Ph.D. Thesis - Physics

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Figure 3-4: The above probe is a Nalorac HFX probe, where X was tuned to 13 C
3.4.4 RF electronics
The rf electronics discussed in this section consist of the transmitter and the receiver. The
spectrometer we used is a custom-modified Varian UNITY Inova unit with four transmitter
channels.
The transmitter consists of four frequency sources (PTS 620 RKN2X-62/X-116) that
are supplied with a 10 MHz reference signal by a temperature-controlled crystal oscillator.
These frequency sources then supply rf signals of up to 1 Vrms in the range of 1-620 MHz,
with frequency resolution of 0.01 Hz. These signals are sent to transmitter boards that
create pulses of the duration programmed by the experimenter. These have a resolution of
50 ns with a minimum pulse length of 100 ns. In addition, the phase may be set with a
ressolution of 0.5 ◦ . A set of fast memory boards, the waveform generator boards, is then
used to shape the pulses as desired.
The power of the pulses is then set by passing through a set of coarse attenuators.
These can attenuate the signals by up to 79 dB in steps of 1 dB. The relative powers of
each pulse are set here, and finally the pulses pass through a set of linear amplifiers. There
are two dual amplifiers that each contain two units, a low-band amplifier that operates
from 6-200 MHz with 300 W maximum pulse power and 60 dB gain, and a high-band
amplifier that operates from 200-500 MHz with 100 W maximum pulse power and 50 dB
gain. The amplifiers are fast; rise/fall times are 200 ns, which is more than sufficient for
our experiments. Also, there are fast “blanking” circuits which shut off all output from the
amplifier to the experiment. Were this not the case, noise from the amplifiers in between
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