Frequency tone spacing for TOI measurement of power amplifier

I have to be able to measure the TOI of a power amplifier. I read on-line that the frequency tone spacing that can be used for this test is 100 MHz with a 1 MHz resolution but this does not apply for amplifiers that don't have that kind of bandwidth. Is there a ROT for determining what the frequencies of the tones should be based on the center frequency? Does the answer change based on spacing?

Evelyn

It is common to use two-tone method with 1MHz spacing to measure OIP3. While for high power amplifier, the measured inter-modulation may be a function of tone space. So people need to measure different tone spacing in stead of the 1MHz space. The inter-modulation must lies inside the PA bandwidth, there the tone spacing limitation may rise.

A good starting point measuring the IM of your PA, is to use for the tone spacing frequency the channel bandwidth frequency of your system.
For example in WCDMA (3G) the channel bandwidth is 5MHz, so the tone spacing is 5MHz.