3.1 Swept Frequency Sources

Depending on the type of measurement, there are various sources that can be used to provide stimuli to the DUT. Reflection and transmission measurements require swept frequency sources, so a swept frequency source is used to test the magnitude or phase response of a device from which we measure insertion loss and phase, device gain, return loss, etc. Continuous wave (CW) sources provide information regarding a device's response at a single frequency.

Swept frequency sources can be categorized as sweepers or synthesizers:

Sweepers: Offer less frequency stability and as such, are better suited for broadband device measurements whose responses do not vary much over frequency. Sweepers lack the phase lock loop block contained in synthesizers and so tend to be less expensive. This type of instrument has been superceded in production by more sophisticated and accurate electronics.

Synthesizers: Provide high frequency stability and little phase noise, making them well suited to narrowband device measurements. Frequency stability is required if phase information is needed and a device's phase response changes rapidly with frequency. This is evident even in passive devices such as long cables.

 

Block Diagram of a Phase-lock-loop-based Frequency Synthesizer


A source's frequency stability will limit a system's ability to make repeatable frequency measurements versus time. Using a phase-locked synthesizer or synthesized sweeper will greatly improve test system source accuracy and stability over a simpler oscillator.

References

  • Application note 1390, "9 Hints for Making Better Measurements Using RF Signal Generators,"
  • Agilent Technologies document no. 5988-5677EN