This tutorial summarizes some of the major points Besser Associates instructor Rick Fornes gave during a brief presentation on RF System Design principles.
One issue that most systems encounter is the fact that the transmitter and receiver share the same antenna. For for simplex or half-duplex systems, where the transmitter and receiver are on at different times, this is not a major problem. For full duplex systems, tremendous care must be taken to isolate the high power transmit signal from the very low level receive circuitry.
In the receiver, the LNA and mixer are the most important blocks (from an RF signal standpoint). When designing LNA’s, a tradeoff must be made between low noise performance and gain. Both parameters are important because while you obviously don’t want to introduce extra noise to your signal (noise figure), gain is needed to boost the signal to a relatively high power level, where the contribution of noise from the other blocks is minimal by comparison.
Intermodulation products – distortions that occur when the receiver encounters a very strong signal – limit the dynamic range of the receiver at the high end of the power scale. Thus both the LNA and mixer should have a high IP3 value, a parameter which describes the strongest signal that can be tolerated before distortion becomes problematic. Unfortunately, high IP3 designs usually require more power from the battery, thus reducing the time between recharges for mobile transcievers. This is another tradeoff in RF system design.
Most receiver designs employ a downconverter that shifts the signal down to an intermediate frequency (IF). The major motivation for this architecture is that filter performance is limited by the Q value of real-world components (bandwidth = f/q). In most cases it is not possible to build a filter to separate individual channels at RF frequencies. A major problem with this architecture is that spurious signals that are separated from each other by the same offset as your IF frequency can enter the mixer and be output on top of your desired signal. Careful planning must be done when choosing the IF frequency to avoid this problem as much as possible.
Another strategy is to simply downconvert the signal all the way to DC. This is called a direct conversion or “zero IF”/ZIF receiver. The advantage of this architecture is that you avoid the spur problems and also reduce the part count in your receiver. A major problem encountered is that since the local oscillator and the desired signal are at the same frequency, LO leakage in the mixer can cause a DC offset at the output. Since the LO level may vary due to reflections, this can wreak havoc with your A/D conversion process.
Yet another architecture is to use a very low IF, on the order of 100 kHz. This offers good selectivity, reduces the problems with spurs, and avoids the DC offset problem. Low IF receivers are gaining in popularity currently.
Other important considerations in RF system design are the modulation format, as well as the type of filtering that is applied to the baseband waveform to constrain its occupied spectrum. The tradeoff with modulation formats is between spectral efficiency and power efficiency. FM based modulation (FSK or MSK in the digital world) uses more spectrum than other formats, but since the RF waveform has a constant amplitude, the power amplifier in the transmitter can be operated at saturation without distorting the signal. This results in more efficient conversion of battery power to RF signal power than other formats. Conversely, other formats such as phase shift keying tranmit the same data using less bandwidth, but the RF amplitude of the signal varies with time and thus the power amplifiers need to operate with more linearity to faithfully reproduce the signal. Since the amplifiers cannot run at saturation, they are less efficient in using the battery power to produce the RF signal for transmission. Power amplifier designers are constantly researching ways to offer more linearity in the output, while achieving the highest efficiency possible.
Besser Associates offers several courses on RF System Design as well as power amplifier design for high efficiency.