In this section, we test the transmit functions of the USRP that we can use later when building a communications system. We will observe the transmitted spectrum, minimum and maximum power level in dBm (dB relative to one milliwatt). You will use both the osciloscope and the spectrum analyzer at your bench to view and measure the output from the USRP transmitter.

  • Review the theory on section 1.4 Spectrum Analyzers. For more detailed information, you may also wish to review Spectrum Analyzer Basics and The Basics of Spectrum Analyzers. The concepts presented here will be applicable to any spectrum analyzer you may use in your career.

  • Open the GRC patch tx_carrier.grc.

  • Observe that the USRP sink center frequency is set to 50MHz. This block represents the USRP transmitter hardware.

  • Observe that the sine and cosine signal sources are configured for 10 kHz.

  • Connect the USRP Tx output to the spectrum analyzer and execute the flowgraph. A scope display will come up along with three buttons that allow you to select different values for Q(t).

  • Set the spectrum analyzer’s center frequency to 50 MHz and the span to 50 kHz by using the FREQUENCY and SPAN buttons. Adjust the LEVEL as necessary.

  • What do you observe on the spectrum analyzer display with Q(t) = 0? Try the other two options for Q(t). What do you observe on the spectrum analyzer?

  • What is the minimum and maximum signal power output from the USRP? The USRP output power level can be set via the dialog box obtained by double‐clicking on the USRP sink in the flowgraph. Measure the power using both the oscilloscope and spectrum analyzer and verify they are the same.

    • Note: Recall from your circuit theory that the output power is relative to the load impedance. On the spectrum analyzer, the input is 50Ω, which matches the expected impedance of a typical antenna. The input to the scope on the other hand is high impedance, effectively open circuit, to prevent damage to the internal circuits. To compare measurement on the scope to that of the spectrum analyzer, the line from the USRP transmitter should be terminated with a 50Ω terminator. Alternatively, some circuit theory can be applied to determine the equivalent power output measured across an open circuit and a 50Ω load.