Voltage variable attenuators (VVAs) provide electronic control over signal levels, ideally with little or no distortion of the signals being controlled. In the case of the new VVA series of attenuators from Mini-Circuits, output waveforms are reduced in level with minimal distortion. The VVAs can provide level reductions as small as a few tenths of a decibel, or as large as typically 30 dB or more. The surface-mount RF VVAs are available in a variety of frequency ranges from 10 MHz to 6 GHz, for control voltages from 0 to 6 V.
Attenuators such as the VVA series can be evaluated in terms of a variety of parameters, including maximum available attenuation, insertion loss, attenuation flatness, and attenuation accuracy. The insertion loss of a VVA is usually measured with the attenuation operating at its lowest attenuation control voltage, and assumes that no attenuation is desired. As a result, it is the residual signal loss through the device's diodes and package circuitry. Attenuation flatness is the peak to peak variation of attenuation through its operating frequency range or for a specified frequency range, while attenuation accuracy is determined by subtracting the minimum attenuation setting from the maximum attenuation setting and dividing the result by two. For an analog-controlled VVA, the transfer function is the relationship between attenuation and control voltage normally specified in dB/volt.
In addition, VVAs can be characterized by a number of other parameters, including the phase shift as a function of attenuation, which is the difference in the transmission phase angle of a test signal at the output for a given frequency and attenuation setting compared to the transmission phase angle at the same frequency and the lowest attenuation setting (the insertion-loss setting of 0 V). The temperature coefficient of attenuation is a measure of how attenuation changes for a given frequency and control voltage setting as the temperature is varied. Typically, compensation networks are added to a VVA to minimize the amount of attenuation drift as a function of temperature. Return loss, which is measured at the input and output ports, compares the amount of direct signal to reflected signal at each port. Finally, a VVA's power-handling capability is a measure of the highest incident power it can handle without destruction or degradation in electrical performance.
The new surface-mount VVAs provide typically better than 30 dB total attenuation across their frequency ranges and control voltages, allowing adjustments to a minimum attenuation setting at a 0-V control voltage, which is essentially the insertion loss of the VVA. These are analog VVAs, with continually adjustable attenuation profiles compared to digitally controlled VVAs, which tune in steps and do not provide continuous attenuation over frequency. In addition to exhibiting flat attenuation as a function of frequency and control voltage, the VVAs are extremely well impedance matched. This allows multiple VVAs to be cascaded for attenuation control of 60 dB or more, without penalties in electrical performance such as attenuation flatness or insertion loss.
The VVAs are provided in two different surface-mount packages (Fig. 1), and are simple to install, requiring no matching networks. The packages are the AH202-1, measuring 0.38 x 0.50 in., for the lower-frequency 10-to-16, 16-to-30, and 150-to-260-MHz VVAs, and the CZ682, measuring 0.375 x 0.375 in. for the two higher-frequency VVAs (see table). The VVAs are ideal for use in automatic-level-control (ALC) circuits, in variable-gain amplifiers, and in power level control applications.
The two lower-frequency models are ideal for intermediate-frequency (IF) adjustments through 30 MHz. Model VVA-13758/1 provides as much as 35 dB typical attenuation from 10 to 16 MHz for control voltages from 0 to 6 V, with typical input return loss of 20 dB and typical output return loss of 22 dB. The insertion loss is typically 1 dB and a maximum of 1.5 dB for a 0-V control voltage. Model VVA-13758/2 offers as much as 32 dB typical attenuation from 16 to 30 MHz for control voltages from 0 to 6 V, with typical input and output return loss of 20 dB. Input and output return loss as a function of frequency and attenuation is better than 20 dB across a better-than 30-dB attenuation control range for the VVA- 13758/2. The insertion loss is typically 0.7 dB and a maximum of 1.2 dB for a control voltage of 0 dB. Both of these lower-frequency VVAs are rated for maximum absolute control current of 10 mA and absolute maximum RF input level of +10 dBm.
The two mid-frequency models are the VVA-13758/6, covering 150 to 260 MHz, and the VVA-13662/2, which operates from 250 to 450 MHz. The VVA-13758/6 provides at least 28 dB attenuation for the maximum attenuation control voltage, and typically as much as 34 dB attenuation, with a control voltage range of 0 to 6 V. The insertion loss is typically only 0.9 dB at the minimum (0 V) control voltage, and no higher than 1.3 dB. The input and output return loss for the VVA- 13758/6 VVA are typically 18 dB. The higher-frequency model VVA-13662/2 delivers at least 27 dB attenuation for the maximum attenuation control voltage, and typically as much as 32 dB attenuation, over a control voltage range of 0 to 6 V. Its measured performance (Fig. 2) reveals consistent attenuation with control voltage across its full frequency range. It exhibits typical input return loss of 22 dB and typical output return loss of 21 dB.
The highest-frequency VVA, model VVA-13662/3, is a true microwave unit operating from 5500 to 7000 MHz and ideal for WiMAX and other wireless applications. It achieves at least 22 dB attenuation at the maximum attenuation control voltage, and typically as much as 27 dB attenuation. The insertion loss reaches a maximum of 2.3 dB at the minimum attenuation control voltage of 0 V, and is typically only 1.6 dB.
Because they are well matched to a 50-Ohm environment, the new VVA series attenuators are ideal for cascaded applications requiring more attenuation than possible from a single unit. When teaming a pair of VVAs, for example, in excess of 70 dB attenuation was possible from 105 to 155 MHz. The VVAs are well suited to both IF and RF/microwave applications requiring repeatable, predictable attenuation as a function of control voltage. Mini-Circuits, P. O. Box 350166, Brooklyn, NY 11235-0003; (718) 934-4500, FAX: (718) 332-4661, Internet: www.minicircuits.com.