MOGlabs DLC202, Manual, Page: 60 / 82

50 Appendix C. Modulation coils
equals the inductive impedance. That is,
ωL =
1
ωC
C =
1
ω
2
L
. (C.3.7)
Using the long-solenoid equation for inductance,
C =
1
ω
2
µ
0
n
2
Al
(C.3.8)
although in practice we find that the inductance is about half the
long-solenoid prediction and hence the capacitance should be dou-
bled, typically about 1 to 5 nF. With this arrangement, energy is
stored in the inductor-capacitor “tank”, and the
DLC
need only drive
a small current (e.g. 50 mA peak-to-peak) to compensate for losses.
WARNING!
The potential across the secondary Zeeman coil can
easily be hundreds of volts! Please ensure that your coil and capac-
itor do not have exposed connections! Also be sure to use capacitors
with adequate voltage rating.
C.4 Tuning
To maximise the current in the secondary, the capacitor should be
chosen to tune the circuit to the
DLC
modulation frequency. A spec-
trum analyser with tracking generator is particularly helpful: con-
nect the coil to the TG output, and to the SA input, and sweep
through the resonance (see figure). Alternately, drive the coil with
a function generator and measure the magnetic field with another
independent coil (e.g. 20 turns of fine wire on a 1 cm diameter for-
mer) connected to an oscilloscope. Adjust the capacitor by adding
or removing small capacitors in parallel, until the detected field is
maximum at 250 kHz. Again, be sure to use capacitors with sufficient
voltage rating.
In some cases the Q of the circuit may be too high, such that a series
resistor of about 0.5 ohm can result in increased current at 250 kHz,
and reduced sensitivity to frequency drifts.