A number of different RF front-end topologies are appropriate for software radios, each with its own advantages and disadvantages. This article explores the tradeoffs involved with each approach.

By Jeffrey H. Reed, Virginia Tech

The most common types of RF front-ends for software radios are dual conversion, single conversion, and tuned radio frequency receivers. The suitability of a particular receiver topology depends on a number of parameters that may include the following.

• Sensitivity defines the weakest signal level that a receiver can detect and is usually determined by the various noise sources in the receiver.
• Selectivity- represents the ability of the receiver to detect the desired signal and reject all others.
• Stability indicates the lack of change in the receiver gain and operating frequency with temperature, time, voltage, etc.
• Dynamic range is the difference in power between the weakest signal that the receiver can detect and the strongest signal that can be supported (either in-band or out-of- band) on the receiver without detrimental effects.
• Spurious response is a receiver’s freedom from interference due to internally generated spurious signals or to their interaction with external signals.

Topologies

Tuned RF

The tuned radio frequency (TRF) receiver, shown in Figure 1, consists of an antenna connected to an RF bandpass filter (BPF). The BPF selects the signal and the LNA with the automatic gain control (AGC) raises the signal level for compatibility with the ADC. This BPF bandwidth relative to the carrier frequency can be quite narrow, while in absolute bandwidth, it may be quite broad. For example, a second-order inductor and capacitor filter would require a filter quality factor of 10⁷ to extract a 30 kHz signal at 900 MHz with 60 dB of attenuation for a channel 60 kHz away, which is highly impractical.

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