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electronic circuit. Wobble mode behavior is similar to the PAD relay version.

RX/ Wob.

RF switch

Duplexer (RF switch)

Figure 3.4: The PAD standard version shown in normal mode, ready to route RF

pulses to the NMR probe. The duplexer changes from TX to RX to receive the sample’s response signal. In wobble mode the duplexer remains in the TX state, the RF switch changes from TX/RX to wobble, and reflected signals are routed through the directional coupler CPL port.

Following the FID or spin-echo signal, the duplexer switches from RX mode back to TX mode in readiness for the next RF pulse. The PAD must be capable of routing these signals quickly, and to their correct destination. The electronic circuitry used to implement the duplexer will be discussed in the next section.

3.5

The duplexer

The duplexer is a radio frequency switch that routes signals depending upon their amplitude. The duplexer used in the PAD designs is constructed from diodes and a lumped element quarter-wave line. These will now be discussed in turn.

3.5.1

Diode switches

Diodes are used to route RF signals because of their ability to behave as electrical switches. When reverse biased, diodes switch off and do not conduct. When

76 Chapter 3. Pre-amplifier duplexer

forward biased, diodes increasingly conduct as forward voltage VF increases. The

diodes used in the PAD design have forward currentIF ≈1µA when VF ≈250µV

and increase to aroundIF ≈1,000mA whenVF ≈700mV.

Two sets of crossed diodes are used on each PAD module as shown in Fig. 3.5. Each set consists of four diodes arranged as anti-parallel pairs. The BAV21 (Fairchild [42]) and BAV103 (Fairchild [41]) diodes appear from the data sheets to be identical except the former is a leaded device and the latter a surface mount device. Leaded devices were used on the PAD relay version to simplify PCB trace routing.

Figure 3.5: A section of circuitry from both PADs showing the crossed diodes and

quarter-wave line arrangement with respect to the amplifiers and NMR probe.

The main function of the BAV crossed diodes D1-4 in series with the TX amplifier output is to isolate the probe and pre-amplifier input from the noisy TX amplifier during receiving the NMR signal. In normal mode the TX pulses are large, enabling two of the crossed diodes to conduct during each half cycle of the RF pulses. The BAV diodes are considerably slower at switching than the LL4448 diodes (50ns compared to 4ns) but have higher peak current handling capability (3.7A per device compared to 1.6A per device). Following a TX pulse the only voltage present on the probe side of the diodes is from the tiny FID or spin-echo response signal (ignoring probe ring-down), which is insufficient to cause the BAV diodes to conduct, therefore the TX amplifier is isolated from the probe.

The LL4448 diodes (Philips [118]) also conduct during the large TX pulses. During each TX pulse they work alongside the quarter-wave line to block the large amplitude TX amplifier pulses from reaching the sensitive pre-amp. At the conclusion of the TX pulse all diodes are switched off. Unlike the BAV diodes the LL4448 diodes do not block the tiny response signal’s path. The response signal has insufficient amplitude to bring the LL4448 diodes into forward conduction and is therefore routed into the pre-amplifier.

In wobble mode the PAD relay version uses its relays to remove both sets of diodes from the circuit. By contrast, both sets of crossed diodes on the PAD

3.5.2. Quarter-wave line 77

standard version remain in circuit in wobble mode, and are driven into conduction during wobble pulses.

3.5.2

Quarter-wave line

A quarter-wave (λ/4) line is a transmission line one-quarter wavelength in length. A λ/4 line with characteristic impedance Z0 has the mathematical property:

ZSZL= (Z0)2 (3.1)

whereZS is the source impedance looking to the transmission line and ZL is the

load impedance terminating the transmission line (Horowitz and Hill (1990) [79]). The value ofZ0 is a fixed property of the transmission line, therefore theλ/4 line

behaves as an impedance transformer for ZS and ZL.

A λ/4 line is constructed from lumped elements C1, C2 and L1 as shown in Fig. 3.5. The capacitors have identical values; at the frequency corresponding to

λ/4 the capacitor and inductor impedances are conjugates, leading to two potential resonant circuits. The first resonant circuit appears if the load impedance ZL is

open circuited. In this instance L1 and C2 form a series resonant circuit, which appears to the source as a short circuit. The second resonant circuit appears if the load impedance ZL is short circuited. In this instance C2 is short circuited,

leaving C1 and L1 to form a parallel resonant circuit that appears to the source as an open circuit.

These properties of the λ/4 line are very useful when coupled with crossed diodes arranged as shown in Fig. 3.5. When the TX amplifier delivers high power pulses to the probe, the BAV diodes and the LL4448 diodes conduct. The LL4448 diodes short circuit the λ/4 line making it appear open circuit to the high power RF pulses. Almost all of the energy from the TX amplifier is therefore reflected to the NMR probe. When the tiny response signal appears from the probe, it is blocked by the BAV diodes and is instead routed through the quarter-wave line to the pre-amp input. The λ/4 line and 50Ω pre-amp input appear together as a 50Ω load.

It should be noted that capacitor C1 experiences the full TX amplifier voltage and is rated accordingly. Capacitor C2 is protected from the high TX amplifier voltages by the LL4448 crossed diodes.