2008 火人节 OpenBTS RF 射频链路

发射链 - Transmit Chain

USRP -> 3318PA -> DB4090 -> 20' LMR-400 -> （避雷针）lightning arrestor -> 20' RG-8X -> 9 dBi 天线

• USRP 的输出被设置为 19 dBm。
• 3318PA 是 Down East 的一个 "33cm" （33 cm -> 902 -928 MHz）的增益在我们的操作频段大致为 14 dB 的功率放大器。这是一款停产的设备，Datasheet 已无从所致。
  
• DB Products 公司的 DB4090 是一个两腔双工器，TX 和 RX 的隔离增益为 65 dB。链接是找到的唯一有关它的资料：Db-4090-installation.pdf

One problem to note is that the USRP puts out spurs and noise in the rx band, about 80 dB down from the tx signal, but still high enough to be a problem without good duplexing isolation.

With cable losses the EIRP of the antenna was about 39dBm.

接收链路 - Receive Chain

9 dBi天线 -> 20' RG-8X ->（避雷针）lightning arrestor -> 20' LMR-400 -> DB4090 ->（前置放大） preamps -> rx filter（滤波） -> USRP

• 前置放大选用的是 Angle Linear PHEMT。使用的是三级串联后得到 24 dB 的增益。
• "rx filter" - 接收链路滤波是自制的
  
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• The "rx filter" was a homemade 2-element interdigital filter and not a very good one. It had 60 dB of suppression in the downlink band, but 20 dB of insertion loss. It would not have worked but for all those preamps.

The Rx chain of the USRP was set to maximum gain, so the RF amp was set to the max of 70dB, and the programmable gain amp in front of the A/D was set to 20dB. According to the USRP RFX900 specs, the low-pass filter has a notch about 50-55 MHz from the center freq. So we twiddled with the RFX900 receive LO, so that our transmit frequency fell into this notch, giving us more transmit isolation. That feature would not work, however, in a multi-ARFCN system.

Getting the Picture

http://openbts.sourceforge.net/FieldTest/layout.jpg