modulation voltage never drives it into forward conduction. You supply that voltage through a high value resistor (>10K), because the diode is reverse biased there will be almost no voltage dropped acrss the resistor but it will act as a barrier so the 433MHz oscillations are not damped by the power source. You apply the modulation signal through another capacitor (again for DC isolation) and another resistor (for RF isolation) in series to the varicap diode. The DC you apply sets the capacitance of the diode and the modulation voltage mixed with it makes the capacitance change according to the modulation signal. This detunes the SAW so it's frequency shifts and produces the FM you are looking for.

It sounds more complicated than it is, all you need is two resistors, two capacitors and a varicap diode, probably fewer components than your original mixer idea.

Brian.

If you wish to transmit baseband FM on carrier, it could be messy with image detection and baseband phase noise. I suggest you consider an FM sub-carrier such that deviation of subcarrier, is greater than audio bandwidth. Pre-emphasis is recommended.

I assume you have the circuit to modulate the resonant frequency.

Another way to go is PM, phase modulation. feed your RF through a series inductor resonated with a varicap diode. Arrange that your audio modulates the reverse bias on the varicap. The audio will swing the phaseshift through the network and generate exactly the same side bands as an FM signal. The only difference is that the audio should go through a frequency tailoring network (either integrates it or differentiates it, I can't remember, so of to Wiki we go). " For indirect FM modulation, the message signal is integrated to generate a phase-modulated signal. This is used to modulate a crystal-controlled oscillator, and the result is passed through a frequency multiplier to give an FM signal.[9] ...From Wiki So an integration network is required ~
Frank

Another way to go is PM, phase modulation. feed your RF through a series inductor resonated with a varicap diode.

Click to expand...

Theoretical, yes. Unfortunately the achievable FM deviation will be in the same order of magnitude as the modulation frequency, in other words about none for lower audio frequencies.

Thank you all for the excellent replies! I sense that I have definitively arrived at a forum with a lot of resourceful members!

Some comments:

Audioguru- I guess this is what would happen if the mixer was a mixer in the traditional audio sense, but I am talking about an RF-mixer that produces sum and difference products of the RF and the LO frequency...

Betwixt- Thank you for the extensive reply! I agree with what you are saying. I guess even a double-balanced mixer would not work, as the RF+LO and RF-LO would still be very close. Maybe there is another way of just isolating one sideband to make the consept work? Thank you for the varicap idea. I was thinking somewhere along those lines, but I thought that the SAW might just try to maintain its resonant frequency even if you threw in a series cap. A SAW has an equivalent circuit, but I guess you cannot just look at the equivalent circuit and throw in whatever cap you want to change the series or parallell resonant frequency as the SAW works on "non-electrical" principles of surface waves, and would not necessarily be affected "electrically" by these changes.

SunnySkyGuy- Thank you for the reply. Need to meditate a bit on that one. Not sure I understand 100%...

Chuckey and FvM - Thank you for the tip regarding PM. Need to read up a bit on that too..

Best regards

Anders

PHASE modulation of an existing source is the best way to go. there are lots of topologies to do that.

There are "Armstrong modulators" that can do FM modulation. but most people would get a saw oscillator with a tuning port, and do low data rate FM with the tuning port.