  One of the most basic quantities relevant to planning observations and
assessing detection bias is the signal-to-noise ratio (SNR). Remarkably, the
SNR of an idealised radial velocity (RV) signal has not been previously derived
beyond scaling behaviours and ignoring orbital eccentricity. In this work, we
derive the RV SNR for three relevant cases to observers. First, we consider a
single mass orbiting a star, revealing the expected result that
$\mathrm{SNR}\propto K \sqrt{T}$, where $T$ is the observing window, but an
additional dependency on eccentricity and argument of periastron. We show that
the RV method is biased towards companions with their semi-major axes aligned
to the observer, which is physically intuitive, but also less obviously that
the marginalised bias to eccentricity is negligible until one reaches very high
eccentricities. Second, we derive the SNR necessary to discriminate eccentric
companions from 2:1 resonance circular orbits, although our result is only
valid for eccentricities $e\lesssim0.3$. We find that the discriminatory SNR is
$(9/8) e^2 (1-e^2)^{-1/2}$ times that of the eccentric planet solution's SNR,
and is thus typically an order-of-magnitude less. Finally, we have obtained a
semi-empirical expression for the SNR of the idealised Rossiter-McLaughlin
effect, revealing the bias with respect to spin-orbit alignment angle. Our
formula is valid to within 10% accuracy in 95.45% of the training samples used
(for $b\leq0.8$), but larger deviations occur when comparing to different RM
models.
