OFDM - real RF output vs. expected output

Hi,

I use an USRP1 together with gnuradio/GRC and would like to transmit an
OFDM
signal, abt. 1 MHz bandwidth at 433 MHz. The FFT display block within
GRC
shows the power mask as expected, a flat line over the bandwidth, and
steep
drops at both ends. What a real spectrum analyzer at the output shows is
something totally different, like a Gaussian distribution of the emitted
energy, with highest amplitude in the center, bandwidth as configured,
but
the amplitude is dropping continuously towards the ends for about 70 dB
or
so. Adjusting the channel bandwidth in the sink block has no influence,
maybe the WBX anyway does not support adjustable filtering?!

Even the simplest possible setup, like random source, OFDM modulator and
URSP, shows this behavior.

What am I doing wrong?

Btw., the spectrum analyzer is a professional device and out of any
doubt :slight_smile:
LTE, DAB and DVB-T is shown as expected.

Ralph.

Ralph A. Schmid
Mondstr. 10
90762 Frth
+49-171-3631223
[email protected]
http://www.bclog.de/

You are probably using an odd interpolation ratio. You need to use an
even
rate in order to get a flat passband.

The WBX does not have adjustable filtering.

Matt

On Wed, Feb 13, 2013 at 10:42 PM, Ralph A. Schmid, dk5ras

Aah, OK, in fact it is odd. So I will modify my parameters and try
again…
Just got confused that theoretical and practical results were so
different
:slight_smile:

Meanwhile I had a look into the WBX schematics, fixed filters
before/after
ADC/DAC, as expected.

Ralph.

From: Matt E. [mailto:[email protected]]
Sent: Thursday, February 14, 2013 7:51 AM
To: Ralph A. Schmid, dk5ras
Cc: GNURadio
Subject: Re: [Discuss-gnuradio] OFDM - real RF output vs. expected
output

You are probably using an odd interpolation ratio. You need to use an
even
rate in order to get a flat passband.

The WBX does not have adjustable filtering.

Matt

On Wed, Feb 13, 2013 at 10:42 PM, Ralph A. Schmid, dk5ras
[email protected]
wrote:

Hi,

I use an USRP1 together with gnuradio/GRC and would like to transmit an
OFDM
signal, abt. 1 MHz bandwidth at 433 MHz. The FFT display block within
GRC
shows the power mask as expected, a flat line over the bandwidth, and
steep
drops at both ends. What a real spectrum analyzer at the output shows is
something totally different, like a Gaussian distribution of the emitted
energy, with highest amplitude in the center, bandwidth as configured,
but
the amplitude is dropping continuously towards the ends for about 70 dB
or
so. Adjusting the channel bandwidth in the sink block has no influence,
maybe the WBX anyway does not support adjustable filtering?!

Even the simplest possible setup, like random source, OFDM modulator and
URSP, shows this behavior.

What am I doing wrong?

Btw., the spectrum analyzer is a professional device and out of any
doubt :slight_smile:
LTE, DAB and DVB-T is shown as expected.

Ralph.

Ralph A. Schmid
Mondstr. 10
90762 Frth
tel:%2B49-171-3631223 +49-171-3631223
mailto:[email protected] [email protected]
http://www.bclog.de/ http://www.bclog.de/

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