Crossed diodes

Fig. 16. The conventional duplexer. Zq = 50 1. The characteristic impedance of the A/2 line from the branching point B to the preamplifier port is Z = 200 ft. The parallel capacitances of the crossed diode packages D, D2, D4, and are tuned by matched coils to 270 MHz that of Dj is tuned to 258 MHz. Note that package has a parallel resistor of 50 ft. D, consists of two pairs of BAW76 crossed diodes all other packages contain three such pairs.

Duplexer Using Quadrature Hybrids The circuit devised by Umathum is shown in Fig. 17 and it works as follows While the pulse is on, the crossed diode packages Dj and Dj are heavily conducting and, because of the A/4 lines, a virtually infinite impedance appears at ports 2 and 3 of quadrature hybrid 1 (QHl) that causes QHl to funnel all the incoming power to port 4, which is connected to the (matched) probe. At the crossed diode packages >, and >2 leak voltages t/ i and each 2 V, will... 

After the pulse, the probe acts as the generator of the (small) NMR signal, which enters QHl at port 4 and is funneled with a phase difference of -90° to ports 2 and 3. From there the signal flows, unaffected by the crossed diode packages Dj and Dj (their capacitances are also compensated by coils), to ports 2 and 3 of QH2. Because the phase difference is now -90°, the signal is funneled to port 1 and the preamplifier. 

Figure 3.7. Lowe-Tarr circuit design for connecting the NMR probe to both the transmitter and the receiver with crossed diodes and /4 cables.

There are two parts to solving the receiver paralysis problem. One is to reduce the pulse feedthrough and the other is to make the receiver recover quickly. The feedthrough may be reduced by a well designed duplexer together with a set of crossed diodes shunting the receiver input. The commonly used crossed diodes limit the input to the amplifier to only about 0.5 volt and no less. A modification to reduce this threshold has been suggested by Stokes (1978). 

We now consider a duplexer like that described on pp. 395-397. The main elements are the two sets of crossed diodes and the A./4 line. After the cross diodes stop conducting, the decay rate will be determined by the unloaded Q of the tank circuit in the absence of any active damping. In a high Q circuit, especially at low frequencies, this could entail an intolerable wait, e.g., in excess of Tg. Lowering the tank... 

In summary, what we have described is a system which recovers from the transmitter pulse in two distinct steps. The first part takes place through some low impedance path usually, but not always, associated with the output circuitry of the transmitter. In order to improve the S/N, the transmitter circuit is passively disconnected by a set of series crossed diodes after which the second phase of recovery proceeds according to an active damping circuit which, if present, leads to better S/N because of the improved recovery even though it is a source of additional noise. Neither sets of... 

The transformer is homemade with 4 and 9 turns on Amidon core as indicated while the cables are 50 ohm coaxial except for the 93Q /4 cable to the receiver. The crossed diodes are 1N3731 and in the transmitter cable there are 6.8 volt rf Zener diodes (Unitrode) with silicon diodes in series to reduce capacitances. (See V.C.6.)... 

In pulse NMR the use of non-linear elements, usually in the form of crossed diodes, is widespread. Consider a pair of silicon diodes with the diodes antiparallel. 

Each diode has the property that no current can flow against the arrow (the reverse direction) but current can flow in the direction of the arrow (the forward direction) provided that the potential driving the current exceeds a fairly well defined threshold which is about v for most silicon diodes. This means that the crossed diode circuit as pictured above and symbolically represented as... 

Such a crossed diode unit is obviously a good thing to... 

A pair of crossed diodes in parallel with the input to the receiver acts as a shunt for signals exceeding the h r threshold while acting as if it were not there at all (ideally) for the small NMR signal. 

It is also possible to make crossed diodes with smaller than normal thresholds (Stokes, 1978). Such a device will be very useful to shunt the receiver input because the threshold voltage of the usual crossed diodes is so big compared to the NMR signal that the receiver can still become overloaded even though it is protected. 

We have emphasized these passive crossed diodes because they are easy to implement and are quite effective. There are more complicated non-linear elements which are active (in the electrical sense), the most noteworthy ones being PIN diodes (Kisman and Armstrong, 1974 Hoult and Richards, 1976). Although we have not tried them, they should work very well so that people who have special requirements ought to know about them. In particular, ordinary crossed diodes do not work at high frequencies, say above 100 MHz, because of their capacitances which cause their impedances to become too small at these frequencies. PIN diodes, on the other hand, do not work well at the lower frequencies, say below 100 MHz, so they complement the silicon diodes very nicely. There are now commercial PIN diode switches which can be used quite conveniently, although it is still an active com-... 

One more comment about crossed diodes the series... 

We could think of more sophisticated variations such as a quarter wave line terminated in a series cross diodes which now acts like a shorted quarter wave line only for large signals and an open one for small signals. This, then, is a passive nonlinear circuit which selectively attenuates small signals while ignoring the large signals. A further variation... 

Figure 5.3. Electric signals of the photodiodes initiated by the reflected laser beam crossing diode channel 2.

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