In a few special circumstances, it is desirable to reduce a signal’s frequency offset from a reference by a fixed integer amount or to reduce its phase noise by a fixed number of dB. Here is an example: a recently introduced Phase Noise Generator is quite versatile in generating arbitrary phase noise profiles due to DDS technology but the ultimate noise floor is above the best crystal oscillators so such oscillators cannot be simulated. The Frequency Difference Divider reduces the entire phase noise profile of the instrument by an integer factor without changing its shape. The block diagram below will reduce the entire phase noise profile for a 10 MHz signal by 20dB, as long as the end result is somewhat more noisy than the oscillator and other circuits.
The Frequency Difference Divider essentially performs the opposite operation of the well-known frequency difference multiplier. The input signal is divided by an integer, say 10, and then mixed with a low noise oscillator signal to bring the frequency back up to the starting frequency. The division process reduces the absolute frequency offset by the division factor and reduces the phase noise by 20 log N (for small angles of modulation). The mixing process restores the original frequency but does not degrade the phase noise or frequency offset if the reference oscillator and mixer do not contribute significant noise. The process may be repeated for more reduction.
The device is limited to a single frequency but a couple of stages should make it possible to simulate crystal oscillators near the state-of-the-art in phase noise. Blue Top modules simplify the realization of the block diagram!
