Metodología en trabajos de campo

Directional couplers are 3-port devices useful for measuring power flow in a single direction. Bi-directional couplers have an additional port, enabling simultaneous measurement of power flow in forward and reverse directions. The majority of

78 Chapter 3. Pre-amplifier duplexer

power transferred through a directional coupler occurs between the mainline in and out ports. The ancillary coupled (forward) and coupled (reverse) ports tap off a small sample of the mainline power depending upon the direction of power flow. Directional couplers have an essential role in protecting RF power amplifiers from load mismatch (Norman and Dye (2001) [51]) and in tuning and matching NMR probe circuits. The directional couplers implemented on the two PAD modules are now discussed in turn.

3.6.1

PAD relay version directional coupler

A schematic drawing of the directional coupler used in the PAD relay version is shown in Fig. 3.6. The only commercial Mini-Circuits part suitable for operation at the Mole frequency and having 50Ω impedance was the SYDC-20-61HP+ bi- directional coupler (Mini-Circuits [108]). Since only reflected power was of interest, the coupled (forward) port (CPL FWD) was terminated with 50Ω as shown in Fig. 3.6. The Mini-circuits coupler has a specified coupling coefficient of 20dB, thus the ratio of mainline to coupled power is 100:1 when all ports are ideally terminated. The power rating of the directional coupler is 15W maximum. This should never be exceeded on the PAD module because the directional coupler is switched out of the circuit by the relays during high power NMR experiments, and only switched into the circuit during low power wobble experiments.

Figure 3.6: A schematic representation of the commercial Mini-Circuits SYDC-20-

61HP+ bi-directional coupler with CPL FWD port terminated with 50Ω. According to the manufacturer’s specifications, optimal performance requires pins 2, 3, 6, and 7 to be

connected to ground as shown (Mini-Circuits App Note [109]).

3.6.2

PAD standard version directional coupler

A schematic drawing of the PAD standard version directional coupler is shown in Fig. 3.7. This directional coupler was not a commercial off-the-shelf component. It was instead built from capacitors C1 and C2, resistor R1, and hand-constructed transformer T1. The design for the directional coupler is not original; Adams used

3.6.2. PAD standard version directional coupler 79

a similar design in his PAD module and similar designs can be found in Radio Frequency Transistors (Norman and Dye (2001) [51]), Radio-Frequency Electronics (Hagen (1996) [66]) and a QST paper from 1959, which appears to be the original

published design (Bruene (1959) [23]).

Figure 3.7: The directional coupler circuit consisting of C1, C2, R1 and T1 is built into

the PAD standard version. A sample of the reflected power is routed to the CPL REV

port. αV andβI indicate the voltages across C1 and R1 respectively.

The directional coupler is always part of the PAD circuit but in normal mode its CPL REV output is ignored. In wobble mode the TX amplifier sends a series of low power RF pulses to the NMR probe. The pulses have ∼30µs duration and 1ms separation, and the lower and upper frequencies within the pulses are specified by the experimenter. The B1 circuit is tuned and matched at only one frequency, so

all other frequencies reflect some incident power back toward the TX amplifier as indicated by the arrows in the mainline in Fig. 3.7. A small sample of the reflected power arrives at the CPL REV port.

The directional coupler circuit calculates reflected power by measuring the transmission line voltage V and current I at a single location. Voltage V is a superposition of incident (forward) and reflected (reverse) traveling waves VF and

VR, and current I is the sum of the incident and reflected currents IF and IR. The

directional coupler circuit in Fig. 3.7 measures only the reverse traveling waves – although it is relatively simple to modify the circuit to measure power flow in both directions. The following analysis of the directional coupler circuit shown in Fig. 3.7 shows how reflected power is calculated (Hagen (1996) [66]).

Voltage V on the mainline consists of forward and reverse traveling components:

80 Chapter 3. Pre-amplifier duplexer

and similarly for mainline current I:

IZ0 =VFe−jkx−VRejkx (3.3)

which can be solved for VF and VR as follows:

VF = (V +IZ0) ejkx 2 (3.4) VR = (V −IZ0) e−jkx 2 . (3.5)

The phase component of the traveling waves ejωt is ignored for simplicity. To measure VR the directional coupler circuit needs to compute Eqn. 3.5 from the

mainline voltage V and current I. The mainline voltage and current are not measured directly; instead a small voltage sample is measured using the capacitive voltage divider and a small current sample is measured through the transformer. The sampled current is converted to a voltage across resistor R1. Assuming the voltage across C2 is αV and across R1 is βI as shown in Fig.3.7, the voltageVCPL

at the CPL REV port is:

VCPL =αV −βI. (3.6)

In the case of β/α=Z0, Eqn. 3.6 becomes:

VCPL =α(V −IZ0)

=α([VFe−jkx+VRejkx]−[VFe−jkx−VRejkx])

= (2αejkx)VR+ 0VF. (3.7)

For β/α = Z0, the forward traveling waves cancel leaving only the reverse

traveling waves in the computation. Capacitor, resistor and transformer values necessary for nulling forward traveling waves are calculated by setting VCPL= 0.

Eqn.3.6 then simplifies to:

αV =βI. (3.8)

VoltageβIis equal to the current through resistor R1 multiplied by its resistance. The resistor current is a fraction of the mainline current by the primary-to-secondary turns ratio, therefore voltage βI is:

βI = I ×NP NS ×R1. (3.9)