for Solid-State NMR.
There are no commercial
equivalents to our transmission-line probes. The strategy of inserting
radiofrequencies only at specific impedance minima along the transmission
line results in isolation between channels of over 80 dB (compared
to 40 dB in standard probes). The isolation means that small REDOR
and 2D CODEX difference peaks are completely trustworthy. We have
performed tests on 13C-labeled solids that contained no fluorine
in which millions of 19F 180-degree pulses were used and obtained
a zero REDOR difference. That is, there was no measurable leakage
HFPN four-frequency transmission-line probe for operation in a 500-MHz
spectrometer. The probe sits on a pallet supported on the legs of
the magnet dewar. The support points are connected to worm gears
which are synchronized by universal joints. The probe is raised
and lowered by driving a single shaft (top of photo) with a hand
drill. The yellow cable is part of the active control circuit for
the power amplifiers. Capacitive coupling to the transmission line
monitors the rf voltage at the coil.
The ratio of inner to
outer pipe diameters is determined by impedance matching to the
power amplifiers. Large pipes are used because the transmission
lines are part of the tuned circuit. High currents are carried on
the surface of the inner conductors, and the larger the surface
area, the greater the efficiency.
The transmission line
permits the tune-and-match circuits of all channels to be remote
from the coil and external to the magnet. This results in temperature
stability (no proximity to spinning gas) and allows the use of a
non-perturbative capacitive pickup near the coil of the rf voltages
generated by the final-stage amplifiers. Test pulses fired on each
channel after each data acquisition period generate the voltages
which are diode detected. The test voltages are compared to a calibration
voltage and the difference is used to correct the drive to the power
amplifiers. In effect, we recalibrate the spectrometer every one
or two seconds which makes the long-term data acquisition periods
practical. Short-term fluctuations are suppressed by REDOR and CODEX
differencing and long-term drifts are removed by real-time calibration
and active control of the power amplifiers.
drawing of the HFPN probe. Point A is the proton-frequency impedance
minimum and is the insertion point for the next lowest frequency
in the cascade (fluorine). Adjustments of the tune-and-match tuning
circuits for F, P, and N have no effect on higher-frequency tuning.
C3 and C5 are part of a fluorine trap. The C1 capacitor in the probehead
ensures a homogeneous rf field. The capacitor C2 allows fine tuning
of the transmission-line length and accurate positioning of point
There are a number of
copper and plastic components required for assembly of the transmission-line
probe. The high-frequency input components also contain specially
machined capacitors to handle high frequency and high power. The
transmission line itself is made from standard copper plumbing pipe.
Machined copper and delrin parts for transmission lines and high-voltage
capacitors. (Right) Probehead assemby for air lines and stator support.
However each pipe has
to be adapted to fit with precision machined copper components and
to locate impedance minima accurately (Figure below, right). Final
assembly is by clamps so that disassembly does not require removing
Section of the transmission-line probe showing the four-way junction
on the left and the proton-fluorine trap in the center. (Right)
Adjustment of phosphorous and fluorine isolation by G. Potter.