OpenBTS Burning Man 2008 RF Chains¶
Transmit Chain¶
USRP -> 3318PA -> DB4090 -> 20' LMR-400 -> lightning arrestor -> 20' RG-8X -> 9 dBi antenna
- The output of the USRP is set to 19 dBm.
- The Downeast 3318PA is a "33 cm" power amp with a gain of about 14 dB at our operating frequency. It's an obsolete product and datasheets are no longer available.
- The DB Products DB4090 is a 2-cavity duplexer with 65 dB of isolation between the tx and rx. Here's the only document we can find on it.
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.
Receive Chain¶
9 dBi antenna -> 20' RG-8X -> lightning arrestor -> 20' LMR-400 -> DB4090 -> preamps -> rx filter -> USRP
- The preamps are Angle Linear PHEMT. We used a cascade of 3 to give 24 dB of gain.
- 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.
