1. Technical Field
The present invention relates to a passive intermodulation (PIM) measurement instrument configured to utilize frequency-dependent plugins.
2. Related Art
A PIM is an unwanted signal or signals generated by the non-linear mixing of two or more frequencies in a passive device such as a connector or cable. PIM has surfaced as a problem for cellular telephone technologies such as Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) systems. Cable assemblies connecting a base station to an antenna on a tower using these cellular systems typically have multiple connectors that cause PIMs that can interfere with system operation.
The PIM signals are created when two signals from the same or different systems mix at a PIM point such as a faulty cable connector. If the generated PIM harmonic frequency components fall within the receive band of a base station, it can effectively block a channel and make the base station receiver think that a carrier is present when one is not. PIMs can, thus, occur when two base stations operating at different frequencies, such as an AWS device and a PCS device, are in close proximity.
The PIMs can be reduced or eliminated by replacing faulty cables or connectors. Test systems are thus utilized to detect the PIMs enabling a technician to locate the faulty cable or connector. The test system to measure the PIMs, thus, creates signals at two different frequencies, amplifies them, and provides them through cables connecting a base station to antennas on a tower for the base stations. A return signal carrying the PIMs is filtered to select a desired test frequency harmonic where PIMs can be detected and the PIM measurement is provided to an operator.
PIM testers to date have used CW signals for the two frequencies used to create the PIM. This is due to the unknown nature of where physically the PIM is located in the transmission path. The PIM is monitored by one technician while the other technician climbs the tower and physically moves the connector joints to see if the PIM changes. Other techniques plot a time graph of the PIM so a single technician can correlate his movement up the tower with results on a graph provided on a plotter below the tower.
FIG. 1 shows a block diagram of components of a prior art test system setup for measuring a PIM. The test system utilizes two signal sources 100 and 102 producing CW signals, with a first signal source 100 producing a signal at frequency F1 and the second signal source 102 producing a signal at frequency F2. When these multiple signals are allowed to share the same signal path in a nonlinear transmission medium, the unwanted signals can occur. The combined 3rd order response is particularly troublesome as it produces an unwanted signal at 2F1-F2 that can pass from one system transmitter into another system's receiver.
The signal at frequency F1 is provided from source 100 to a high power amplifier (HPA) 104. The signal at frequency F2 is provided from source 102 to a high power amplifier 106. Both the high power amplifiers 104 and 106 are shown as 50 W amplifiers, and receive a DC power supply input shown ranging from 100 to 125 Watts to produce a 50 Watt signal output.
The output of each of the amplifiers 104 and 106 is provided through respective isolators 108 and 110 to the input of a hybrid combiner 112. The hybrid combiner 112 assures the two carrier signals F1 and F2 are isolated from each other. If they are allowed to combine without isolation, intermodulations would appear due to power output stage nonlinearities. The isolators 108 and 110 are inserted after the power amplifiers 104 and 106 to give additional isolation from any return signal from the hybrid combiner 112. The intermodulations are the same frequency as the PIM (2F1-F2), so isolation using both the hybrid combiner 112 and the isolators 108 and 110 is critical.
The outputs of the hybrid combiner 112 are provided to a commercial duplexer 114. The commercial duplexer 114 includes a transmit filter 116 and a receive filter 118. Signals F1 and F2 are provided to a first terminal via the transmit filter 116, the first terminal is connected to the front panel 120 of the instrument using a Low PIM Cable 122. Although the low PIM cable 122 is designed to minimize PIM, the low PIM cable can introduce additional PIM signals into the system. The PIM signal is provided to a second terminal via the receive filter 118. The PIM signal can be provided to a digital receiver or spectrum analyzer for measurement, such as a tuned receiver 124 which is connected to the second terminal via a FPIM filter 126 and Pre Amp 128.
The power needed to create the PIM is a standardized 20 W per carrier. Overall for the PIM test circuit of FIG. 1, the DC power supplied to the amplifiers 4 and 14 needs to be 50 to 60% higher to create the 20 W output due to DC and RF inefficiencies. This translates to a continuous DC power consumption of 200 to 250 Watts. The loss of power combiner 20 is 3 dB, so 25 W carriers (F1 and F2) can emerge from the combiner while other 25 W carriers that are not needed are dissipated in an internal load 130. The 50 Watt power output of the two amplifiers 104 and 106 is further reduced a total of at least 1 dB above the theoretical 3 dB loss through the hybrid combiner 112 due to the losses through cabling. Further losses in the isolators 108 and 110 and commercial duplexer 114 reduce total power so that 20 Watt carriers F1 and F2 are produced from the output of commercial duplexer. PIMs introduced by the low PIM cable 122 or other sources receiving the signals F1 and F2 will generate a return PIM signal that is provided back through commercial duplexer 114 and directed to the tuned receiver 124 for processing.