在變頻器應(yīng)用的無(wú)源混頻器

　　而大多數(shù)的下變頻混頻器設(shè)計(jì)通常使用主動(dòng)或電流整流的拓?fù)浣Y(jié)構(gòu)，新的無(wú)源混頻器電路，可以提供更好的性能比線性顯著。

　　無(wú)源下變頻混頻器的ltc554x家庭覆蓋頻率從600 MHz到4 GHz，并提供高轉(zhuǎn)換增益和低噪聲系數(shù)（NF）具有高線性度。這些混頻器的目標(biāo)是在無(wú)線基礎(chǔ)設(shè)施接收器，需要高增益混頻器，以克服高的插入損耗，今天的高選擇性，如果SAW濾波器。而傳統(tǒng)的無(wú)源混頻器的變頻損耗通常有7分貝，新ltc554x混頻器集成中頻放大器，如圖1所示，產(chǎn)生整體轉(zhuǎn)換增益8分貝。這允許一個(gè)額外的15分貝中頻濾波器的損失，同時(shí)仍然使接收器。

　　Active versus passive mixers

　　Most integrated-circuit mixers are based on an active or current commutating topology. Linear Technology has a wide portfolio of active mixers， such as the LT5527 and LT5557， which are widely accepted due to their ease of use and low power consumption. Nevertheless， their 2 dB to 3 dB of conversion gain is not enough for some wireless infrastructure designs. Furthermore， active mixers typically exhibit higher NF than passive mixers with comparable linearity. LTC554x mixers employ a passive mixer core to achieve the lowest NF with high linearity. Table 1 compares the performance of the LTC5541 passive mixer to the LT5557 active mixer. As shown in the table， the passive mixer has approximately 5 dB higher gain， 2 dB lower NF， and 1.7 dB higher IIP3. The LT5557， however， has much lower DC power consumption.