El Real Decreto 630/2013

Fig. 3.1 shows a photograph of the completed PAD relay version. The schematic diagram and printed circuit board layout is shown in AppendixAon page213. The photo shows the DIN 41612 connector that attaches the PAD to the backplane and the SMA (SubMiniature Version A) and SMB RF connectors that attach to the RF front-end components. A tin wall surrounds the quarter-wave line and parallel crossed diode circuit, and extends behind the pre-amp module. The tin strip walls are mounted through holes in the PCB and attached to the PCB and to each other with solder. The lid for the box is not shown in the photograph. The pre-amp module was attached to the tin strip with screws and mating SMA connectors. The input to the pre-amp is connected to the circuits inside the tin box. The output of the pre-amp connects to a right-angle SMA-to-RG-316 connector and carries signals via 50Ω cable to the tiny RF switch IC located at the crossroads of three 50Ω PCB traces. The black 15V voltage regulator and white optocouplers are located near the backplane connector, spatially separated from the other components. The two RF relays, directional coupler and series crossed diodes are located near to the

70 Chapter 3. Pre-amplifier duplexer

front-end RF connectors and interconnected with top layer 50Ω traces. Eight shiny PCB pads indicate the future mounting position of a low-pass filter.

Figure 3.1: PAD relay version 3.1. The shielding box around theλ/4 circuitry is shown

without the tin lid fitted. Earlier PAD versions were not part of this thesis.

Fig. 3.2 illustrates the functional behavior and routing of signals within the PAD relay version. The TX amplifier transmits a RF pulse toward the PAD module. In normal mode the two relays have their contacts set to the TX/RX position and the duplexer routes the signal to the NMR probe. The duplexer then switches to RX mode and routes the sample’s response signal from the NMR probe to the pre-amp, RF switch, and low power RX amplifier, which is on the Kea receiver amplifier module. During normal mode operation the directional coupler is not part of the signal path.

In wobble mode the TX amplifier transmits a series of RF pulses toward the NMR probe. The two sets of relay contacts are in the wobble position so the pulses pass through the directional coupler en route. In the absence of a perfect match, power is reflected from the NMR probe B1 circuit back toward the TX amplifier.

A small sample of the reflected power is tapped off by the directional coupler and routed through the RF switch to the RX amplifier.

3.3.1. Relay considerations 71

RF switch _Duplexer

(RF switch)

Figure 3.2: The PAD relay 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 relays and RF switch change from TX/RX to wobble, and reflected signals are routed through the directional coupler CPL port.

3.3.1

Relay considerations

The PAD relay version uses the two single-pole double-throw (SPDT) mechanical RF relays to switch between normal mode and wobble mode. Relays were chosen for switching between modes for several reasons. First, they provide excellent electrical isolation through their mechanical contacts. This allows either the normal mode or wobble mode electronic circuitry to be cleanly switched into the circuit while the other is cleanly switched out of the circuit. The relay coils are de-energized in normal mode rather than wobble mode to reduce power consumption and to reduce the possibility of coupling between the relay coils and low-level FID or spin-echo signals that are passing through the relay contacts. A second benefit provided by relays is isolation of the directional coupler from the mainline when in normal mode. This means the directional coupler cannot introduce any mainline insertion loss or impedance changes and is unable to interfere with the minute FID or spin-echo signals. A third benefit is that the directional coupler is only required to carry the low power wobble mode signals and not the high power RF pulses that are part of

72 Chapter 3. Pre-amplifier duplexer

normal experiments. The directional coupler can therefore be a small low power device.

The two-relay design provides an elegant solution for changing between normal and wobble modes. Unfortunately the relays also provided a design dilemma because relay manufacturers offer few small RF relays rated to handle several tens of watts of RF power as required for the Mole probe. RF relays form a small subset of the total relay market and options were found to be very limited. A higher power RF relay that could be sourced was the 250 watt PCB mountable TOHTSU CX-120P but it was physically large and expensive. An alternative was the Omron G6Z RF relay, which was small and reasonably priced. This relay, however, illustrated the second design dilemma, which is interpreting the manufacturer’s continuous power rating specifications for pulsed operation. For example, the relay specifications state a maximum carry and switching power of 10W. For (π/2) and (π) RF pulse amplitudes shown in Fig. 2.4, the Mole requires about 18W for a duration of 40µs at a duty cycle of around ten percent. The average power delivered to the Mole during a long CPMG sequence is therefore about 18W×10% duty cycle = 1.8W. A second example is that the maximum switching voltage is specified as 30VAC RMS and DC while the Mole’s (π) pulse requires a peak voltage of about 42.3V. But maximum relay contact voltage is rated under switching conditions while the PAD relays are never switched when voltages are present. A third example is that the carry and switching current ratings are also less than required by the Mole. But again these ratings are for continuous power transfer rather than pulsed operation. In addition to these ratings, the G6Z relay contacts are designed to operate up to several gigahertz. At higher frequencies skin effects lead to increased resistance, which would reduce the power handling capability of the relay contacts. This fact is probably implicit in the data sheet’s insertion loss specification, which shows increased insertion loss with frequency but is irrelevant at the low operating frequency of the Mole probe. Finally, the electrical and mechanical endurance ratings for the switching contacts are rated as hundreds of thousands and one million respectively. The number of switching operations the PAD relays would experience during their lifetime is likely to be a tiny fraction of these numbers. Given the lack of applicability of the relay specifications to the PAD requirements, it is not possible to know whether or not the relays would be reliable for Mole operation over a long time period. Omron Relay and Devices Corporation was contacted regarding rating the contacts for pulsed operation but no specifications were available. The PAD relay version received limited testing during the course of this project for reasons to be discussed in§3.14 and showed no evidence of failure.

Of course the relays may experience significantly worse conditions if the source and load are mismatched. Under fault conditions the Tomco RF amplifier can deliver significantly more than 18W of pulsed power. Power through the relay