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IMD/Diplexer

Fri, 11/07/2003 - 6:07am

By James Penny, Filtek Corporation

This article is intended to acquaint and provide system users and subsystem designers insight to the importance of IMD (Intermodulation Distortion) suppression in bi-directional amplifier systems.

Many types of bi-directional amplifiers are in abundant use in cellular systems. In cellular networks, these configurations are commonly termed as repeaters or range extenders.

One of the drawbacks of these systems is the need to balance gain with suitable linear dynamic range. As to be shown later in this article, the reception of the transmitted IMD products at the common antenna normally causes many problems and reduces the efficiency of repeaters.


Figure 1 depicts a standard gain (u) amplifer symbol


Figure 2 depicts the gain amplitude (A) of a perfect amplifier


Figure 3.

Figure 3 illustrates the spectrial response of a perfect amplifier. F1 and f2 are two equal amplitude (two tone) signals closely separated in frequency. The driving amplitude is selected to keep the amplifiers output reasonably linear.


Figure 4

Figure 4 illustrates a standard amplifier output due to non-linearity and is related to an amplifiers defined 1 dB compression point.


Figure 5

Figure 5 shows the response one might expect from a typical amplifier gain block at drive levels well below the 1 dB compression point. A reasonable figure of merit can be fairly stated: when A2 is at least 60 dBc (dB below carrier) below A1.

Similar response may apply to the typical spectral response output of the amplifier illustrated in Figure 4 when the drive level is forcing the amplifier output into a very non-linear region.

Note that the output response resembles harmonic output and seems to be only increasing in frequency. In real-life applications, the same spectrum also exists to the left of f1. The amplitude difference between A1 and A2 is critical to proper operation.

We will now introduce diplexers into this discussion and examine the roles of these devices in


Figure 6

Figure 6 depicts a standard Diplexer connected to a directional antenna load.

In this application, the diplexer serves to isolate the transmitted signals from the receiving signals.

The operation of the diplexer is straightforward. In figure 6. as shown, the diplexer consists of two bandpass filters connected at one end, namely the common port or the antenna port.


Figure 7

Figure 7 depicts typical common port bandpass spectrum response to frequency swept signals at either the high frequency port or the low frequency port. Note the diplexer allows band pass filter operation at two frequency separated bands.

The spectrum diagram, Figure 8, was selected to show the common AMPS Cellular bands response. The frequencies are in MHz and amplitude in dB.


Fig. 8: Diplexer Spectrum, AMPS Cellular Band


Fig. 9: Complete Repeater Block Diagram including antennas.

In the setup illustrated in Figure 9, physical spacing of the antennas is required to achieve at least an amount of attenuation between them that is approximately equal to the amplifier gain.


Fig. 10: Amplified cell signals re-broadcasted to fill area.

From Figure 10, which illustrates a fairly common repeater implementation, a possibility for oscillation is not difficult to spot. If either gain block, u1 or u2 has more gain than the attenuation (isolation) of the diplexers, a portion of the signal will be re-amplified in the bi-directional amplifier loop. Insertion loss of the diplexers will provide minor gain margin, obviously.

Fig. 11.

However, if IMD products exceed the gain margin, they too will be re-amplified and induce the bi-directional amplifier to re-broadcast them along with legitimate signals.

This problem is depicted in Figure 11, which is the same bandpass diplexer as in Figure 8 but with the IMD spectrum that can introduce various problems into the system application being discussed here.

The transmitted IMD products spread through the frequency spectrum and eventually significant products may arrive in the receiving frequency band. Since some of these products are in the proper passband of the diplexer, they are amplified to the adjacent diplexer and amplifier. This process continues until the system shuts down as an effective repeater system due to the saturation of the amplifiers.

There are many preventive measures including the use of ALC (Automatic Level Control), AGC (Automatic Gain Control), gain adjustments, and, of course, proper placement of the repeater with good antenna directivity. Another effective preventive measure is the utilization of high isolation / rejection diplexers and balanced amplifier gain, along with high dynamic range thus preventing amplifier compression.

The location of a repeater installation is equally essential to a solid system design. A poorly surveyed and planned location can lead to the possibility of the system being captured by a single user that is operating his or her mobile phone in close proximity of the system.

Another essential aspect of a successful system implementation that was briefly discussed above is the isolation between the repeater antennas. Such isolation needs to be at a physical distance, at least, equals to the diplexer rejection.

A relatively safe isolation figure is: for a 70 dB gain specification, a repeater needs to employ diplexers with isolation of 80 dBc or better.

James Penny is CEO and Chief Engineer of Filtek Corporation. Mr. Penny has published many articles concerning Diplexers and Filters and was issued U.S.Patent No. 5502715 for tower mounted Diplexed amplifiers. Mr. Penny can be reached at email: engpenny@hotmail.com or at Filtek, Email: filter@ipa.net. Tel. (479) 306 4076 Fax (479) 361 2296

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