Obtención y preparación de clases de entidad

The RF switches discussed in §3.3 and §3.4 are Analog Devices ADG918 8-pin surface-mount integrated circuits (Analog Devices [47]). The ADG918 is similar to a SPDT switch with pins for common, normally open and normally closed. The Analog Devices data sheet denotes the pins RFC, RF1 and RF2 as shown in Fig. 3.11. The position of the switch changeover contact is controlled by the optocoupler circuitVrf-switch. A logic “0” from the optocoupler connects RF2 and

RFC while isolating RF1, and a logic “1” connects RF1 and RFC while isolating RF2. The isolated port is terminated internally with 50Ω.

From this description, the ADG918 would appear to be a simple switch compo- nent that routes signals depending upon the state of its logic input. This is indeed true when signals at the switch inputs remain within specified bounds. However, when signals stray outside the boundaries the switch becomes less than ideal.

An example encountered in this project was signals at the off channel input affecting signals passing through the switch’s on channel. According to a footnote in the ADG918 data sheet, the absolute maximum voltage between RF1 or RF2 and ground in their respective off states is between−0.5V and VDD −0.5V, where

3.11. RF switch biasing 87 Bias circuit RF1 ADG918/ SA58643 RF Switch RF2 RFC VRF-switch 2k7 2k7 100nF 10uF 100uH +5V

Figure 3.11: The RF switch circuit used for both PAD designs, shown in wobble mode.

Modifications to the PAD standard version included changing the ADG918 to a SA58643 and addition of the bias circuit as discussed in the text.

VDD = 2.5V. This defines a voltage window of −0.5V minimum to +2.0V maximum.

When testing the PAD standard version in wobble mode it was found that the lower limit was being exceeded, resulting in loss of channel isolation and distortion of wobble signals at the RF switch output.

The loss of channel isolation was tested separately by attaching a signal generator to the off-channel RF2 while sending wobble pulses through the switch from RF1 to RFC. The signal generator frequency was set to 3.25MHz and the AC amplitude to 400mVp-p. No obvious distortion of the wobble pulses was observed even with +6V

DC offset on the signal generator, but below a DC offset of −300mV the wobble pulses became distorted.

The reason the lower window threshold was being exceeded on the PAD standard version in wobble mode was that the λ/4 line and shorted LL4448 diodes cannot fully block the wobble signal from reaching the pre-amp. The pre-amp boosted the signal and delivered it to RF2, where it was measured at voltages between −1.3V and +0.9V (into the internal RF switch 50Ω resistor).

One means of ensuring signals did not exceed the minimum window value at RF2 was to diode clamp them either side of ground using fast back-to-back LL4448 diodes. The LL4448 forward voltage however is marginal in terms of the −0.5V lower window limit.

An alternative solution was to add a DC bias to the AC coupled pre-amp output to shift the signal within the window, but this would have been poor design practice as the signal magnitude was close to the window limits and the necessary DC bias was outside the recommended upper limit of 0.5V. This upper limit is due to CMOS switch insertion losses that increase exponentially with increasing bias

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voltage (Analog Devices [46,48]). In addition, as the signal frequency decreases below 50MHz, the ADG918 1dB compression point decreases dramatically from around 16dBm to 12dBm. This is due to two factors: The first is that the parasitic diodes in the NMOS transistors becomes forward biased. They have longer to turn on for lower frequency signals and therefore cause greater signal attenuation. The second reason is that at lower frequencies, the shunt transistors in the off state partially turn on thereby passing some of the signal power to ground. The issues are discussed in the Analog Devices references.

A third solution was to reduce the signal amplitude at RF2 by fitting the attenuator components, but this would have led to the undesirable consequence of also reducing FID and spin-echo signals.

The fourth solution was to find an RF switch with better power handling capability. The Philips SA630D and NXP SA58643 (NXP [119]) SPDT switches were alternatives with higher supply voltage and power ratings. Both devices appeared to have identical specifications, differing only in pinout and package. The SA630D/SA58643 supply voltage VDD was +5V compared to +2.5V for the

ADG918, and the absolute maximum power rating for the three switch inputs and outputs was 20dBm (6.3Vp-p) compared to 18dBm (5.1Vp-p). The 1dB compression

point near Mole frequencies is 18dBm for the SA58643 compared to 12dBm for the ADG918. The older SA630D was available in both through-hole and SO8 surface mount packages; the newer SA58643 was available in the tiny TSSOP8, which was the same component footprint as the ADG918. It was decided to use the NXP device rather than the Philips device as it was cheaper, and the PCB footprint and pinout were closer to the existing ADG918.

Testing the NXP device using a function generator revealed similar behavior to the ADG918. Reducing the off-channel input voltage below −500mV affected the on-channel signal, but no side effects were observed when increasing the offset to +7V. Unfortunately there was no information regarding DC biasing in the NXP data sheet, and no application note. A DC bias level of around +2.5V was therefore applied as shown in Fig.3.11, and used successfully for the remainder of this project. The 100µH inductance is capable of passing 10mA and presents an impedance of around 2kΩ at Mole frequencies, thus isolating the DC bias capacitors from RF signals.

The NXP device was carefully soldered above the circuit board as shown in Fig. 3.12 because of slightly different pinouts. The bias circuit consists of two leaded resistors, two capacitors, and one inductor as shown. The LP2992 voltage regulator was changed from a +2.5V to a +5V version to power the new RF switch.

The bias circuit worked well but needs refining in future revisions. If the SA58643 switch behaves like the ADG918 the bias level should be reduced to reduce signal compression. The bias components should be surface mounted to

3.11. RF switch biasing 89

minimize parasitic inductance and capacitance. The 100µH inductor (Radio Spares 182-6695) has a specified self resonant frequency of 12MHz, which must be kept distant from the operating frequency. RF switch inputs should be also be diode clamped for input protection as transient voltages at the pre-amp input may appear at the pre-amp output.

The PAD relay version did not experience the problems described above for the PAD standard version because signals arrive at either RF1 via the directional coupler or at RF2 via the pre-amp. The arrangement of relay contacts does not allow both signals to be present at the RF switch simultaneously, thus the off- channel voltage window can never be exceeded. The problems also did not occur in the PAD standard version in normal mode. This is because the off-channel RF switch port receives only reflected power signals, and these are insignificant under normal matched conditions, and temporally separated from the FID or spin-echo signals.

Figure 3.12: Pre-amp duplexer module version 4.0 showing replacement RF switch and

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