  We present a simple method for producing a low-drift atomic frequency
reference based upon the Zeeman effect. Our Zeeman Shifted Atomic Reference
`ZSAR' is demonstrated to have tens of GHz tuning range, limited only by the
strength of the applied field. ZSAR uses Doppler-free laser spectroscopy in a
thermal vapor where the vapor is situated in a large, static and controllable
magnetic field. We use a heated $^{85}$Rb vapor cell between a pair of
position-adjustable permanent magnets capable of applying magnetic fields up to
1 T. To demonstrate the frequency reference we use a spectral feature from the
Zeeman shifted D1 line in $^{85}$Rb at 795 nm to stabilize a laser to the
7S$_{1/2}$ $\longrightarrow$ 23P$_{1/2}$ transition in atomic cesium, which is
detuned by approximately 19 GHz from the unperturbed Rb transition. We place an
upper bound on the stability of the technique by measuring a 2.5 MHz RMS
frequency difference between the two spectral features over a 24 hour period.
This versatile method could be adapted easily for use with other atomic species
and the tuning range readily increased by applying larger magnetic fields.
