  We present a MHz frequency-division dc-SQUID multiplexer that is based on
flux ramp modulation and a series array of $N$ identical current-sensing
dc-SQUIDs with tightly coupled input coil. By running a periodic,
sawtooth-shaped current signal through an additional modulation coil being
tightly, but non-uniformly coupled to the individual SQUIDs, the voltage drop
across the array changes according to the superposition of the flux-to-voltage
characteristics of the individual SQUIDs within each cycle of the modulation
signal. In this mode of operation, an input signal injected in the input coil
of one of the SQUIDs and being quasi-static within a time frame adds a constant
flux offset and leads to a phase shift of the associated SQUID characteristics.
The latter is inherently proportional to the input signal and can be inferred
by channelizing and down-converting the sampled array output voltage. Using a
prototype multiplexer as well as a self-developed high-speed readout
electronics for real-time phase determination, we demonstrate the simultaneous
readout of four signal sources with MHz bandwidth per channel.
