Always Design the PA for Good Linearity
The active linearization capability shown in Figure 4 is a regulation loop that can reduce the effects of nonlinearities from all elements between the RF generator and the antenna. It must be carefully designed with respect to its bandwidth; otherwise, it may increase the noise outside its bandwidth where broadband noise begins. Even with this capability in a transmit path, any amplifier should always be designed to provide good linearity.
Nonlinearities of amplifiers can be based on a variety of mechanisms and are often described with very complex formulas, but the practical view is much simpler. If, for example, a digitally modulated single carrier is used as a transmit signal, then linearity is just how fast and how accurately the output signal of an amplifier can follow the input signal in phase and amplitude.
The AM frequency response of an amplifier is a major parameter in the context of nonlinearities, and this AM frequency response is set and limited in most of the cases by the circuitry of the input stages, not by the transistor. For a Class AB amplifier, the gate voltage may increase with an increasing RF drive signal, which opens the transistor for a higher output power. This leads to a dynamic shift of bias voltage toward Class A. It is important that the average DC bias voltage at the gates follow the average level of the drive signal fast enough; otherwise, the output power of the transistor cannot follow amplitude changes at the input fast enough. Figure 5 shows a simplified schematic of a push-pull amplifier, which may limit its AM frequency response.
Figure 5 Input circuit limiting AM frequency response.
The incoming RF signal is changed from unsymmetric to symmetric by the balanced-to-unbalanced (BALUN) circuit. Relative to GND, one transistor is driven with the positive half of the RF carrier sinewave, while the second transistor is driven with the negative half, and vice versa.
The envelope of the modulation signal represents the level of the RF carrier and should look identical on both gates. The average instantaneous level of the envelope is responsible for opening the gate to the required source gate voltage (shown as dotted lines in Figure 5).
The elements C1/C2 and R1/R2 set a lowpass characteristic. If the time constants are set too low, then the average DC voltage of fast modulation peaks cannot follow these fast changes in the envelope. Consequently, the gates are supplied with gate voltages that are too low (shown as red dotted lines) during fast modulation periods while the RF envelope is reaching full amplitude.
This leads to the suppression of fast amplitude signal changes, which looks like amplifier saturation. This mechanism creates almost the same intermodulation distortion as a saturated amplifier, but it is simply caused by a bandwidth limitation of the DC paths within the input circuits of the PA.
It is therefore recommended to first optimize the AM frequency response of a PA according to the required bandwidth of the transmitted signal and use the feedback loop only for further optimization of the remaining nonlinear behavior, if necessary.
It is a good choice to design an amplifier for an AM/FM frequency bandwidth response that is at least 2x or 3x higher than the highest modulation frequency of the signal to be transmitted. For an HF amplifier operating an SSB Voice signal, the AM frequency response shall be at least at 6 to 9 kHz. This method of improving amplifier linearity is easy to achieve and is easily validated because only an AM-modulated signal is required for testing.
In a push-pull configuration, it is also important that the gates of both transistors see identical circuits. Within Figure 5, a DC path through the matching network connects the gates to both capacitors C3 and C4. These capacitors likely have different values because they have been selected based on different goals. Additionally, C4 is connected to a series impedance, while C3 is directly connected to GND. As a result, AM frequency responses at the two gates may be different due to different capacitive loads, which leads to unequal transistor drives.
With a good design for the PA in combination with linearization, the following two-tone intermodulation values can be expected if the transmitter is driven with a two-tone signal level 6 dB below peak envelop power for each tone. The IM3 values represent the difference between the intermodulation products and the two tones:
- IM3 ≥ 30 dB – basic linearity
- IM3 ≥ 40 dB – enhanced linearity using digital predistortion
- IM3 ≥ 45 dB – superior linearity using digital active linearization.
On the receive side, an HF receiver (whether using direct sampling or intermediate frequency (IF) sampling) may be used as an IF receiver after a VHF/UHF down-converter as part of an overall superheterodyne concept.
Splitting into sub-bands makes sense for components close to the antenna. However, for an RF generator, it is possible today to create highly linear signals with low phase noise up to very high frequencies, where a split into sub-bands does not provide any advantage.
CONCLUSION
A transceiver architecture comprises circuits for transmit and receive that can be completely independent of each other. An important step in optimization, however, is to harmonize key parameters of the transmit path with those of the receive path for best system performance where these functions are co-located with low antenna decoupling.
It should be a goal for the designer that the robustness of the receiver is matched to the noise performance of the transmitter with a given antenna decoupling. The harmonization of the receive path with the transmit path within a transceiver architecture is then the basis for sharing valuable resources.
Figure 6 High-end configurable V/UHF software-defined radio (SDR).
These shared resources may be high-quality filters that act as pre-selectors for the receive path or as co-site filters to reduce the noise of the transmit path. Another sharable resource is a low noise signal generator providing a clean signal in transmit that is also used as a clean sampling or mixing signal in receive for best desensitization performance.1
For high-end installations, such as military communication sites, radios can be adapted to the co-site situation (see Figure 6). These radios can be equipped with special co-site options like hopping filters. Co-site filters are not only used as pre-selectors for enhanced receiver robustness, but they are also re-used as bandpass filters within the transmit chain.
The co-site filters are active in all operational modes including fast frequency hopping modes, ensuring a highly robust communication link, even in the presence of intentional jamming. The RF specifications enable interference-free operation in complex antenna systems with antenna decoupling values as low as 25 dB.
Transmitters can additionally be equipped with optional circulators to reduce transmitter backward intermodulation effects and, in combination with a frequency planning, spurious-free operation can be achieved.
ACKNOWLEDGMENT
We thank Ludger Korsmeier, Manfred Fleischmann, Robert Traeger and Harald Wickenhäuser DK1OP (all Rohde & Schwarz) for their support during the creation of this article.
References
- U. L. Rohde and T. Boegl, “The Perfect HF Receiver. What would it Look Like Today?” Microwave Journal, Vol. 65, No. 5, May 2022, pp. 68–82.
- H. L. Hartnagel, R. Quay, U. L. Rohde and M. Rudolph, Fundamentals of RF and Microwave Techniques and Technologies, Chapter 9.6, Springer, 2023.
For Further Reading
