  This work demonstrates the first non-reciprocal network based on switched
low-loss acoustic delay lines. The 4-port circulator is built upon a recently
reported frequency-independent, programmable, non-reciprocal framework based on
switched delay lines. The design space for such a system, including the origins
of the insertion loss and harmonic responses, is theoretically investigated,
illustrating that the key to better performance and low-cost modulation signal
synthesis lies in a large delay. To implement a large delay, we resort to
in-house fabricated low-loss, wide-band lithium niobate (LiNbO3) SH0 mode
acoustic delay lines employing single-phase unidirectional transducers (SPUDT).
The 4-port circulator, consisting of two switch modules and one delay line
module, has been modularly designed, assembled, and tested. The design process
employs time-domain full circuit simulation and the results match well with
measurements. A 18.8 dB non-reciprocal contrast between insertion loss (IL =
6.6 dB) and isolation (25.4 dB) has been achieved over a fractional bandwidth
of 8.8% at a center frequency 155 MHz, using a record low switching frequency
of 877.19 kHz. The circulator also shows 25.9 dB suppression for the
intra-modulated tone and 30 dBm for IIP3. Upon further development, such a
system can potentially lead to future wide-band, low-loss chip-scale
nonreciprocal RF systems with unprecedented programmability.
