USRP2 Transmit Power

GNU Radio
1

Dear all,

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:

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

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?

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?

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?

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

Thankyou in advance for any help you can provide.

Best regards,
Marc.

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2

On Fri, Aug 6, 2010 at 9:54 AM, MarcW [email protected] 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

3

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.

You are connection the output of one RFX directly to the input of
another one ??? Without attenuators ?

that doesn’t sound wise.

Cheers,

Sylvain
4

On 08/06/2010 02:32 PM, Sylvain M. wrote:

Indeed, the LNA in the receive chain on many of the boards will have a
hard time
coping (device damage possible) with anything greater than roughly
0dBm, although the
MGA82563 lists an absolute maximum input power of +13dBm, you’ll get
nasty clipping long
before then, and it wouldn’t surprise me if you’d end up blowing-out
the mixer. Which is why
Matt generally recommends no more than -10dBm into the front end of
the Rx boards. And really,
for a general-purpose receiver, a -10dBm signal is what we technically
call “thunderin’ loud”.

The Rx side of most boards includes an LNA, and the intention is that
they be used as off-air
receivers, which means that they “expect” a fairly low-level signal.
Connecting the Tx side
directly to an Rx is a way to potentially have the magic smoke come
out, or at least get quite
undesirable non-linear responses from the receive chain.

For doing lab tests, with direct connections, a set of attenuators can
be your best friends:
10dB, 15dB, 20dB, 30dB. I’d use the 20dB minimum if I was
connecting the Tx side directly to the
Rx side–that way, even with +17dBm “full power”, the Rx would only
see -3dBm, which is still
a lot, but it likely wouldn’t damage anything.

–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium