The designer of a receiver front-end can set the intermodulation performance of the front-end to reasonable values by hiding the expected receiver intermodulation products behind those generated by the transmitters. Consequently, the operational system limit of the radio station is not limited anymore by the receiver intermodulation performance, but by cross-modulation.

Cross-modulation is a critical effect in communication systems with waveform amplitude changes or amplitude modulation (AM). This is the case, for example, in systems used for air traffic control because the communication waveform used is a double-sideband AM signal.

Cross-modulation occurs when a strong interferer drives the front-end into saturation. If an amplifier is saturated, its gain is reduced and any other low-level signal passing through the amplifier at the same time also sees some reduction of gain. As a result, the low-level signal is inversely modulated by those parts of the interferer’s envelope that saturate the front-end.

An air traffic controller operating on a voice channel cross-modulated by another communication channel will not be able to communicate with pilots. Even a very high receive level will not help in this situation. It is therefore essential that all interferers are kept below the cross-modulation threshold of a receiver system to keep it within an area of linear operation. Below this limit, the quality of the reception is determined by key parameters of the receiver and those transmitters acting as interferers.

With additional frequency planning, operation on channels blocked by intermodulation products is avoided and the entire station can operate nearly free of self-created interference.

STARTING WITH THE TRANSMITTER RF CHAIN

The transmitter RF design can be simply seen as an RF signal generator with some following amplifiers that increase the low signal level of the generator to the required high output power at the antenna.

A high-end transmitter is characterized by very low unwanted emissions outside the transmit channel. These unwanted emissions are either discrete signals or are represented by a noise floor. Discrete spurs can be caused by a variety of effects, such as the internal coupling of clock signals into the RF path. Other effects that can create discrete spurs may be based on nonlinearities in the transmit path, mainly within the final power amplifier (PA).

Before the minimization of discrete spurs, the basic transmitter design must first provide a low noise floor; otherwise, the radiated unwanted noise may already significantly interfere with adjacent channels.

Figure 2 Improved transmitter RF path.

The unwanted noise floor can be divided into two areas, which also represent two different noise-creating mechanisms. Close to the carrier noise is normally called phase noise and far from the carrier noise is called broadband noise. While phase noise is determined by the quality of the RF generator, the broadband noise floor is mainly determined by the design of the amplifier chain after the RF generator.

For a high-end transmitter, the required broadband noise floor might be required to be so low that even with a “no noise RF generator,” the resulting noise floor after the amplifiers may be too high. This means that a significant improvement can only be achieved by inserting filters in the RF path. The optimization of this filter-amplifier chain must then be done by going backward from the antenna through all stages and analyzing the gain of each amplifier stage.

Figure 2 shows a RF path improved by inserting two bandpass filters in front of each of the transmit amplifiers. The minimum broadband noise floor at the output of the transmitter is mainly the noise at the input of Amplifier 2 that is increased by its gain.

Figure 3 Spectrum of the improved transmitter RF path.

For a first optimization, the elements within the RF path can be seen as ideal, e.g. the filters have no insertion loss and the amplifiers only have a NF plus a flat frequency response with a given gain. It is also assumed that a filter in front of an amplifier completely suppresses all broadband noise coming from previous stages. With this simplification, it is clear that the broadband noise floor after the final amplifier is just the input noise floor represented by the amplifier‘s NF, which is then increased by its gain.

While the output power of the RF generator inside the filters‘ passbands is increased by the amplifiers, the broadband noise floor outside the filters‘ passbands is first suppressed by the filters; then, a following amplifier just increases its input noise floor by its gain. Consequently, the output noise floor always remains quite low while output power increases with the total gain of the transmitter‘s RF path.

In Figure 3, the dotted purple line shows the suppression of broadband noise by the filters, which is then increased by the gain of the following amplifiers. The defining parameters of broadband noise performance are the maximum allowed gains for each amplifier stage, which also define the power levels appearing at all filters.

It may be that the filter in front of the final PA must handle a power level of some Watts due to the quite low allowed maximum gain of the amplifier following it. Such a high-power filter may be difficult to realize, especially because it must be tunable in frequency as well.

On the receiver side, a similar high-power filter may be required to ensure sufficient receiver performance in the presence of strong interferers. Therefore, the dual use of relevant resources like filters and other components for the receive and transmit paths should be considered during the process of optimizing the overall RF transceiver architecture.

The RF Generator (Exciter) is the Heart of the Radio

The most important building block of a transceiver is the RF generator (see Figure 4). It provides a low phase noise modulated RF signal that represents the complete transmit signal of the transceiver. It starts with a sufficiently high level, providing an already low broadband noise as a starting point to the following filter/amplifier chain of the transmit path. In receive, the unmodulated RF signal is either used as a local oscillator for superheterodyne receivers or is used as a clock signal for digital direct sampling receivers.¹

Figure 4 Transceiver RF generator.

Today it is technologically possible to realize the core part of the RF generator, represented by the box with the blue dotted lines in Figure 4, with a fully integrated solution. Even digital-to-analog converters (DACs) can be part of such a chip. Only some manufacturers currently provide fully integrated solutions, but with a split between purely RF functionality and the core direct digital synthesizer (DDS), a sufficiently higher number of components is available.

The information signal (e.g., voice or data) to be transmitted is filtered and adjusted in level and is then digitized. The digitized signal passes through a baseband processing stage that creates a digital I/Q signal stream representing the modulation, including filtering. This digital I/Q modulation information is complex multiplied with the digital unmodulated carrier signal from the numerically controlled oscillator (NCO). This fully I/Q modulated digital RF signal is then input to a DAC and appears at its output as an analog RF signal to be transmitted.

Smart Power Control Loop

It is recommended to use a configuration for the RF generator where two parallel outputs are available, one with the modulated RF transmit signal, and one with the same, but still unmodulated, RF signal, derived from the same digital oscillator. This second unmodulated signal can be used to supply two feedback paths that provide phase-coherent demodulation of the RF signals present at the directional coupler after the final PA.

The signal from the forward output of the directional coupler can be used to set up a power regulation loop by monitoring the level of the transmit signal. This feedback signal can also be digitally compared with the transmitted signal to identify any signal distortions. A so-called active linearization loop is now able to pre-distort the transmit signal, compensating for all influences which happen on the way to the antenna through all stages.

The second feedback demodulator is connected to the reflected output of the same directional coupler and allows a highly capable analysis of all signals traveling from the antenna back to the final amplifier.

Due to the phase-coherent nature of the demodulation, it is now easy to distinguish between signals that are reflections of the transmit signal and those that have been radiated backward into the transmit antenna by other transmitters. Signals from other transmitters are characterized by a frequency offset when compared with the transmitted signal and can now be isolated. This enables an optimized strategy, keeping the transmit power constant or reducing it not any more than necessary, independent of the source of the reflected signals.²