DUPLEXER AND FILTER TERMINOLOGY
Decibel (dB):
A decibel is a logarithmic scaling value that is used in most RF engineering work because of its universal acceptance and simple manipulation in calculating signal and power levels. A decibel is a relative number, not an absolute value.
For example, a +20 dB difference in a power level is the same as saying the level change is 100 times the starting value. If the change was -20 dB, the change would be 1/100th the original level.
Decibel/one milliwatt (dBm):
This signifies an absolute (real) value, with 0 dBm being one milliwatt of power.
TX RX Systems provides a dB chart in their engineering catalogs.
Selectivity:
Selectivity is a measurement of the ability of the filter to pass or reject specific frequencies relative to the center frequency of the filter. Selectivity is usually stated as the loss through a filter that occurs at some specified difference from the center frequency of the filter.
For example; “- 3 dB bandwidth is +/- 250 KHz.” means the output level of a signal frequency at + or - 250 KHz from the center (tuned) frequency of the filter will be at least 3 dB less than the level of the same signal IF it was at the center frequency.
The greater the selectivity the greater the attenuation of frequencies other than the center frequency.
The greater the selectivity the narrower the lowest loss ‘window” of the filter and the need for good temperature and mechanical tuning stability in the filter design.
The larger the diameter of a cavity filter the greater the selectivity, assuming similar materials and construction of the filters being compared.
Tuning Stability:
Tuning stability is the ability of the filter to remain at tuned at the desired frequency over time and variations in temperature, orientation and vibration. Many aspects of filters are designed to overcome these variables, such as the use of temperature compensating metals, elimination of threaded tuning rods which can store mechanical torque stresses, added cooling, fine tuning adjustments, etc.
Insertion Loss:
Insertion loss is the minimum amount of loss to the signal passing through a filter at a designated frequency. For example, a filter may have 1 dB insertion loss at its center frequency and if two filters are used in series in a duplexer, the duplexers insertion loss would be 2 dB.
Insertion losses occur in both the transmit and receive paths of a duplexer and they may be different amounts.
The greater the insertion loss, the less the output level.
Higher insertion losses generally increase the selectivity of cavity filters. (i.e. A filter bandpass might be +/- 200 KHz at 1.5 dB insertion loss and +/- 100 KHz at 2 dB insertion loss.
The greater the insertion loss, the greater the power dissipation and temperature rise of the filters. High insertion losses may reduce the power capacity of a filter.
Receiver Desensitization:
Receiver desensitization, commonly called ‘receiver desense’, is caused when high RF signal levels enter a receivers antenna input.
When desense occurs, the usual symptom is as though the desired signal was reduced; the signal becomes noisy or even fades out completely.
The frequency of the desensitizing signal can be considerably different than the frequency the receiver is tuned to. The interfering signal can be wideband noise and/or spurious emissions from the associated transmitter or other nearby transmitters.
The susceptibility of a specific receiver to off-frequency signals is dependent upon the receiver design and any external filtering added to the receiver.
Transmitter Noise:
Every transmitter emits signals other than those on the desired frequency. The frequencies and amplitudes of these undesired signals varies greatly and is dependent mainly upon the transmitter design and the modulation used. The amount of transmitter noise can be reduced by external filters and/or physical isolation between the transmitter and any receivers.
TYPES OF DUPLEXERS
There are many ways to combine filters to perform duplexer operations. The more common approaches are:
Bandpass Duplexers:
Bandpass duplexers use several filters to reduce the bandwidths of the transmitter output and the receiver input frequency bands.
Cavities 1, 2 and 3 tuned to pass 458 MHz. 80 dB loss at 453 MHz.
Cavities 4, 5 and 6 tuned to pass 453 MHz. 80 dB loss at 458 MHz.
Cable lengths between cavity 3 and cavity 6 to "T" are tuned lengths.
The amount of isolation between the transmitter and receiver may be reduced or increased by changing the number of cavities and the size (efficiency) of the cavities. Note that since the transmitter output passes through bandpass filters, therefore transmitter noise and spurious emissions are also attenuated in a bandpass type duplexer, which can help reduce interference to other receivers at the same site.
Bandpass type duplexers are best suited for moderate to wide transmit/receive frequency separations. Close spaced frequencies may require additional notch filters and/or separate antennas.
Notch Type Duplexers:
Notch type duplexers may appear physically similar to bandpass duplexers but their operation and tuning is very different.
There are two types of notch filters that may be used in a notch type duplexer:
- The series notch filter, which has two ports (in and out).
- The shunt (or common) notch filter which has one port and is linked to the other filter sections by a “T” connector. NOTE: Do not confuse this with the TX RX Systems “T-Pass” filter which is a specialized bandpass filter.
Cavities 1, 2 and 3 are tuned to notch out 453 MHz.
Less than 1 dB loss near and at 458 MHz.
Cavities 4, 5 and 6 are tuned to notch out 458 MHz.
Less than 1 dB loss near and at 453 MHz.
Cable lengths between cavity 3 and cavity 6 to "T" are tuned lengths.
Note that since the transmitter output passes directly to the antenna, therefore transmitter carrier and noise is only attenuated near the 453 notch frequency.
This offers minimal reduction of interference to other receivers at the same site.
Notch type duplexers are cost effective and operate at much closer transmit/ receive frequency separations than bandpass type duplexers.
Shared sites may require additional bandpass filters and/or separate antennas.
Bandpass/Band Reject (BP/BR) type duplexers:
These types of duplexers are combinations of the two preceding duplexer types, having many of the benefits of both and usually at some increase in cost. An bandpass/band reject example; (Actual combinations vary widely)
Cavity 1 and 3 tuned to pass 458 MHz
Cavity 2 tuned to notch (reject) 453 MHz.
Cavity 4 and 6 tuned to pass 453 MHz.
Cavity 5 tuned to notch (reject) 458 MHz.
Cable lengths between cavity 3 and cavity 6 to "T" are tuned lengths.
The TX RX Vari-Notch (c) type Duplexer:
The Vari-Notch type duplexer is a very popular, low cost and small size duplexer that is only available from TX RX Systems. It is very similar to a bandpass/ band reject type duplexer in operation and tuning.
The major difference is the use of TX RX Systems exclusive “Vari-Notch” (c) filter designs which incorporate the equivalents of a broad bandpass filter and a notch filter within the same cavity.
The result is the elimination of separate bandpass sections in most duplexer requirements and an inherent increase in the number of notch filters for a given number of cavities. It is important to remember that there can be interaction between the bandpass and notch frequency tuning of any combination duplexer, especially when the duplex frequencies are close spaced.
Additional details and technical notes about Duplexers, Filters and other
TX RX Systems products are available at www.TXRX.com.
Last Update: September 11, 2005
(c) 1996 - 2005 Jack Daniel Co.
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