Equation 1 and Figure 1 show that VSWR can be calculated if the reflection coefficient is known.

where,
Vi = incident wave; Vr = reflected wave; Z0 = characteristic impedance; ZL = load

Figure 2 shows a directional coupler placed between the source and the load to isolate and sample the incident and reflected waves from the load. With high directivity, the ratio of incident wave to reflected wave is equal to the reflection coefficient, as shown in Equation 2. Thus, with the help of a directional coupler and detectors, the reflected and incident waves can be detected and post-processed (performing division of reflected and incident waves) to measure the reflection coefficient.

where,
C = coupling coefficient; D = directivity

Θ and Φ = unknown phase delays through the coupler

Vc = voltage on the coupled port (Port C) of the coupler, a sample of the incident wave

VD = voltage on the reflected port (Port D) of the coupler, a sample of the refelcted wave

Once the incident and reflected signals have been sampled and isolated, the magnitude of these signals needs to be detected, which requires dual detectors. The best detection method is determined by considering measurement accuracy and detection range over temperature.

Accuracy of the detection method will determine the accuracy of the VSWR measurement. The accuracy of the output used to detect the incident and reflected waves is degraded due to the coupling between the two channels, especially when the two channels are operating at different power levels. This implies that isolation is one of the primary criteria in detector selection. This isolation criterion is two-fold: isolation between the two RF channel inputs, and isolation from one RF channel's input to the other RF channel's output. Isolation between the two inputs can be easily measured using a network analyzer, but input-to-output isolation is more important. Input-to-output isolation is measured by increasing the power level on one channel until it starts to affect the power detection accuracy of the other channel (at a much lower power level within its dynamic range) by 1 dB. The difference between the two power levels is the input-to-output isolation. Different valued couplers and attenuators can be used to position the power levels for a minimal difference at the inputs in order to reduce coupling. Coupling on the PC board also affects isolation. Care should be taken in the layout to isolate the RF inputs from each other.

The detection range for the incident signal is equivalent to the transmitter's output power range, but detection of the reverse propogating signals that have been reflected from the interface needs to be greater. The reflected power level can range from a very small signal level (when there is a good impedance match between the PA and the antenna), to a signal level as large as the maximum level of the incident signal (when there is an open or short on the transmission line) requiring detectors with high dynamic range.

The fact that logarithmic subtraction is equivalent to division makes it easy to perform the complicated math of signal division, one important reason for selecting log amps for VSWR detection. For VSWR measurements using log amps, both detectors should be on the same chip to provide good matching for temperature and process variations. Log amps also have greater dynamic range than other detector types. All of this shows that the best detection method to use for VSWR applications is a dual log amp with high dynamic range and good accuracy over temperature.

In addition to the difference outputs it is preferable to have access to the individual log amp outputs since most RF designers use this information to simultaneously determine the output power of the Tx section. The ADL5519 is a good example of a dual log detector that has individual channel outputs and also the difference between the two channels. As shown in Figure 3, the ADL5519 offers a 54 dB dynamic range from low frequency to 8 GHz, with temperature drift less than ±0.5 dB, making it an ideal solution for detecting the incident and reflected waves and simultaneously controlling the output power. The excellent input-to-input and input-to-output channel isolation specs of the ADL5519 (>30 dB), shown in Figure 4 and Figure 5, make this part suitable for dual RF channel systems. The AD8302 can be used where there is no need for individual log outputs.

There are many ways to protect an amplifier from potentially damaging high VSWR. High VSWR conditions are generally catastrophic at high output powers, so the goal of the protection circuit should be to lower the output power, thereby placing the amplifier in a safe operating mode. The VSWR detection methodology is independent of the amplifier's architecture, but the power control scheme of the architecture does influence the choice of the amplifier's protection mechanism.

For the case where the amplifier's power is controlled by an external pin, the output power can be easily reduced when a VSWR event exceeds a pre-determined reference level. The proposed protection scheme has the capability of altering this reference level, extending VSWR protection to several different PA architectures.

Previous 1 2 3 Next

RSS    Save to Del.icio.us  Digg This