RF samplers and directional couplers
To performance-test radio communications equipment, many small RF test devices are necessary. I carry a little toolbox reserved for small RF test devices that I often need in performing test and measurement procedures. RF sampling and coupling devices are some of those small tools that are needed in certain test procedures.
Perhaps the simplest signal-sampling device is the homemade UHF tee connector that has been modified to form an isolated tee, often called the isotee. Figure 1 above shows how an isotee is used to sample a small portion of the transmitter RF output signal to feed the input of an RF measurement instrument. In this arrangement the isotee is a signal sampler.
The isotee is made from a regular tee connector such as a UHF tee connector. Remove the center pin of the male part of the connector and cut it short. Cut a small slot into the sawed-off end of the pin so that a screwdriver can be used to reinsert the pin into the tee connector. Connect an RF measuring instrument to the isotee through a barrel connector. It is important that the center pin of the tee connector does not make direct dc contact with the barrel’s center connector. If it does, it will not provide isolation, and it might damage the RF measuring equipment. To be safe, use an ohmmeter to ensure that no direct contact exists between the barrel’s center pin and the center of the tee connector.
Figure 2 at the right shows how the isotee is used to make a signal coupler. This can be used to inject a signal into the line ahead of the receiver input. The setup shown in Figure 2 can be connected as shown in Figure 3 on page 36 to test for noise or interference degradation to the receiver. First, connect the 50V termination to the isotee, and set the signal generator to produce 12dB SINAD at the receiver’s audio output. Then, remove the 50V termination and connect the antenna instead. Again, the signal generator is set to produce 12dB SINAD at the receiver’s audio output. The increase in signal generator level indicates the severity of noise or interference degradation at the site. Similar isotee connectors can be constructed using type N or type BNC connectors.
A directional coupler can have three or four ports. For our purposes, we will limit the discussion to the three-port directional coupler. We will treat the device as a black box and avoid its internal workings. The photo at the right shows a directional coupler that can be used for low-power or small-signal applications.
One of the most important specifications of a directional coupler is directivity. The simplest way to describe this is to show how it is measured. Figure 4 on page 38 shows a spectrum analyzer connected to the coupled (CPL) port. A signal generator is connected to the input port and set to an output level of 0dBm with no modulation (CW). The output port is terminated in a 50V impedance. The difference in the signal level of the input port and the level of the coupled port is equal to the coefficient of coupling. This is stated in decibels. For example, with 0dBm applied to the input port and a signal level of -20dBm measured at the coupled port, the coupling coefficient is 20dB. This will vary somewhat with the frequency of the signal, and it will be stated in the specifications of the directional coupler. The signal level measured at the coupled port is recorded as a reference level for the signal in the forward direction.
Next, reverse the input/output connections and note the signal level at the coupled port with the spectrum analyzer. Suppose that the signal level at the coupled port is now -60dBm. This means that the signal level at the coupled port is 40dB below the reference reading in the forward direction. Therefore the directivity of this directional coupler at this operating frequency is 40dB.
If the 50V termination is removed from the input, the signal level at the coupled port will increase to nearly -20dBm, the same as it was in the forward direction. Such an arrangement could be used as a reflectometer.
The input power rating of the device must be observed to avoid damaging the directional coupler. The mainline insertion loss is the loss from input to output, or vice versa. It should be small. To use the directional coupler as a method of signal injection in place of the isotee discussed previously, use the arrangement shown in Figure 5, above. Because this is the reverse of the normal use of a directional coupler, the input/output connections are reversed. The output port becomes the input port as far as the direction of signal flow is concerned.
When using a directional coupler, pay particular attention to such specifications as input power level, mainline insertion loss, directivity, coupling coefficient and operating frequency.
Special in-line tap-offs are available, as are sampling taps on some in-line wattmeters. Some are adjustable; others are fixed in the coupling level.
Whatever coupling method you use, make sure that you don’t exceed the power input limit to either the measuring instrument or the coupling/sampling device itself. You will “let the smoke out,” and you know what happens then.
Until next time — stay tuned!
Contributing editor Kinley, MRT’s technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. Kinley’s email address is firstname.lastname@example.org .