An RF Preselector (a.k.a an RF Tuner or “front-end”) is used to prevent out-of-band signals from entering the radio receiver. In a super-heterodyne type of receiver, front-end filtering is essential to prevent unwanted signals from degrading the performance of the receiver. Without filtering, the frequency conversion stage(s) of the receiver will convert all signals received by the system, and the resulting unwanted mixing products can fall on or near the intermediate frequency (IF).
Figure 1. The HiPerTuner™, a 2012 RF Globalnet Editor’s Choice Award winner, is a high performance RF Preselector covering a frequency range of 200 – 2500 MHz.
Consider this example provided by Prof. Jim Stiles of the University of Kansas. For this example, the desired RF frequency is 100 MHz, and the IF frequency is 30 MHz. If the only input signal to the receiver is the 100 MHz signal, the receiver downconverts the input signal to the 30 MHz IF and all is well. Unfortunately this is never the case outside of the controlled environment of the laboratory, as the receiver will be bombarded by a multitude of RF signals across the spectrum in normal operation. Prof. Stiles goes on to demonstrate that without front-end filtering, the following conditions can cause detrimental interference to the IF signal:
– An RF signal at 30 MHz can cause a first-order product at the IF frequency, if the signal bypasses the mixer.
– RF signals at 15 MHz or 160 MHz can cause a second-order product at the IF frequency.
– RF signals at 10 MHz, 50 MHz, 80 MHz, 230 MHz, or 290 MHz can cause third order products at the IF frequency.
– RF signals at 15 MHz or 160 MHz can cause a second-order product at the IF frequency.
– RF signals at 10 MHz, 50 MHz, 80 MHz, 230 MHz, or 290 MHz can cause third order products at the IF frequency.
In this simple example, there are eight opportunities for receiver degradation due to signals far outside the desired input frequency. Clearly, a preselector is key to the maximum receiver performance. Please see Prof. Stiles’ full text here for a detailed and informative presentation of the need for RF preselection.
NuWaves Engineering’s HiPerTuner™ RF Preselector features a triple-cascade of varactor tuned filters, providing a tuning range of 200 – 2500 MHz with a 3 dB bandwidth of 4 – 8%. Additional features include 45 dB of gain control (selectable in 1 dB steps), a 6 dB noise figure, and a standard RS-232 interface for frequency and gain configuration. The HiPerTuner™ received the RF Globalnet’s “Editor’s Choice Award” for the month of May, 2012.
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