Transient properties of the N200

[EtuelDue]

In my department we are working on a full-duplex transceiver with with two N200 boards and SBX daughterboards (200k bandwidth). As a first step we build the program shown in the first picture: A transmitter-session is initialized and inside the while-loop we are transmitting one single set of ofdm-data (one trainingsequence-bock and ten ofdm-blocks) and afterwards wait for about 2000ms.

On the receiver we get the following data:

The upper left picture shows the autocorrelation function and the lower left picture shows the received data signal. You can see that in addition too the ofdm-data the signal is overlaped with a sine, which also results in additional results in the autocorrelation-function (red square). This additional sine-function is only present if we use the interruped transmission scheme shown in the first picture. In case of a continuous transmission this "transient" property doesn't appear.

Do you know the reason for this transient problem or any idea on how to fix this?

[psisterhen]

There is some transient in the DSP as well as some transient when powering on the front end.  Given that you are not seeing the issue in packets during continuous transmission, this looks like transients through the amplifier and mixer when a signal is first sent.

Do your packets start with valid data immediately?  If so, I would pad the beginning of the first packet with zeros... this may reduce the amplitude of the transient.  If you can synchronize your Rx and Tx, either start the Rx after the transient or throw out the first  250 us or so of transient samples.

[EtuelDue]

 Thank you for your time.

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psisterhen wrote:

Do your packets start with valid data immediately?  If so, I would pad the beginning of the first packet with zeros... this may reduce the amplitude of the transient. 

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We are already padding the beginning with zeros. 

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@psisterhen wrote:

If you can synchronize your Rx and Tx, either start the Rx after the transient or throw out the first  250 us or so of transient samples.

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 Can you explain this in more detail? 

[psisterhen]

The Rx and Tx sessions can be configured to have a common timebase (a common timer so they can synchronize when Rx and Tx start and stop).  You can configure the Rx to start a fixed amount of time after the Tx to avoid capturing any transients.  See the following examples for more information:

<LabVIEW dir>\examples\instr\niUSRP\niUSRP EX Rx Multiple Synchronized Inputs (MIMO Expansion).vi

<LabVIEW dir>\examples\instr\niUSRP\niUSRP EX Tx Multiple Synchronized Outputs (MIMO Expansion).vi