Duplexer Tuning Using the EAGLE Return Loss Bridge
1.0 Introduction
A duplexer allows the simultaneous operation of a transmitter and receiver with a single antenna. The duplexer provides enough attenuation of the transmitter signal, and transmitter parasitics, to prevent overloading or desense of the receiver. A duplexer is divided into a transmit and receive section. The duplexer has three ports they are the Antenna port, TX port and RX port. A properly tuned duplexer meets the following requirements:
- Transmitter to Antenna port-minimum loss at transmitter frequency.
- Transmitter to Antenna port-maximum loss at receiver frequency.
- Antenna to receiver port-minimum at receiver frequency.
- Antenna to receiver port-maximum at transmitter frequency.
- Return Loss better than 15 dB @ respective ports and frequencies.
While duplexers are factory tuned, in most cases, field retuning sometimes necessary. This may be because of aging, or to set up on new frequencies. Duplexer manufacturers often use RF network analyzers to set up duplexers. This type of equipment is not often found in a two-way shop so technicians resort to using signal generators, spectrum analyzers, wattmeters, general coverage receivers and who knows what else.
With the advent of modern communications service monitors, which include a spectrum analyzer/tracking generator, it is now possible to do swept measurements of duplexers with something two-way shops already have. With the addition of the EAGLE return loss bridge reflection measurements can also be made. This makes retuning duplexers quick and easy!
2.0 Equipment Setup
This application note assumes that you have a communications service monitor that is equipped with a spectrum analyzer and tracking generator. In order to accomplish the reflection measurements a return loss bridge is also required.
You will also need three coaxial cables with the proper connector, a coax feed thru and a termination. The termination, when checked with the bridge, should yield 30 dB of return loss or more at the frequencies of interest. All of this equipment is available from EAGLE in our standard bridge kits. It is recommended that the cables be of the double shielded type. Adaptors, if used should also be high quality and in good condition. If possible adaptors should be avoided. EAGLE also makes cables that have different connectors on each end to avoid the use of adaptors.
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2.0 Equipment Setup-continued
There are two types of setups that will be used to measure the duplexers. The first one, illustrated in figure 2.1, will use the tracking generator hooked thru the duplexer to the spectrum analyzer in the service monitor. This one will be used whenever the stop band of the duplexer is being measured.
The second one, illustrated in figure 2.2, will use the return loss bridge and will measure the reflected power of the duplexer. This one will be used when the passband is being tuned and to measure the reflected power (return loss) in the passband.
Should you not be familiar with return loss measurements and the use of The return loss bridge please refer to EAGLE application note: Return Loss Bridge Basics This is located on our website at: www.eagle-1st.com/notes/note_toc.htm This note covers return loss basics. It shows how to hook the bridge up and what the measurements mean.
