Portsdown Sweeper

Both the Portsdown 4 and Portsdown 2020 include a Frequency Sweeper that uses the LimeSDR Mini and an RF Power Sensor to provide an indication of the frequency response of filters and amplifiers.

General Principle

The frequency sweeper uses a software controlled signal source (currently a LimeSDR Mini) to generate a signal on a specific frequency which is input to the device under test. The output power from the device under test is measured and displayed on a graph. The frequency is then increased and the process repeated to take multiple meaurements and plot the frequency response of the device under test.

Equipment Required

The sweeper requires a Portsdown, a LimeSDR Mini and the power meter sensor that was described in CQ-TV 273. The ciruit of the power meter head is very simple:

Making a Measurement

Connect the system up as shown above. Note that the LimeSDR will produce a calibration pulse of up to 50 mW, so it is recommended that 5-10 dB attenuators are used on the input and output of the device to protect the device and power head, and also to minimise the effect of cable reflections.

Select the Sweeper from Menu 2, Test Equipment. The sweeper will calibrate the LimeSDR and run straight away. You can change the centre frequency and the frequency span from the Set-up Menu.

The Marker menu allows the measurement of peak, null or manually-chosen levels.

Normalisation can be used to set the sweeper to display a flat response without the device under test in circuit, and then the device can be connected to show its response without any effect from the measuring setup.

Limitations

Because the LimeSDR Mini takes a finite time to move to each frequency, plotting with many points is slow, and difficult to use for real-time alignment. The number of sample points can be reduced to allow fast coarse alignment of circuits and filters. The output level of the LimeSDR can also be adjusted.