# How to increase frequency modulation index?

I'm thinking of making a low power FM transmitter. Here's a diagram.

It seems an absolute pull range of a VCXO will limit the modulation index. Is there possibly a way to extend it?

It seems an absolute pull range of a VCXO will limit the modulation index. Is there possibly a way to extend it?

You could use a colpitts oscillator and varactor - you could "wrap" around it a PLL so that centre-frequency stability is as good as a reference crystal. I've done this a few times on applications ranging from 80 MHz to 400 MHz and got FM deviations of +/- 5MHz no problem and data rates of up to 15 MBpsec.

EDIT

Here's a PLL that takes 6mA and works upto centre frequencies of 400MHz: -

You need a small MCU for setting up the division ratio to make lock plus a simple varactor controlled colpitts will do the trick. Or, try the LTC 6905: -

I particularly like this device because it's really simple and doesn't go belly-up on large G-forces (unlike cheap xtals)

• The thing is I want to power this transmitter from a 3V battery or so and PLLs consume too much power and voltage.
– ivan
Jan 11, 2014 at 20:29
• What current does the VXCO use and more importantly who much current does the RF power amp use? What centre-frequency stability are you after and what are your modulation data or signal rates. There are always other ways and quite possibly simpler!!! Jan 11, 2014 at 21:11
• VCXO - 10mA; RF amp - 40mA; 3V 90mAh battery. 100ppm stability; 20kHz max modulating signal with up to 1.5V rms. Thanks for your response.
– ivan
Jan 11, 2014 at 21:31
• @ivan what centre frequency are you looking for? If less than 160MHz then I might have something that'll be of interest. I assume 100ppm means 100ppm/degC. Jan 11, 2014 at 21:47
• I'm looking for 110 MHz centre frequency. Yes, 100ppm means 100ppm/degC (it is cheap).
– ivan
Jan 11, 2014 at 22:01

You are correct. The pull range of the VCXO will limit the modulation index. A VCXO is not a good choice for use here.

There are many other types of VCO's that have wider pull range than a VCXO that you might use instead.

• Can you recommend some? Preferably low-power ones.
– ivan
Jan 11, 2014 at 20:28
• A google image search for "fm transmitter schematic" turns up a bunch of similar designs that seem to be a Colpitt's oscillator, like mentioned in Andy's answer. Jan 11, 2014 at 20:34