Empathy List Archives

LS

Ludwig Stephan (CR/AEH4)

Thu, Feb 18, 2016 4:29 PM

Hello List,

I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged.

I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate.

Could you give me some hint for an explanation, please? Any help in changing this?

Best regards

Stephan Ludwig

Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000;
Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner,
Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel,
Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller

Hello List, I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged. I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate. Could you give me some hint for an explanation, please? Any help in changing this? Best regards Stephan Ludwig Communication Technology (CR/AEH4) Robert Bosch GmbH | Renningen | 70465 Stuttgart | GERMANY | www.bosch.com Tel. +49(711)811-8809 | Mobile +49(172)5630639 | Fax +49(711)811-5187845 | Stephan.Ludwig2@de.bosch.com Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000; Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner, Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel, Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller

MD

Marcus D. Leech

Thu, Feb 18, 2016 4:56 PM

On 02/18/2016 11:29 AM, Ludwig Stephan (CR/AEH4) via USRP-users wrote:

Hello List,

I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged.

I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate.

Could you give me some hint for an explanation, please? Any help in changing this?

Best regards

Stephan Ludwig

When you use the coax, do you add attenuation? You MUST do this, not
just for signal fidelity, but to prevent possible damage to the
receiver. My guess is that the receiver is being driven into
non-linearity in both of your scenarios. Put attenuation in your cable
(30-40dB), and back off the RX gain and/or TX gain to keep things
linear in the analog stages.

On 02/18/2016 11:29 AM, Ludwig Stephan (CR/AEH4) via USRP-users wrote: > Hello List, > > I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged. > > I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate. > > Could you give me some hint for an explanation, please? Any help in changing this? > > Best regards > > Stephan Ludwig > When you use the coax, do you add attenuation? You MUST do this, not just for signal fidelity, but to prevent possible damage to the receiver. My guess is that the receiver is being driven into non-linearity in both of your scenarios. Put attenuation in your cable (30-40dB), and back off the RX gain and/or TX gain to keep things linear in the analog stages.

IB

Ian Buckley

Fri, Feb 19, 2016 1:40 AM

Stephan,
If I understand you correctly, you are decimating by 10 in the N210 to get 10 MSamples/Sec before passing data to GNURadio, correct? Possibly you are also doing this on the transmit side also?
This will result in a less than ideal digital filter configuration…basically a CIC filter cascaded into a 7tap half band FIR filter. The CIC is approximately flat for Fs/4 in the band center before it starts to roll off significantly. The FIR is also not very flat beyond about ~0.35Fs where the stop band roll off starts.
I suggest you repeat you experiment using 12.5MSamples/Sec instead. This will result in a cascade of CIC into 7tap FIR into 31tap FIR. This configuration should be very flat out to about ~Fs0.45.
You will never see a flat response across the whole bandwidth because real world practical filters have to have start rolling off before the band edge to give a reasonable stop band rejection to prevent aliasing from adjacent frequencies.
-Ian

On Feb 18, 2016, at 8:29 AM, Ludwig Stephan (CR/AEH4) via USRP-users usrp-users@lists.ettus.com wrote:

Hello List,

I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged.

I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate.

Could you give me some hint for an explanation, please? Any help in changing this?

Best regards

Stephan Ludwig

Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000;
Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner,
Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel,
Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller

<CableTransfer.png>_______________________________________________
USRP-users mailing list
USRP-users@lists.ettus.com
http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

Stephan, If I understand you correctly, you are decimating by 10 in the N210 to get 10 MSamples/Sec before passing data to GNURadio, correct? Possibly you are also doing this on the transmit side also? This will result in a less than ideal digital filter configuration…basically a CIC filter cascaded into a 7tap half band FIR filter. The CIC is approximately flat for Fs/4 in the band center before it starts to roll off significantly. The FIR is also not very flat beyond about ~0.35*Fs where the stop band roll off starts. I suggest you repeat you experiment using 12.5MSamples/Sec instead. This will result in a cascade of CIC into 7tap FIR into 31tap FIR. This configuration should be very flat out to about ~Fs*0.45. You will never see a flat response across the whole bandwidth because real world practical filters have to have start rolling off before the band edge to give a reasonable stop band rejection to prevent aliasing from adjacent frequencies. -Ian On Feb 18, 2016, at 8:29 AM, Ludwig Stephan (CR/AEH4) via USRP-users <usrp-users@lists.ettus.com> wrote: > Hello List, > > I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged. > > I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate. > > Could you give me some hint for an explanation, please? Any help in changing this? > > Best regards > > Stephan Ludwig > > Communication Technology (CR/AEH4) > Robert Bosch GmbH | Renningen | 70465 Stuttgart | GERMANY | www.bosch.com > Tel. +49(711)811-8809 | Mobile +49(172)5630639 | Fax +49(711)811-5187845 | Stephan.Ludwig2@de.bosch.com > > Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000; > Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner, > Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel, > Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller > > > > <CableTransfer.png>_______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

LS

Ludwig Stephan (CR/AEH4)

Mon, Feb 22, 2016 7:22 AM

Good Morning all,

thank you, Marcus and Ian, for providing this valuable information. I will test both, but it will take 2..3 days. I will report the results back here.

Best regards

Stephan Ludwig

Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000;
Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner,
Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel,
Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller

-----Ursprüngliche Nachricht-----
Von: USRP-users [mailto:usrp-users-bounces@lists.ettus.com] Im Auftrag von Ludwig Stephan (CR/AEH4) via USRP-users
Gesendet: Donnerstag, 18. Februar 2016 17:30
An: usrp-users@lists.ettus.com
Cc: support@ettus.com
Betreff: [USRP-users] N210+SBX40MHz: Transfer Function of RF path?

Hello List,

I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged.

I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate.

Could you give me some hint for an explanation, please? Any help in changing this?

Best regards

Stephan Ludwig

Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000; Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner, Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel, Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller

Good Morning all, thank you, Marcus and Ian, for providing this valuable information. I will test both, but it will take 2..3 days. I will report the results back here. Best regards Stephan Ludwig Communication Technology (CR/AEH4) Robert Bosch GmbH | Renningen | 70465 Stuttgart | GERMANY | www.bosch.com Tel. +49(711)811-8809 | Mobile +49(172)5630639 | Fax +49(711)811-5187845 | Stephan.Ludwig2@de.bosch.com Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000; Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner, Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel, Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller -----Ursprüngliche Nachricht----- Von: USRP-users [mailto:usrp-users-bounces@lists.ettus.com] Im Auftrag von Ludwig Stephan (CR/AEH4) via USRP-users Gesendet: Donnerstag, 18. Februar 2016 17:30 An: usrp-users@lists.ettus.com Cc: support@ettus.com Betreff: [USRP-users] N210+SBX40MHz: Transfer Function of RF path? Hello List, I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged. I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate. Could you give me some hint for an explanation, please? Any help in changing this? Best regards Stephan Ludwig Communication Technology (CR/AEH4) Robert Bosch GmbH | Renningen | 70465 Stuttgart | GERMANY | www.bosch.com Tel. +49(711)811-8809 | Mobile +49(172)5630639 | Fax +49(711)811-5187845 | Stephan.Ludwig2@de.bosch.com Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000; Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner, Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel, Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller

LS

Ludwig Stephan (CR/AEH4)

Wed, Feb 24, 2016 12:55 PM

-- sorry, I forgot to reply to the list/SL

Thank you Marcus, for this valuabel advice. Of course I checked the output power and applied an attenuator. But I came pretty close to the max. specified input power.

After adjusting the sample rate, I indeed saw some non-linear effects: The transfer function is in total flat, but has a lot of small spikes.
Then, reducing the input power helped a lot.

Thanks again for your help.

Best regards

Stephan Ludwig

Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000;
Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner,
Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel,
Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller

-----Ursprüngliche Nachricht-----
Von: USRP-users [mailto:usrp-users-bounces@lists.ettus.com] Im Auftrag von Marcus D. Leech via USRP-users
Gesendet: Donnerstag, 18. Februar 2016 17:56
An: usrp-users@lists.ettus.com
Betreff: Re: [USRP-users] N210+SBX40MHz: Transfer Function of RF path?

On 02/18/2016 11:29 AM, Ludwig Stephan (CR/AEH4) via USRP-users wrote:

Hello List,

I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged.

I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate.

Could you give me some hint for an explanation, please? Any help in changing this?

Best regards

Stephan Ludwig

When you use the coax, do you add attenuation? You MUST do this, not just for signal fidelity, but to prevent possible damage to the
receiver. My guess is that the receiver is being driven into non-linearity in both of your scenarios. Put attenuation in your cable
(30-40dB), and back off the RX gain and/or TX gain to keep things linear in the analog stages.

USRP-users mailing list
USRP-users@lists.ettus.com
http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

-- sorry, I forgot to reply to the list/SL Thank you Marcus, for this valuabel advice. Of course I checked the output power and applied an attenuator. But I came pretty close to the max. specified input power. After adjusting the sample rate, I indeed saw some non-linear effects: The transfer function is in total flat, but has a lot of small spikes. Then, reducing the input power helped a lot. Thanks again for your help. Best regards Stephan Ludwig Communication Technology (CR/AEH4) Robert Bosch GmbH | Renningen | 70465 Stuttgart | GERMANY | www.bosch.com Tel. +49(711)811-8809 | Mobile +49(172)5630639 | Fax +49(711)811-5187845 | Stephan.Ludwig2@de.bosch.com Registered Office: Stuttgart, Registration Court: Amtsgericht Stuttgart, HRB 14000; Chairman of the Supervisory Board: Franz Fehrenbach; Managing Directors: Dr. Volkmar Denner, Dr. Stefan Asenkerschbaumer, Dr. Rolf Bulander, Dr. Stefan Hartung, Dr. Markus Heyn, Dr. Dirk Hoheisel, Christoph Kübel, Uwe Raschke, Dr. Werner Struth, Peter Tyroller -----Ursprüngliche Nachricht----- Von: USRP-users [mailto:usrp-users-bounces@lists.ettus.com] Im Auftrag von Marcus D. Leech via USRP-users Gesendet: Donnerstag, 18. Februar 2016 17:56 An: usrp-users@lists.ettus.com Betreff: Re: [USRP-users] N210+SBX40MHz: Transfer Function of RF path? On 02/18/2016 11:29 AM, Ludwig Stephan (CR/AEH4) via USRP-users wrote: > Hello List, > > I am using a 2 x USRP N210 + SBX40MHz daughter board for measuring channels. I use a OFDM (802.11g) transmission signal @2.45 GHz carrier frequency for estimating the channel in frequency domain. Everything works fine, I even receive data packets without error. But when I use a coax cable between Tx and Rx, I do not obtain a flat transfer function as I expect (cp. Attached image). From the center frequency, the magnitude drops by some 8 dB at the band edge. This is also true for a short (2m distance) over-the-air transmission in an office scenario, where we also expect a flat channel, because the echos all fall within the first tap. Du to my limited processing power, I scaled the transmit signal to 10 MHz bandwidth (each OFDM symbol then has double the length) instead of 20 MHz by changing the sample rate accordingly, leaving the GNU Radio DSP unchanged. > > I checked with different hardware (we have 4 USRPs) in different Tx/Rx combinations. When I further decrease the used bandwidth/sample rate, I get the a similar transfer function but only the corresponding portion of the one at 10 MHz band width - in the picture the share from -2.5 MHz to +2.5 MHz at same magnitude. Thus the problem is not in the signal processing done in GNU radio, but must be either in the DSP on FPGA/N210 or in the RF path of the SBX. I cannot imagine that such a large frequency selectivity in DSP/RF hardware is wanted. The SBX is specified for 40 MHz bandwith. And I doubt that this is a sinc D/A-C distortion, because 10 MHz is a tenth of the D/A-C sample rate. > > Could you give me some hint for an explanation, please? Any help in changing this? > > Best regards > > Stephan Ludwig > When you use the coax, do you add attenuation? You MUST do this, not just for signal fidelity, but to prevent possible damage to the receiver. My guess is that the receiver is being driven into non-linearity in both of your scenarios. Put attenuation in your cable (30-40dB), and back off the RX gain and/or TX gain to keep things linear in the analog stages. _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com