Spread Spectrum Oscillators

Every clocked system produces electromagnetic emission at the frequency of the clock generator and the harmonics of this frequency. The idea of spread spectrumis to modulate the frequency of the clock generator in a way that the energy of the disturbance is distributed over a certain bandwidth. This way the energy disturbing RF systems operating at the same frequency as the clock generator can be reduced by:

    P/P0 = RBW/SBW

with:

P: Power inside the bandwidth of the RF system being disturbed using spred spectrum.
P0: Power inside the bandwidth of the RF system being disturbed not using spred spectrum.
RBW: Resolution bandwidth, bandwidth of the RF system being disturbed.
SBW: Width of the band the energy is distributed using spread spectrum methods.

Here comes an example to make it easier to understand the usage of spread spectrum:
Bandwidth of the victim RF system: RBW=10kHz
Frequency of the system: 10MHz
FM modulation maximum frequency shift: 1%

Reduction of the disturbance:

P/P0 = 10kHz/10MHz*0.01 = 10kHz/100kHz = 0.1

The difference in magnitude is about

10lg(RBW/SBW) = -10dB

How does a spred spectrum oscillator look

The basic idea is to simply perform a frequency modulation. Since FM modulation produces side bands of

f_{side_band} = f_osc +- f_mod

as well as weaker bands (Intensity decays according to bessel functions) at multiples of the modulation frequency. FM modulation with only one frequency is not too effective. The modulation must be carried out using a noise signal. In stead of real noise quasi noise comming from a digital source will do the job as well as long as the line spacing is less or equal the resolution bandwith of the victim RF system for which the disturbance should be reduced. (If the line spacing is bigger only a fraction of the spread bandwidth really is used and the spread spectrum approach performs not as good as it could using propper design.)

Fig. 1: Simple spread spectrum oscillator

Since analog noise generators are cumbersome to design using digital solutions should be considered. So the design slightly changes using the oscillator as a clock source for the digital noise generator.

Fig. 2: Spread spectrum oscillator with digital noise generator.

The line spacing calculates as approximatelly

df = f_mid / 2ⁿ

with n being the length of the random generator shift register

Example:
fmid = 10MHz
n = 8
df = 10MHz/256 = 39kHz

Well, not quite optimum (df of 10kHz would lead to about 6dB better results because the energy would be distributed over 4 times more lines without pushing more lines into the RBW) but already a good staring point.
A simulation of the oscillator output and an FFT (fast fourrier transformation) of the signal shows the difference with and without spread spectrum.

Fig. 3: Simulation of a spread spectrum oscillator with spread spectrum active (red) and spread spectrum disabled (green). The benefit is about 10dB