針對市售GPS車用天線易受到外在環境、基地台或手機等GSM信號干擾。本論文設計一低通濾波器(Low Pass Filter)設計一低通濾波器在低雜訊放大器前面提高GPS收訊，改善GPS主動天線的CN訊雜比及電源雜訊，觀察其受干擾的優點。 在射頻電路的技術上，並搭配衛星導航GPS接收機驗證結果。GPS陶瓷天線位於PCB板正上方，設計射頻電路在PCB板另一面。單一GPS陶瓷天線接收時會收到些許GSM訊號及1GHz以下頻段訊號。因此，本設計是先以一低通濾波器放在放大器 前濾除1GHz以下低頻頻帶的訊號，SAW濾波器位於雙極低雜訊放大器的中間，再次降低GSM信號，使GPS主動天線(GPS Active Antenna)不易受到干擾而斷訊提高訊雜比。 設計一低通濾波器在低雜訊放大器前面的優點: 1.可以濾除頻率1GHz 以下干擾，衰減可達到40dB ~ 80dB。 2.濾波器價格很便宜(約台幣1 元)，但是面積大(至少10*10mm2)。 對於傳統市售車用GPS 天線易受到手機GSM 信號干擾，電路設計以雙極放大器中間放置Dielectric 濾波器的方式實現。Dielectric 濾波器可濾除1675.42MHz以上的頻帶訊號及1475.42MHz 以下的頻帶訊號，衰減至少可達到30dB，價格約NT5~8 元，元件尺寸面積小(約5*5mm2)。使用15dBm GSM 訊號干擾，搭配衛星導航GPS 接收機，比較此設計及市售GPS 天線傳統設計的收訊效果。此設計的GPS 信號強度CN 值比市售天線高4dB 以上。 Vehicle GPS Antennas in markets are easily influenced by GSM signals. GSM signals are transmitted by the GSM base transceivers or mobile phones. In this thesis, design a low pass filter in front of amplifiers in GPS active antenna. Use this antenna to improve GPS reception, CN ratio and power noise to view with following advantages of interference event. GPS Patch is above the PCB board, and RF circuits are designed on the other side of it. GPS Patch can receive GPS signals and little signals under 1 GHz frequency bands. Our design is a low pass filter in front of amplifiers. It is used to reduce signals under 1 GHz frequency bands. SAW filter is in the middle between cascade amplifiers. It is used to reduce GSM signals again. Then, CN ratio is less influenced by GSM signals and other signals under 1 GHz frequency bands. A low pass filter of our design has advantages as follows： 1. Reduce influence under 1 GHz frequency bands, and out band rejections are even to 40dB ~ 80dB decayed. 2. Reduce cost each to approach to NT.1 per chip but layout area are lager about 10*10 mm2. For traditional products in markets, the input of RF circuits in vehicle GPS antenna is designed to feed cascade amplifiers with band-pass filter in the middle between them. The constitution of RF circuits in vehicle GPS antenna are including first amplifier following a band-pass filter with second amplifier. The band-pass filter is dielectric filter. Out-band rejections of frequency 1475.42MHz ~ 1675.42MHz are about 30dB decayed. Its cost approximate NT.5~NT.8, but its layout size is smaller about 5*5mm2. Use 15dBm GSM signals as the experiment, and our design improves CN ratio over 4dB in comparison to traditional products.
