Patent Grant
7271682
Related applications are application Ser. No. 10/912,726, filed Aug. 5, 2004, for “Wideband Temperature Variable Attenuator,” and application Ser. No. 11/107,586, filed concurrently herewith, for “Voltage Controlled Attenuator with No Intermodulation Distortion,” both of which disclosures are incorporated by reference herein.
The invention relates to wideband temperature-variable microwave attenuator.
Attenuators are used in applications that require signal level control. Level control can be accomplished by either reflecting a portion of the input signal back to its source or by absorbing some of the signal in the attenuator itself. The latter is often preferred because the mismatch which results from using a reflective attenuator can create problems for other devices in the system such as nonsymmetrical two-port amplifiers. It is for this reason that absorptive attenuators are more popular, particularly in microwave applications. The important parameters of an absorptive attenuator are: attenuation as a function of frequency; return loss; and stability over time and temperature.
It is known that variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures. In many cases, it is impossible or impractical to remove the temperature variations in some Radio Frequency (RF) components. For example, the gain of many RF amplifiers is temperature dependent. In order to build a system with constant gain, a temperature-dependent attenuator is placed in series with the amplifier. The attenuator is designed such that a temperature change that causes the gain of the amplifier to increase will simultaneously cause the attenuation of the attenuator to increase such that the overall gain of the amplifier-attenuator system remains relatively constant. However, prior art temperature-dependent attenuators do not offer the bandwidth needed for certain wideband applications.
Voltage controlled attenuators (VCAs) are a fairly common element of almost any RF or microwave circuit. Their function is to change the amplitude of a signal based on some external signal, usually a voltage or current. A common use is the leveling of a signal so that both strong and weak signals can be adjusted in amplitude to provide a constant level signal to the next stage of the circuit. Another use is the balancing of multiple signal paths so they all have the same gain. A third use would be to use a VCA to control the gain of an amplifier over temperature by varying the control voltage based on a measurement of the ambient temperature. This last use is to counter undesired changes to the gain of the amplifier when the ambient temperature changes.
The vast majority of presently available VCAs include either diodes, transistors, or FETs (field effect transistors). These active devices have non-linear transfer characteristics which result in distortion to RF and microwave input signals. This causes additional and unwanted signals to be generated which are not present in the original signal. These additional signals have the potential of causing interference to other services, like police or fire departments that use the same frequencies as the additional signals.
U.S. Pat. No. 5,332,981, issued to Joseph B. Mazzochette, et al., issued Jul. 26, 1994, entitled “Temperature Variable Attenuator,” which is incorporated herein by reference, describes an attenuator that includes temperature variable resistors (thermistors) in the attenuating path. As shown in
In the temperature variable attenuator of the '981 patent, the temperature coefficient of resistance (TCR) of at least one resistor is different such that the attenuation of the attenuator changes at a controlled rate with changes in temperature while the impedance of the attenuator remains within acceptable levels.
While such prior art temperature-dependent attenuators have enjoyed considerable success in many applications, they do not offer the bandwidth needed for certain wideband applications.
The present invention is a wideband temperature-dependent attenuator. In a preferred embodiment the attenuator is a modified Tee attenuator having first and second resistors connected in series at a first node and third and fourth resistors connected in shunt between the first node and ground. In a physical implementation of the attenuator, the third and fourth resistors are on opposite sides of the first and second resistors. Preferably, each of the four resistors is formed as a thick film resistor.
To provide the desired temperature dependent characteristics, at least one of the resistors and preferably more has a resistance that varies with temperature. The temperature coefficients of resistance (TCR) are selected such that the attenuator changes attenuation at a desired rate with changes in temperature while the impedance of the attenuator remains within acceptable levels over the operating temperature and frequency ranges of interest. In some cases acceptable levels are such that the voltage standing wave ratio is less than 2.0 to 1.
In one embodiment, the temperature-dependent attentuator has a negative TCR, and in another embodiment it has a positive TCR. One or more of the resistors may have negative TCRs, and one or more of the resistors may have positive TCRs. At least one resistor should have a TCR that differs from the TCRs of the other resistors.
Advantageously, numerous attenuators are made simultaneously by printing thick-film resistors on an insulating substrate such as alumina and firing them. To maximize the number of attenuators that can be formed on the substrate, the attenuators are aligned in a rectangular array. Starting with a bare substrate, in a preferred embodiment via holes are drilled at the 6 o'clock and 12 o'clock positions for each attenuator and are then filled with a conductive ink. Next, one surface of the substrate is printed with a metallization layer. The opposite surface is then printed with five contact areas at the location of each attenuator. One of these contact areas is in the center of the attenuator and the other four are at the 3 o'clock, 6 o'clock, 9 o'clock and 12 o'clock positions. Thick film resistors having a positive TCR are then printed followed by printing of thick film resistors having a negative TCR. Each attenuator is then tested to determine the resistance of its resistors and the resistors are laser trimmed to meet the resistance specifications for the attenuator. Finally, a protective coating is applied to the upper surface of the attenuators, the substrate is scribed, and the individual attenuators are separated from the substrate.
These and other objects, features and advantages of the invention will be more readily apparent from the following detailed description in which:
For a nominal 3 dB attenuation, each series resistor 210, 220 has a resistance of 8.55 ohms at 25° C.; and each shunt resistor 260,270 has a resistance of 283.8 ohms at 25° C. In a preferred embodiment, each of the four resistors has a non-zero temperature coefficient of resistance (TCR) and in a preferred embodiment the series resistors have a positive TCR and the shunt resistors have a negative TCR.
Other arrangements may also be used. In general, at least of the resistors must have a TCR that is different from that of the other resistors in order to meet the impedance requirements for the attenuator. However, the impedance requirements can be met in some attenuators in which at least one of the resistors has a TCR of zero. As will be appreciated, the impedance that is observed over the operating frequency range and/or temperature range of the attenuator will not be precisely constant and the variation in impedance will depend on the amount of attenuator provided by the attenuator. At low attenuation, deviation from the desired impedance may be within +/−a few percent of the desired impedance over the operating range. At higher attenuations, deviation from the desired impedance can be expected to be higher, for example +/−10%, +/−20% and even +/−50% or more in some cases. In practice, considerable variation in impedance may be tolerated depending on the specific application in which the attenuator is used and the temperature and frequency range of use. As a rule of thumb, the variation in impedance of the attenuator should be such that the Voltage Standing Wave Ratio (VSWR) of the RF power is no more than 2.0:1 over the operating range of the attenuator.
In one embodiment, the resistors 310, 320, 360, 370 are thick film resistors produced by inks combining a metal powder, such as, bismuth ruthenate, with glass frit and a solvent vehicle. This solution is printed on the substrate and then fired at an appropriate temperature in the range of 600° C. to 900° C. When the resistor is fired, the glass frit melts and the metal particles in the powder adhere to the substrate, and to each other. This type of a resistor system can provide various ranges of material resistivities and temperature characteristics can be blended together to produce many different combinations.
The resistive characteristics of a thick film ink are specified in ohms-per-square (Ω/□). A particular resistor value can be achieved by either changing the geometry of the resistor or by blending inks with different resistivity. The resistance can be fine-tuned by varying the fired thickness of the resistor. This can be accomplished by changing the deposition thickness and/or the firing profile. Similar techniques can be used to change the temperature characteristics of the ink.
The temperature coefficient of the resistive ink defines how the resistive properties of the ink change with temperature. The Temperature Coefficient of Resistance (TCR) is often expressed in parts per million per degree Centigrade (PPM/C). The TCR can be used to calculate directly the amount of shift that can be expected from a resistor over a given temperature range. Once the desired TCR for a particular application is determined, it can be achieved by blending appropriate amounts of different inks. As with blending for sheet resistance, a TCR can be formed by blending two inks with TCR's above and below the desired TCR. One additional feature of TCR blending is that positive and negative TCR inks can be combined to produce large changes in the TCR of the resulting material.
Some thermistors exhibit a resistance hysteresis as a function of temperature. If the temperature of the resistor is taken beyond the crossover point at either end of the hysteresis loop, the resistor will retain a memory of this condition. As the temperature is reversed, the resistance will not change in the same manner observed prior to reaching the crossover point. In one embodiment, to avoid this problem, the inks used in producing a temperature variable attenuator are selected with crossover points that are beyond the typical operating range of −55° C. to 125° C.
As shown in
Advantageously, numerous attenuators are made simultaneously by printing the contacts, center pads and resistors on an insulating substrate in a process depicted in
Each attenuator is then tested at step 560 to determine the resistance of its resistors and the resistors are laser trimmed at step 570 to meet the resistance specifications for the attenuator. Finally, a protective coating is applied to the upper surface of the attenuator at step 580 and the substrate is scribed and individual attenuators are separated from the substrate at step 590.
The attenuators are then ready to be mounted in a circuit. At the time of mounting, the metal layers on the bottom surface of the substrate are soldered to a ground place on a circuit board thereby connecting the ground contacts 360, 370 to ground and the RF contacts are connected by wire bonding to transmission lines.
As shown in
The percent invention may be implemented in a variety of forms without departing from the spirit and scope of the invention. For example, thin film resistors can be used in place of thick-film resistors. And the film resistors can be printed on low temperature co-fired ceramic substrates instead of the ceramic substrate described above.
| Number | Date | Country |
|---|---|---|
| 2000196395 | Jul 2000 | JP |
