Polarisation
6.0 RF front end
6. 1 Introduction
The digital transceiver can provide and process all the signals within an NMR system, but only for a narrowly defined range of signal amplitudes. Therefore, before the transceiver can be connected to the probe some other modules are required to provide the appropriate signal levels. These modules are called the "RF front end" as they deal directly with the probe and usually consist of a transmit RF power amplifier, a very sensitive and low noise preamplifier, a variable gain receiver amplifier and a RF transmit/receive switch (duplexer). A typical arrangement of the modules is shown in figure 6.0. RF P ower Amp RF o utp ut RF Am p RF P re a m p D u p l exer P ro be
Figure 6.0 RF front end block diagram.
Probes operate at different frequencies and with various RF transmit power levels and so it is difficult to produce modules for a wide range of applications. Therefore the RF front end is considered as application specific, unlike the DSPfUSB board and digital transceiver. However, the variable gain receiver amplifier module within the RF front end can be used for most applications if it is designed to operate as a broadband amplifier. This approach was chosen to minimise the effort when modifying the system for new applications and hence a digitally-controlled variable-gain broadband receiver module was designed. The developers of the NMR-MOUSE have also produced RF power amplifier and preamp/duplexer modules to support their probe and a set of these modules was obtained and interfaced to the system. This was a faster and easier way to get a complete NMR system and allowed the early evaluation of the system' s performance. For the NMR-MOLE probe a simple RF front end was put together using a commercial RF power amp and preamplifier. Again this sped up the construction and evaluation of the system. It is intended that in the future a set of RF modules will be designed to support the various probes that are produced.
6.2 Variable gain receiver amplifier
The design of the variable gain receiver is based on the Analog Devices AD8369 [ 1 10] amplifier. This amplifier has a frequency range from about 1 kHz to 600 MHz and a variable gain range from - 1 0 dB to +35 dB. A schematic of the device is shown in figure 6. 1 . The setting of the gain is achieved through the use of either a 4-bit parallel
input or a three-wire serial interface and the device operates with both differential inputs and outputs.
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Figure 6.1 AD8369 variable gain amplifier functional diagram. Figure taken from reference 1 1 0. The receiver amplifier module was designed to be a standard 100 mm x 1 60 mm sized PCB board and the schematic is shown in figure 6.2
Figure 6.2 Variable gain receiver amplifier module.
The modules in the RF front end need some TTL control lines so two 8-bit data latches were included. The latches connect to the DSPIUSB board data lines D8-D23 via a backplane connector and act as write only memory locations. The address of the latches is the same as for the 8-bit latch used within the digital receiver module. That latch was connected to DO-D7 so when the DSP writes to address $20000, the lower 8 bits go to the digital transceiver board and the upper 1 6 bits go to the receiver
amplifier board. The outputs of the latch connected to D 16-D23 are brought out onto a 9 pin D connector and are made available as general purpose TTL outputs. 390 n
resistors are put in series with the outputs to prevent overloading. The outputs of the latch connected to D8-D 1 5 are routed back onto the backplane connector and are intended to be used by the other RP front end modules. During the acquisition of an NMR signal it is necessary to minimise the residual output of the RP amplifier. Therefore RP power amplifiers require TTL signals to control the operation of the amplifier' s output stage and two TTL lines (TTL6 and TTL 7) are allocated for this task. TTLO and some extra logic is included to safeguard against the accidental enabling of the RP power amplifier during system start up.
The receiver amplifier should have a maximum gain of at least 60 dB so two AD8369 devices are cascaded to provide a gain of 70 dB . The gain is set using one of the DSPIUSB board's synchronous serial interfaces. These lines are buffered with separate outputs for each amplifier device to minimise crosstalk. The serial bus can be used for other devices within the NMR system and U3 is used to decode the lines PEO-PE2. The board is powered from the 7 V backplane supply which is regulated down to 5 V for the logic devices. Each of the amplifier devices has its own 5 V regulator to minimise parasitic coupling between them and therefore avoid oscillations. The arrangement of the amplifier devices can be seen more clearly in
figure 6.3. The AD8369 devices are operated with 200 n differential inputs and
outputs and so transformers (Tl and T2) are used to convert to 50 n. The two amplifier
stages are coupled to each other and the transformers using 0. 1 JiF AC coupling capacitors. Since transformers are used, the amplifier input and output connections can be differential. This is an advantage when using multiple modules as often Earth loops can occur. If necessary one side can be tied back to ground using links R 1 3 to R22. These links are distributed along the PCB signal line and when used ensure that the line is well connected to the ground plane.
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The digital receiver requires that the input signal be band limited. Therefore provision is made to apply filtering at the receiver amplifier output. The filter would need to be chosen for each application so the circuit provides support for a wide range of Minicircuits filters. The filters have a common pin configuration, except for the output pins. Either pin 6 or 8 is used as the output, and if it i s not an output it must be tied to ground. Four optional links (R9, R IO, R H , R 12) are used to provide the two options. A two layer design with extensive ground planes was used. The composite artwork is shown in figure 6.4 and the assembled board is shown in figure 6.5. The amplifier devices are each housed in individual screening cans. This minimises the coupling of interference and further reduces the coupling between the devices.
Figure 6.4 Receiver amplifier peB artwork.
6.3 NMR-MOUSE RF front end
The manufacturer of the NMR-MOUSE also provides an optimised RP front end to support their probe. It consists of a 250 W RP power amplifier module and another module containing the preamplifier, duplex er and tune/match circuit. A block diagram is shown in figure 6.6 and a picture of the modules together with a probe is shown in
figure 6.7 Blanking (TTL6) Du lexer RF in ut Power Amp Reflected Power RF Switch RF output Preamp Tune/Receive (TTL4)
Figure 6.6 NMR-MOUSE RF front end block diagram.
Figure 6.7 NMR-MOUSE with RF front end [22] .
Directional Coupler
Tuning
The RF output of the digital transceiver is connected to the input of the RF power amplifier and the output of the RF tune/receive switch is connected the input of the variable gain RF amplifier. The RP power amplifier has a blanking capability where the final high power output stage of the amplifier can be shut down. This is necessary to minimise the output noise of the amplifier during the receive phase of an experiment. A TTL input controls this and a low level blanks the amplifier. The
duplexer is the transmit/receive switch which automatically switches into transmit mode when it senses any RF power. Being automatic always provides protection for the very delicate preamplifier. This RF front end also has a built in tuning/matching circuit so that the probe can be "wobbled" while remaining attached to the system. During wobbling, the system provides a low level frequency swept signal to the probe and a directional coupler is used to sense any reflected power. It is important that the probe be tuned to the resonant signal and matched to 50 (2 as any mismatch will result in poor signal to noise performance of the probe and could also lead to the damaging of the preamplifier. If the high RF power can not be absorbed by the probe due to mismatch then it will have to be absorbed elsewhere. A TTL line is used to select between the tune/match and recei ve mode.
6.4 NMR-MOLE
RF
front endThe NMR-MOLE probe and its applications are still in development and the final RF front end requirements are yet to be determined. Therefore, at this stage, it was pointless to consider designing custom modules so some standard off the shelf modules were purchased. A 250 W, 250 kHz-8 MHz RF power amplifier was purchased from Tomco [ 1 1 1] and a picture of it is shown in figure 6.8.
Figure 6.8 Tomeo 250W RP power amplifier [ I l l ] .
The amplifier operates i n class AB and i s designed for pulse applications with the maximum allowable pulse width of 1 ms. The amplifier requires a 48 V power supply, a 0 dBm signal for maximum RF output and a TTL signal for blanking.
An AU- 1 583 low noise preamplifier was purchased from Miteq [1 1 2] and is shown in figure 6.9. It has a typical noise figure of 1 .2 dB, a gain of 36 dB and a bandwidth from 20 kHz to 400 MHz.
Figure 6.9 Miteq preamplifier [ 1 12].
A duplexer was designed and constructed using the common technique of a quarter wave line with crossed diodes, this is shown in figure 6. 10. The diodes D1 are included to reduce the coupling of noise from the power amplifier during the receive phase of the experiment. The diode that was used for this purpose is the BA V21 from Philips [ 1 13]. During the transmit phase, the diodes D2 will conduct due to the high voltages present at the output of the RF amplifier. They will therefore act as a short on the end of the quarter wave line which will result in the other end of the line becoming a high impedance input. This high impedance then prevents any power from reaching the input of the preamplifier thus protecting it. During the receive phase, the quarter wave line acts as a low pass filter. 1 N4148 diodes were used for D2, they are fast, have low capacitance and can be obtained from many manufacturers.
Power Amp
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Figure 6.10 Quarter wave dupJexer.
A schematic of the complete RF front end is shown in figure 6. 1 1 where the quarter wave line has been implemented with a lumped element circuit consisting of an inductor (L1 ) and two equal capacitors (C l and C2) [ 1 14]. When (1)2 Le = 1 , the
lumped element circuit is at resonance and behaves as a quarter wave line with a characteristic impedance of For a 3 .3 MHz design, C = Cl = C2 = 965pF and
cue L = LI = 2.4 1 J.!H . RF in put Power Am p Matching Tu ning RF output Preamp
Figure 6.1 1 NMR-MOLE RF front end block diagram.
Since the RP front end was only constructed for the trialling of the new NMR-MOLE probe, it was unnecessary to include circuitry for the tuning and matching of the probe. The probe can be easily tuned and matched using a spectrum analyser that has a
RP sweep output capability and a directional coupler like the ZFDC- 1O-6 from
Minicircuits. The preamplifier and duplexer were placed in a metal enclosure which can be seen in figure 6. 1 2.