What is the role of the preselector filter in a receiver front end?
Preselector Filter Design Considerations
The preselector is often the most critical filter in a receiver chain because it operates at the RF frequency where component performance is most challenging and its losses directly impact the system noise figure. Its design requires careful tradeoffs between several competing requirements.
As an image rejection element, the preselector must provide sufficient attenuation at the image frequency to prevent false responses. For a superheterodyne with a 70 MHz IF receiving C-band signals, the image is 140 MHz from the desired signal. A cavity filter with 3% fractional bandwidth at 6 GHz can provide 40+ dB rejection at this offset, while a microstrip filter might achieve only 20 dB.
For out-of-band blocking, the preselector prevents strong interfering signals from reaching the mixer and LNA, where they could generate intermodulation products or cause gain compression. In dense signal environments (urban, military), the preselector may need 60+ dB rejection at frequencies only 5 to 10% away from the passband, requiring high-Q filter technologies.
The preselector insertion loss adds directly to the system noise figure. A waveguide cavity filter at 10 GHz might have 0.3 dB insertion loss; a microstrip filter might have 1.5 dB. This means placing the preselector before the LNA increases the system NF by the filter loss, while placing it after wastes LNA dynamic range on signals the preselector would have rejected.
Frequently Asked Questions
Should the preselector go before or after the LNA?
Before the LNA provides better intermodulation and blocking performance because signals are attenuated before amplification. After the LNA provides better noise figure because the LNA gain suppresses the filter's noise contribution. The choice depends on whether the system is sensitivity-limited or dynamic-range-limited.
Can I use a tunable preselector?
Yes. YIG-tuned filters are commonly used as tracking preselectors in wideband receivers, providing excellent selectivity that follows the LO tuning. YIG filters offer constant absolute bandwidth (typically 20-40 MHz) across multiple octaves of frequency, with insertion loss of 1 to 3 dB.
What about switched filter banks?
Switched filter banks use multiple fixed filters selected by PIN diode or MEMS switches to cover different frequency bands. This approach provides better insertion loss than a single wideband filter and allows optimized rejection for each sub-band, but increases complexity and size.