Re: [Discuss-gnuradio] USRP2 Transmit Power

[Jason Abele]

On Fri, Aug 6, 2010 at 9:54 AM, MarcW <address@hidden> wrote:
> We are trying to transmit a carrier using a USRP2 board (done using the
> USRP2 constant source example in the UHD branch) and then measure the
> received power by using the FFT via another USRP2 board directly connected
> to it. However, the fft shows a lower power than specified in the data sheet
> (11dBm instead of 17dBm). We have a few questions regarding this:

Unless you have calibrated the receive chain on your USRP2, the units
in the FFT window are not dBm ... I think we might have been moving in
the direction of dBFS for the units, but without calibration, you can
only treat the FFT units as useful for relative measurement.

Also, since you are using UHD and RFX2400, you should know there will
be an update to UHD soon with a bug fix for the Receive Gain range.
The set_gain() function accepted a range of 0-45dB, but then produced
gains of 0-70dB (ie, nearly 2dB/dB).  This has been fixed and should
appear in the public UHD git soon.

>
> 1.) What are the main factors in hardware and software which affect the Tx
> and Rx gain?
>

The gain setting on the Sink/Source blocks affects the variable gain
in the hardware, the digital amplitude of your modulated input to the
Sink/Source also affects your transmit power.

> 2.) We read in the datasheet that the RFX2400 has a transmit power of 17dBm.
> However, we observe from the schematic of the RFX2400 that there are two
> amplifiers (MGA82563 and RF3315) with output power of 17dBm and 23dBm
> respectively. How is the 17dBm figure in the datasheet derived?
>

17dBm is derived from the pass/fail testing of each unit before
shipping.  It is the lowest maximum transmit power we ship for that
board.

> 3.) In software, what is the point of setting the set_gain parameter in a
> USRP2 sink? How does this change the transmit power? Does this change occur
> in hardware or in software?
>

On daughter boards with variable gain in hardware on the transmit
path, it sets the appropriate registers or control voltages for the
requested gain setting.  The RFX2400 has no variable TX gain, you must
do power control through your digital amplitude.

> 4.) At the receiver side, is there a variable gain amplifier? How does the
> gain parameter in the USRP2 source affect the received signal and is this in
> hardware or in software? What controls the amplifier?
>

Same as on the transmit, if there is variable gain (RFX2400 has
0-70dB), then the appropriate settings are applied in hardware.  In
the case of RFX2400, the gain is a control voltage for the LNA in the
AD8347

> 5.) How do we make sure that we always transmit at the highest possible
> power?
>

Send in the maximum complex digital amplitude, probably +-1.0 if you
are on a USRP2, but check the range for the input of the USRP2 Sink.

Jason