Testing repeater sensitivity degradation

How well is your repeater performing under actual operating conditions? Unless you check the receiver sensitivity under real operating conditions, you are not getting the true picture. Antenna noise, duplexer mistuning and other on-site problems can cause the actual sensitivity to be far worse than the on-bench sensitivity figure stated in the service manual specifications. Here is how to determine just how well your repeater is performing under real-life conditions.

The “isotee” In order to perform the necessary tests and measurements, you can make an isotee coupler from a simple “tee” connector. This is probably most easily done with a UHF connector, but it can be done with other connectors as well. An isotee connector provides a high degree of isolation between the through-line portion and the coupled port. It can be used for coupling in either direction. That is, it can be used to couple a signal into a transmission line from a signal source or to couple a sample of the signal from the transmission line into a measuring instrument such as a spectrum analyzer, frequency counter, deviation meter or other device.

The construction of a simple UHF isotee has been described in past columns in MRT, but it will be repeated here for the sake of those who missed those columns. The center pin of the male portion of the UHF tee connector is removed by unscrewing it from the connector. The pin is then cut with a hacksaw. Next, a slot is made into the end of the pin so that a screwdriver can be used to reinsert the pin into the connector. The slot can be cut with the hacksaw blade as well. Once the pin is reinserted into the connector, screw the connector onto a barrel or straight-through connector. This will leave you with a connector with three UHF female ports.

The barrel will be the isolated, or coupled, port. Make sure that no dc continuity exists between the isolated port and the other ports. If dc continuity exists, the pin is too long, and it should be removed and cut again to a length that will prohibit direct contact between the barrel connector and the modified tee connector.

The isolated port of the isotee connector will be isolated by 30dB-40dB from the through-line portion of the connector. You can determine the amount of isolation provided by using the setup shown in Figure 1 below left. Increase the generator signal level to produce 12dB sinad at the receiver audio output. Record the signal generator level in dBm. Now, subtract the normal receiver sensitivity from the generator level required through the isotee. The result is the amount of isolation provided by the isotee connector. For example, if the normal receiver sensitivity is -119dBm, and the signal level required to produce 12dB sinad through the isotee was -85dBm, then the isolation of the isotee is:

-85 – (-119) = -85 + 119 = 34dB

Be aware that the amount of isolation provided is frequency-sensitive. Thus, the isolation test should be conducted at the frequency at which you are going to be testing.

The isotee can operate in the presence of RF power and still provide enough isolation to protect sensitive devices such as signal generators from overload. Check to make sure that the instrument you are using will not be overloaded by excessive RF coming through the isolated port of the isotee.

Suppose that the output of a repeater is 100W (50dBm) and that the isotee provides 35dB of isolation. This means that the RF level getting into the attenuator pad is 50 – 35 = 15dBm. Be sure that the signal generator or pad can handle this power level. If not, you must provide more isolation.

Checking repeater performance There are several things that must be known in order to determine just how well a repeater is performing and how much degradation is caused by an external problem and how much by an internal problem. The external problem would be the site noise coming in through the antenna. The internal problem would be duplexer tuning and transmitter and/or receiver performance.

First, we need to know the receiver’s bench sensitivity figure for 12dB sinad. This is the basic starting point. It is easier to work with dBm than microvolts in this case. Let’s suppose that the receiver has a 12dB sinad sensitivity of -119dBm. This will serve as our basic reference point. Let’s call this reference level #1.

Next, we want to know how much insertion loss the receiver side of the duplexer is causing. Use the setup in Figure 2 on page 8 to determine the amount of insertion loss in the receiver leg of the duplexer. Adjust the signal generator to produce 12dB sinad again at the receiver output. Let’s call this reference level #2. Subtract reference level #2 from reference level #1 to get the duplexer insertion loss. Suppose that reference level #2 is -117dBm. Subtracting this from reference level #1 (-119dBm) yields

-119 – (-117) = -2dB

This is the insertion loss of the duplexer in the receiver leg. Make certain that the transmitter is either not activated during this test or that the transmitter output is directed into a dummy load.

The next piece of information needed is the effective site sensitivity of the receiver. The setup is shown in Figure 3 at the left, where the antenna is replaced with a 50V dummy load. The signal generator is adjusted to produce 12dB sinad at the receiver output and the signal generator level is noted as reference #3. Next, in Figure 4 on page 50, the antenna is connected, and the signal generator is readjusted to produce 12dB sinad at the receiver output. The level is noted as reference level #4, and the difference between reference levels #3 and #4 is the site noise degradation of the receiver sensitivity. (Again, the transmitter is disabled for this test.)

Finally, the transmitter is reconnected or enabled, and with the repeater operating normally, set up as shown in Figure 5 on page 50, the signal generator level is again adjusted to produce 12dB sinad at the receiver output. The level is noted and recorded as reference level #5. The difference between reference levels #5 and #4 is the amount of degradation caused by the transmitter.

If the degradation is several decibels, then the duplexer might need retuning, or there might be a problem with a connection in the antenna line or antenna that is creating excessive noise that is transferred back into the receiver input.

Effective net system sensitivity The bottom line to all of this is the effective “net” system sensitivity–that is, the system sensitivity in the fully operational mode. The effective net system sensitivity takes into account many factors including site noise, transmitter noise, receiver desense, duplexer tuning and antenna/connector noise. Since the transmitter and receiver must operate at the same time (duplex) then the only way to truly test the “system” sensitivity is in the full duplex mode. The table at the right is a sample of the calculations involved.

It is important to note here that the attenuation of the signal generator padding must be taken into account in the measurements. It is also important to note that the cable connecting the isotee and the straight tee connectors should be half-wavelength. If quarter-wavelength cable were to be used, the open circuit at the isotee might be reflected back to the regular tee as a short circuit.

Until next time — stay tuned!