An apparatus includes a controller having differential modulation control outputs and is configured to provide differential modulation signals having a particular frequency at the differential modulation control outputs. A differential modulator circuit is coupled between first differential terminals and second differential terminals. The differential modulator circuit has differential modulation control inputs coupled to the differential modulation control outputs. The differential modulator circuit is configured to: modulate first differential signals at the first differential terminals with the differential modulation signals having the particular frequency, provide the modulated first differential signals at the second differential terminals, modulate second differential signals at the second differential terminals with the differential modulation signals having the particular frequency, and provide the modulated second differential signals at the first differential terminals.