Why two switches and two Rx in RFX series daughter boards?

GNU Radio
1

I am wondering why there are two switches in the RFX series daughter
from
schematics. It seems their purposes is different: one for switch between
Tx
and Rx, another switch between Rx and Rx2. So, my questions as:

  1. we may use one tx and rx enough, why need to use two Rx?
    2.I follow the python code into usrp.pick_rx_subdevice(u) and the return
    value is side and subdev. Since there are two rx there, how may I know
    subdev refer to rx or rx2?
  2. if we call set_auto_tr(), it refer to which switch?

I also test usrp_fft.py and find both Tx/Rx and Rx2 have signal input
(by
using the same usrp_siggen.py), while input from Tx/Rx is stronger than
Rx2.
Since Tx/Rx is followed by a filter, it should be less than Rx2, so,
what
happens here?

I use two sets of USRP with RFX900 to test a FDD( frequency devision
duplex)
by cable connection. According to my experience, the cable loss should
be
less than 0.5-1 db, right? (which come with RFX900) So, If I set the
same
gain for two RFX900 tx, say -a 100, the received signal is much smaller
than
the coupled signal from itself, whatever the different frenquency is.
So,
why there are both input signal? it is coupled signal or intentionally
designed? I think it is intentional designed, and since the tx signal is
routed to Rx2 directly, so it is much stronger than come from its peer
end,
is it right?

Thanks for your reply!

BR,
Zhifeng

2

zhifeng chen wrote:

I am wondering why there are two switches in the RFX series daughter
from schematics. It seems their purposes is different: one for switch
between Tx and Rx, another switch between Rx and Rx2. So, my questions as:

  1. we may use one tx and rx enough, why need to use two Rx?

The RX2 port is for those people using external amplifiers who need RX
and TX on separate ports.

2.I follow the python code into usrp.pick_rx_subdevice(u) and the
return value is side and subdev. Since there are two rx there, how may
I know subdev refer to rx or rx2?

The subdev is not used on this board. The select rx antenna function is
used to choose between the 2 antennas.

  1. if we call set_auto_tr(), it refer to which switch?

It controls all switches.

I also test usrp_fft.py and find both Tx/Rx and Rx2 have signal input
(by using the same usrp_siggen.py), while input from Tx/Rx is stronger
than Rx2. Since Tx/Rx is followed by a filter, it should be less than
Rx2, so, what happens here?

The RX2 signal is weaker by about 25dB because it is disconnected. The
switches don’t have infinite blocking.

You should NEVER connect the TX output to an RX input without a lot of
attenuation in between. You should also not be using RX2 in your
situation.

Matt

3

Hi Matt,
Thanks a lot! So, you means I just need connect between Tx/Rx and leave
Rx2
unused for RFX 900. But I want to implement FDD (frequency division
duplex),
it should use Rx2. Do I need to make change to hardware? how?

Thank you!

2007/7/6, Matt E. [email protected]: