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