# What does it mean to have two phase coherent signals at different frequencies?

Just as the title states, I don't understand what it means to have two different signals that are phase coherent but are at different frequencies.

I am attempting to implement a MSK modulator in octave based on an article in the IEEE Communications Magazine, "Minimum Shift Keying: A Spectrally Efficient Modulation" by Subbarayan Pasupathy. At the beginning of the modulator the description reads "The multiplier produces two phase coherent signals at frequencies $f+$ and $f-$." Those frequencies are 1200Hz and 1800Hz for my application.

The image shows the signals $\cos{(2\pi \times fc \times t)}$ and $cos{(\frac{\pi \times t}{2T})}$ as inputs to a multiplier and on the output there is the note $fc = +/- \frac{1}{4} T$. At this point they are split into two different signals.

• You can still talk about a phase relationship between two frequencies when they have a integer ratio, such as 3:2 in your example. In that case, you assume some reference, like the positive zero crossing, in each signal. – Olin Lathrop Sep 25 '14 at 20:30