January 11, 2011

Using a network analyzer as a makeshift signal generator

Figure 1: Agilent E8361C PNA Microwave Network Analyzer can go up to 67 GHz

A network analyzer is an instrument that characterize electrical network, devices or components up to very high frequencies, typically using ratio measurements. It is essentially made up of signal source/generator and receivers. Refer to Figure 2 for an example of a simplified network analyzer architecture using Transmission/Reflection test set. The source is outlined in red, while the receivers are outlined in green.

Figure 2: A T/R test set architecture (taken from Agilent AN1287-2)
Since there is a signal source/generator built into the network analyzer, you can use it as a simple makeshift signal generator with no fancy features (no analog/digital modulation, etc). What you can expect is a simple CW signal output. The frequency and power range will be in accordance to the instrument's limitation. 

Here's what you can do to turn your network analyzer into a signal generator:
  1. Assign or identify output port. For example, if you have selected S21, the signal would be coming out from port 1, while port 2 would be the receiver.
  2. Set the frequency range of interest. If you only want a specific frequency, set that as the center frequency and select zero span.
  3. If you've set a frequency range, you can skip this step, but if you have set a specific frequency with zero span, you need to set the sweep time to the maximum value. The reason is that there is a processing time between each sweep which may momentarily cut off your signal. This would make your signal appear to be unstable. Stretching the sweep time allows you to have the signal turned on continuously for the duration of the sweep time. 
  4. Set the desired power level. Bear in mind that a network analyzer's primary function is not to produce very accurate power levels but to have accurate ratio measurement. To have more accurate power level, some network analyzers (such as the PNA series Network Analyzer, see Figure 1) provide the capability to do source power calibration using power sensors.
  5. Connect the network analyzer source port to the DUT.

Knowing this trick can be helpful to save cost if you do not really need a powerful signal generator. It can also be "life-saving" when you have a network analyzer and needed a signal generator urgently.

Recently, Agilent released a new feature in their N9912A FieldFox Handheld RF Analyzer, which enables independent source. The FieldFox is a handy instrument that could contain many-in-one instrument functions such as network analyzer, spectrum analyzer, cable and antenna analyzer and more. This could also be another viable option.
Figure 3: Agilent N9912A FieldFox Handheld RF Analyzer