DUPLEXERS
An Introductory Tutorial by
The Jack Daniel Company
1-800-NON-TOLL
Copyright 1996 -2005 Jack Daniel Company
WHY ARE DUPLEXERS USED?
Radio receivers can be damaged if high level RF signals, like those directly from a transmitter output, is applied to the receiver antenna.
Additionally, receivers may become ‘desensitized’ (or ‘de-sensed’) and not receive weak signals when high noise levels or another signal near the receive frequency is present at the receivers antenna input.
Obviously, radio receivers and transmitters cannot be directly connected to the same antenna without some device being used to:
(1) switch the antenna between the transmitter and receiver so that they are never connected to the same antenna at the same time.
(2) When the transmit and receive frequencies are different, filters may he used to reduce the transmit signal levels to an acceptable low level at the receivers antenna input. Naturally, you cannot filter out the transmitter signal when it is the same as the receiver frequency.
Definition of a duplexer:
A device which allows a transmitter operating on one frequency and a receiver operating on a different frequency to share one common antenna with a minimum of interaction and degradation of the different RF signals.
Duplex Operation
Duplexers are often the key component that allows two way radios to operate in a full duplex manner. Full duplex means the transmitter and receiver can operate simultaneously as opposed to the ‘push-to-talk’ manner used in non-duplex (or ‘simplex’) operating modes.
Remember, The radio system must use two frequencies per ‘channel’ to use the kinds of duplexers we are discussing.
Recently, some very specialized digital radio systems that are under development are emulating duplex operation by switching the transmitter and receiver off and on extremely rapidly. This is not real full duplex operation but appears similar to the radio users. This discussion does not address this approach, but instead deals with more common accepted land mobile practices.
Duplexers are the devices that allow a mobile telephone to operate like a wired telephone, with either or both people speaking at any time without using a microphone switch to enable the radio transmitters.
REPEATERS
Most radio systems today use repeaters located on top of buildings, towers or on hill tops. These repeaters use two frequencies in a duplex fashion to extend the range of the radio system and make signals much stronger. In most cases, a duplexer is used as part of the repeater station.
The duplexers at repeaters may serve several objectives:
Reduce the number of antennas required due to cost or space limits.
Reduce the transmission line costs or allow a better and more expensive single cable to be used instead of two.
Reduce the potential of intermodulation generated from the transmitter.
Reduce the nearby broadband noise generated from the transmitter.
Improve the receiver ‘front-end’ rejection of off-frequency interference.
WHY NOT USE TWO ANTENNAS?
Two antennas may be used instead of a duplexer, provided the antennas are placed far enough apart that the transmitter signals do not interfere with the receiver. Two transmission lines will also be required.
The isolation required between the transmitter and receiver is a complex issue and influenced greatly by the specific transmitter and frequencies used, the bandwidth of the channel, the difference in frequencies of the two frequencies to be used and the minimum amount of receiver degradation that is acceptable to the user.
It is not unusual to have a radio system require as much as 80 to 100 dB isolation between the transmitter output and the receiver input.
When two antennas are used, the type of antennas, the physical spacing and the orientation of the antennas to one another are also major concerns.
The antenna to antenna isolation can also be influenced by the presence of other antennas on the same tower as well as other nearby transmitters and mechanical structures. These factors may change over time and be out of the control of the repeater operator.
Antenna separation designs should also consider any additional receiver protection that may be required for other transmitters that may be present on the same tower.
In some extreme cases, duplex filters AND antenna separations may both be required to obtain satisfactory operation. This generally only occurs at lower frequencies with small differences between the transmit and receive frequencies or when closely spaced channels are combined.
Typical Antenna Spacing Isolation Values: (In dB) (Based on vertically polarized half-wave dipoles. Actual experience will vary due to local conditions, antenna variations, etc.)
TYPES OF FILTERS USED IN DUPLEXERS
There are several ways to implement a duplexer, but all rely upon the characteristics of different types of RF filters. Specifically:
- Bandpass filters which allow a specific range of frequencies to pass through them. The filters are designed and tuned to a specific ‘center frequency’ and ‘pass band’ with relatively low losses to desired frequencies and higher losses that increase as the deviation from the center frequency increases.
- Reject or Notch filters which operate opposite of a bandpass filter. These are designed to cause high losses at the center frequency and lesser losses as the frequencies increase from the center frequency.
- Specialized filters such as TX RX Systems “Vari-Notch” (c) filter, which has characteristics of both a bandpass and notch filter in one device.
The filters are usually tubular or square cavity type filters but other types of construction such as combline, ceramic, etc. may also be used in some cases.
Cavity type filters offer the best overall balance of performance, simplicity and costs.
Combline and ceramic filters have some space and size advantages at higher frequencies.
Ceramic filters may have power limitations and higher cost.
Although this discussion centers on the more commonly used cavity type filters, the basic principals will apply to any type of filter used in duplexers.
