Frequency locking of coupled multivibrators with wide tuning range

A current-/voltage controlled quadrature oscillator with wide frequency tuning range is investigated. It consists of two coupled multivibrators. Two forms of coupling called here soft and hard, are differentiated. Simulations are performed to obtain the tuning range, the synchronization transients, and the influence of mismatches in timing capacitors and charging currents on synchronization. It is found that hard coupling allows a lower quadrature error (below 1°) and low phase-noise (4 dB improvement) with respect to soft coupling, yet wide tuning range may be achieved for both forms of coupling. A constituent multivibrator operating either alone or coupled can be locked to an external synchronizing harmonic, and the locking range is investigated.