WTD: Advanced FM mod techniques

[E-design]

I am looking for material on ultra low distortion (< 0.05%), wide-band-analog FM modulation techniques, that does not involve any digital processing. I have done a lot of searching but could not really come up with anything useful.

I am doing a project on a high quality FM Stereo broadcast exciter and want to see how low I can get the distortion using only analog methods. I have reached a limit of around 0.04% THD at full deviation (tested at 400Hz and 1kHz mod frequency) with my own design and optimizations. My test tone's distortion measure lower than 0.01% so that should not be a problem. Analyzing the distortion shows mainly a 2nd harmonic component remaining.

Signal-to-noise ratio's of my modulated carrier is over 80dB,
Phase noise is good (126db@10k), so I am not sure if any of this is my limiting factor. Any help or ideas appreciated.

Thanks E

 

[biff44]

Phase noise in FM systems is seldom a problem--30 dB easier than a qpsk system for instance. How are you currently modulating, as VCO? What frequency is the carrier?

 

[E-design]

I am modulating the VCO's varactor diodes. Carrier is between 87-108MHz

 

[flatulent]

One trick is to use feedback. A pulse counting detector is very linear. The output from this is fed to the signal doing the modulation.

 

[biff44]

Good point, feedback would be the way to go.

 

[Borber]

Another possibility is to use two VCO's, one at 800MHz modulated and other at 700MHz for frequency shift, both connected to mixer.
Mixer output deviation is not dependent on carrier frequency.

 

[E-design]

Thanks guys for all the ideas and the offers to design custom circuitry for me. I forgot to mention that the project is for my personal use and chasing the limits is only for personal satisfaction. So I don't want to pour big money in to this as it will have to come from my own pocket. The performance is already very good, but it is always exciting to push the envelope.