The 56 signers of the Declaration of Independence declared on July 4, 1776 that “… all men are created equal …” It's too bad that the same cannot be said of termination resistors for RF and microwave transmission lines. The same resistors or networks that perform termination functions adequately at lower frequencies can often exhibit poor performance at higher frequencies due to unwanted capacitance and inductance present in the various package designs. Measuring how far from “ideal” the resistor performance these devices exhibit will predict how they will behave in a high-frequency design. If there are sufficient changes in the impedance of line terminators at RF and microwave frequencies, the resulting mismatch can cause signal reflections on the transmission lines.

So, what types of resistors make good line terminators? Which types of resistors make poor line terminators? To answer these questions, the high-frequency characteristics of an axial leaded discrete resistor, a 0603-size chip resistor, a QSOP resistor array and a BGA resistor array were examined. These packages were selected based on their popularity. The models for each package type were generated from measured return loss data taken from an HP 8753 network analyzer capable of 30 kHz to 6 GHz operation and configured with S-parameter test ports.

Axial leaded resistors typically have the construction shown in Figure 1(a). The device is constructed with wire leads that are soldered, welded or brazed to an end termination attached to a resistor element that is typically laser trimmed in a spiral fashion and then encapsulated in epoxy or plastic.

At low frequencies the device acts like an ideal resistor but at high frequencies the inductance of the leads and laser trim pattern, as well as the capacitance between the end terminations, comes into play. Figure 1(a) shows the high-frequency parasitics of a typical axial leaded resistor. At high frequencies, the unwanted reactive elements cause the impedance of the device to change from the low-frequency nominal value to a different value, resulting in an impedance mismatch with the transmission line.

The next logical question is: How bad will the mismatch be at the specified design frequency? For a typical axial leaded resistor, the lead inductance is about 1.5 nH for each lead, the inductance of the resistor element is about 5 nH and the shunt capacitance across the resistor element is about 0.35 pF, as shown in Figure 1(b). These unwanted reactive elements result in the return loss magnitude chart shown in Figure 5. Recalling that a return loss of -10 dB equates to about a 30% signal reflection, this terminator is only suitable for designs with significant spectral content below 500 MHz; and it becomes almost purely reactive above 2 GHz.

QSOP arrays typically have the construction shown in Figure 2(a). The resistor element is deposited and patterned on the surface of a silicon die with a layer of glass separating the resistor element from the silicon semiconductor material. The silicon die is mounted to a lead frame paddle and connected electrically to the external leads with a 1 mil diameter gold wire. The assembly is then encapsulated in a molded epoxy jacket.

This construction creates a distributed capacitance across the resistor element with the resistor forming the first plate of the capacitor, the glass forming the dielectric of the capacitor and the bulk silicon forming the second plate of the capacitor. Another shunt capacitance is formed between the leads and bond pads of the device. Series inductances are present at the leads and the bond wires used to connect the bond pads on the die to the lead frame inside the plastic package. These elements shown in Figure 2(a) result in the high-frequency model shown in Figure 2(b). The leads and wire bond wires contribute about 1.2 nH of series inductance to the device. The contributions of the die shunt and the lead shunt capacitances are about 1.3 pF and 1.2 pF, respectively.

The return loss for a well-designed QSOP array terminator is shown in Figure 5. From this figure, we can conclude that the QSOP package is useful for transmission line termination to about 500 MHz and maybe 1 GHz in a pinch. After that, the parasitic elements dominate. Note also that in this example, parasitic resonance occurs at a frequency slightly above 5 GHz.

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