Numerous articles and application notes have been written on a PI configured positive intrinsic negative (PIN) diode VCA. This type of VCA is used in a wide variety of RF systems. One typical application is in outdoor radio transceivers to adjust transmit or receive levels. Outdoor radios are typically subjected to tough environmental conditions and wide temperature ranges.

One major limitation of the basic VCA is the stability of the attenuation over temperature at a fixed control voltage, predominantly at high attenuation. Modern communication systems are controlled by a microprocessor that generates the VCA control voltage using D to A. Variation of attenuation over temperature reduces the system ability to fix the transmit or receive level.

This article discusses a modified version of the VCA with better attenuation stability over temperature.

Figure 1 is the original circuit from Lucent Technologies Inc.'s (www.lucent.com) RF engineer Louis Fan Fei's article, “A Low Cost, Compact, Pi-Configured PIN Diode VCA,” in the November 2000 issue of Applied Microwave & Wireless¹. The circuit is also shown in Agilent Technologies Inc. (www.agilent.com) “A Low-Cost Surface Mount PIN Diode π Attenuator,” February 2000 Application Note 1048². The circuit operates between 2 GHz and 3 GHz with measurements taken at 2.5 GHz.

The VCA attenuation and return loss are set by the PIN diodes impedance. The diodes impedance is almost pure resistive and is a function of the current flowing through them. Temperature variations affect the attenuation due to two reasons:

• Variation in diode impedance over temperature³; and
• Variation in the diodes current due to diode voltage drop changes over temperature.

High attenuation is obtained at low Vadj. The current through the pass diodes (connecting to L3) is low, and the current through the shunt diodes (connecting to C2 and C3) is high. At low Vadj the currents are more sensitive to the diodes voltage drop changes.

A measurement of the HSMP3814 diode impedance, as a function of current and temperature, was performed at 100 MHz. A single diode was fed by a 9.3 K resistor, and the impedance was measured with a network analyzer. The results are presented in Table 1.

Table 2 lists the circuit attenuation and return loss (dB) as a function of voltage and temperature.

The variation of attenuation over temperature increases as the attenuation reaches 20 dB and above. The variation of the PIN diodes impedance over temperature has lower affect on the attenuation variations and a noticeable affect on the return loss. This is because the series and shunt diodes impedance is changing in the same direction with temperature.

One way to improve the stability over temperature is to use higher value of resistors and higher control voltage. This, however, has the disadvantages of the need for high voltage power supply, such as 15 V.

Outdoor radios and similar system are designed with low internal supply voltage (5 V) to save power. A better solution is the use of current sources.

The circuit in Figure 2 is a modified version of the original. The control voltage was separated between the shunt and series diodes. High value resistors and voltage are used to approximate current sources. The tabulated series and shunt current are empirically found to achieve best return loss at each attenuation settings.

The currents in Table 3 are used to synthesize two current sources approximating the current relationship between the shunt and series diodes.

The circuit is using a single low voltage supply of 5.0 V. The attenuation control voltage Vadj spans 0.0 V to 5.0 V. The series current source provides 0.0 mA to 20.0 mA as the emitter voltage of Q2 spans 2.5 V to 5.0 V.

Maximum current at minimum insertion loss is at Vadj = 0.0. The shunt current source spans 0.0 mA to 2.0 mA with minimum current at Vadj = 0.0.

The current sources were synthesized using SPICE simulation (evaluation version of MC7).

Table 4 lists the attenuation and the return loss versus voltage over temperature. The attenuation is stable over temperature even at high attenuation.

Some applications require higher range of attenuation control. Cascading two of the above VCAs is the simple approach, but it doubles the part count, space and current requirement.

The circuit in Figure 7 provides approximately double the attenuation range with the addition of a dual diode package. The stability over temperature is somewhat compromised due to the use of four series diodes and two shunt diodes and the addition of transistor break point.

The transistor Q3 with R9, R12, R10, and R11 are added to provide a breakpoint and reduces the non-linearity in the control voltage versus attenuation curve.

This article presents a simple circuit modification that improves the performance of a standard PI VCA circuit.

The use of the current sources enables operation with a single low voltage power supply and good attenuation stability over temperature. The attenuation range can be doubled with the addition of a dual diode package. Linearization of the control voltage versus attenuation curve can be done by adding a transistorized breakpoint.

1. Louis Fan Fei, “A Low Cost, Compact, Pi-Configured PIN Diode VCA,” Applied Microwave & Wireless, 2001.

2. “A Low-Cost Surface Mount PIN Diode π Attenuator,” Application Note 1048, Agilent Technologies Inc., February 2000.

3. Robert Caverly and Gerald Hiller, “The Temperature Dependence of PIN diode Attenuators,” IEEE MTT-S Digest, 1993, pp. 553-556.

Moti Kabelly is an RF design leader for Wiseband Communications Ltd. (www.wiseband.com) where he develops the RF processing for feed-forward and digital pre-distortion MPAs. Moti holds an M.S.E.E. degree from Santa Clara University and has over 20 years of experience in RF and radio development. He can be reached at moti_k@wiseband.com.