Fine-tuning for the RF isolator
Last month, the operation of RF isolators was discussed in detail. This article focuses on the proper tuning of the RF isolator. Several techniques can be used, depending upon the type of test equipment available. Preferably, the isolator should be tuned in its final installed position. The objective is to tune the isolator so it presents the minimum insertion loss in the forward direction and maximum insertion loss (isolation) in the reverse direction.
Figure 1 shows a block diagram of a simple single isolator. The arrow indicates forward flow of RF power. Any reverse or reflected power flow will be dumped into the RF dummy load at port 3. Any reflected power at port 3 will be transferred back to port 1, the input port. It is important that the trimmer at the input port (port 1) is tuned for minimum reflected power at port 1 so that the transmitter “sees” very little reflected power. There will always be some residual reflected power at the input port, even when the load connected to the output port (port 2) is a perfect 50-ohm load.
One method of tuning the input port is to use a return loss bridge (RLB). The test setup for using this method to tune the input port is shown in Figure 2. It is important to terminate the output of the isolator with a 50-ohm dummy load. This ensures that the reflected signal at the reflected port of the RLB is caused predominately by the mismatch at port 1 of the isolator and not by any reflected signal from the output port of the isolator.
A simple method of tuning a single isolator is shown in Figure 3. A through-line wattmeter is connected between the output port of the isolator and a 50-ohm dummy load. Using the setup in Figure 3A, the input and output ports (1 and 2) are tuned for maximum power on the wattmeter. Using the setup in 3B, the load port (3) is tuned for minimum power on the wattmeter. Using the setup at B, it is important that the transmitter power not exceed the power rating of the dummy load connected to port 3 of the isolator. Also, when using the setup in 3B, it may be necessary to use a low-power element in the wattmeter to tune for minimum power. Actually, when using this method, tuning the isolator to the deepest null may be difficult, at best. Suppose that the isolator has an isolation of 30 decibels. If the transmitter output power is 100 watts, then the power appearing at port 2 will only be 0.1 watts or 100 milliwatts. Obviously, the wattmeter element would have to be very sensitive in order to get a sufficient reading to properly tune port 3 for the deepest null.
A better way to tune port 3 for maximum isolation is shown in Figure 4. The logarithmic display of the spectrum analyzer/tracking generator allows much greater range for tuning the isolator for maximum isolation.
The situation gets more interesting when a dual isolator is tuned (Figure 5). The forward response of the isolator is tuned by adjusting the trimmers at ports 1, 2, 3 and 4. The peak of the response should be at the operating frequency, and the curve should be symmetrical. The forward response of the isolator is very broadband. Check the insertion loss at the operating frequency against the manufacturer’s specifications.
The reverse response or isolation tuning is done using the setup shown in Figure 6. First, load 1 is removed, and the trimmer at port 6 is adjusted for maximum isolation (deepest null). Then, load 2 is removed and the trimmer at port 5 is adjusted for maximum isolation. Never adjust the trimmer at the port where the load is removed. Rather, always adjust the trimmer with the load connected to that port. With both loads connected, trimmers at ports 5 and 6 may be tweaked for maximum isolation.
If you don’t have a tracking generator/spectrum analyzer combo, you can use a signal generator with a spectrum analyzer. You won’t be able to get a swept response but you will be able to tune for minimum insertion loss in the forward direction and maximum isolation in the reverse direction. If you don’t have a spectrum analyzer, you can use a receiver tuned to the correct operating frequency. Even a scanner can be used.
To tune for minimum insertion loss in the forward direction, an AC voltmeter is connected across the speaker terminals of the FM receiver (Figure 7). Then, the signal generator is set to produce a measurable amount of noise across the speaker terminals. All the input/output ports are tuned for minimum noise level on the voltmeter. As these ports are tuned, it may be necessary to reduce the signal generator level to maintain a useable noise level at the receiver output. To get a reference benchmark, the signal generator is connected directly to the FM receiver and adjusted to produce a reference noise level on the AC voltmeter. The reference noise level as well as the signal generator level should be recorded. The insertion loss of the isolator can be determined by comparing the signal generator level required to produce the reference noise level with the recorded benchmark.
To tune for maximum isolation, the connections to the isolator are reversed. Then, the signal generator is set to produce a measurable noise level on the AC voltmeter. The load ports are then tuned, as previously described, for maximum noise level on the AC voltmeter. As the load ports are tuned, it may be necessary to increase the signal generator level to reduce the noise level at the receiver output. The isolation can be determined by comparing the signal generator level required to produce the reference noise level with the recorded benchmark.
The exact method you use to tune the isolator will be determined by the type of test equipment you have available. It is best to tune the isolator in the final installed position. Some isolators might be detuned by installing them on a ferrous metal surface. Check the manufacturer’s instructions to avoid improper installation. Minimum insertion loss in the forward direction and maximum isolation in the reverse direction is the goal. An occasional check of these performance specifications is recommended.
Until next time — stay tuned!
