Temperature Variable Attenuator for Millimeter-wave Applications
EMC Technology Inc.
Cherry Hill, NJ
The Thermopad® temperature variable attenuator (TVA) has gained widespread popularity among RF and microwave circuit designers as a simple solution to compensate gain variation over temperature. The TVA is a special case of the absorptive attenuator wherein the attenuation characteristic varies in a prescribed manner in response to the device temperature, all the while maintaining substantially constant characteristic impedance. TVAs find application in gain stages, mixers, local oscillators and other signal processing components to stabilize gain over temperature. System applications include anything from satellites to broadcast transmitters to cell towers. The TVA can replace closed- or open-loop temperature compensation schemes with a single, passive device requiring no external sensor, bias or control. Since it is a passive, low power, resistive device, the TVA produces no distortion products and can increase system reliability and reduce total system cost. EMC introduced the temperature variable attenuator in 1992 (US Patent No. 5,332,981) with the TVA product line (DC to 6 GHz), and has since expanded the product offerings to include the CTVA (75 W), the MTVA (DC to 12.4 or 18 GHz) and HTVA (DC to 20 GHz). The new KTVA further expands the Thermopad product line with wideband performance in the 18 to 42 GHz frequency bands.
As with all lumped-parameter designs, the effects of parasitic components limit high frequency performance. These parasitics are typically dealt with by modifying the geometry of the device or by tuning the device, internally or externally, to achieve the required return loss performance at the frequency of interest. At frequencies above 14 GHz, these parasitic components become severe liabilities, almost impossible to tune out and difficult to minimize due to physical limits of size and geometry. The TVA, MTVA and HTVA Thermopad products all share the same basic circuit topology, that being the classic T or Pi attenuator configuration. The evolution of these products resulted in overall size reduction from the TVA at 0.145" x 0.122" x 0.020", to the 0.075" x 0.060" x 0.012" MTVA, to the HTVA at 0.055" sq x 0.016", as well as not so obvious changes in the internal structure.
Fig. 1 The KTVA schematic.
Each TVA is identified by its 25°C attenuation value and its temperature coefficient of attenuation (TCA). The TCA is the change in attenuation with respect to temperature normalized to the 25°C attenuation value. For example, part number TVA0300N04 is a 3 dB25°C TVA with a negative shifting attenuation characteristic of 0.004 dB/dB/°C. This part decreases its 25°C attenuation value by 0.12 dB per 10°C increase in temperature. When working in the 14 to 20 GHz range, the MTVA and HTVA parts may exhibit a roll off of TCA with frequency. This is usually not a setback since a higher TCA part may be used when working in this band. For operation beyond 20 GHz, this effect will severely limit performance.
To overcome this effect, a different design was used for the KTVA. Instead of fighting the problems of geometry and parasitics, a distributed parameter approach was devised. The KTVA is designed using coupled microstrip lines on an Al2 O3 substrate, as shown in Figure 1 . Thermistors are deposited in the same microstrip configuration to couple the line sections. These thermistors, shown as R1 and R2, act as lossy lines and vary the coupling between the adjacent lines as their resistance shifts with temperature. Figure 2 shows the KTVA's outline configuration.
Fig. 2 The KTVA outline.
The result is a distributed parameter design that yields a temperature variable attenuator suitable for use above 20 GHz. Devices designed in this manner exhibit stable TCA performance over a minimum 20 percent bandwidth at center frequencies from 18 through 42 GHz. Wider bandwidth operation is also possible. Figure 3 shows the S21 performance of a KTVA0400P06 TVA (4 dB25°C and 0.006 TCA) at 30 GHz center frequency.
Fig. 3 S21 performance of a KTVA0400P06 TVA.
It can be seen from the data that there is less than 0.25 dB variation from 28 to 32 GHz for any given temperature. Total attenuation shift is approximately 2 dB from 0° to 80°C, or approximately 0.25 dB per 10°C. This gives a TCA of 0.006. Figure 4 shows TCA stability over frequency. Although not shown, return loss is better than 20 dB across the band. The KTVA has wire-bondable gold input and output terminations and a brazable gold back metallization that is suitable for eutectic bonding.
Fig. 4 TCA vs. frequency.
As with all Thermopad products, the Thermopad Selection Tool can be used to determine the proper attenuation and TCA to compensate the desired circuit or device. The tool is available on CD or on the company's Web site at www.emct.com, and is shown in Figure 5 .
Fig. 5 The Thermopad Selection Tool.
The KTVA product line of the Thermopad family provides stable temperature compensation in designs above 20 GHz and should find particular use in K- and Ka-band satellite applications. This product, as well as the other Thermopad products, is protected under US Patent No. 5,332,981.
1. J.B. Mazzochette, "Passive Temperature Compensation for RF Amplifier Gain," Proceedings of the 1996 Microwave Theory & Techniques Symposium .
2. J.B. Mazzochette and J.R. Steponick, "Temperature Variable Attenuator," US Patent No. 5,332,981, July 1994.
3. M. Marie, J.B. Mazzochette, A.H. Feingold, P. Amstutz, R.L. Wahlers, C. Huang and S.J. Stein, "Thick Film Temperature Variable Attenuators," Proceedings of the 1997 IMAPS Philadelphia Symposium .
EMC Technology Inc., Cherry Hill, NJ (856) 429-7800.
Circle No. 300