How does a stub filter work page 3

Measurement Set Up and Calibration

6) A signal generator capable of pulse modulation and an oscilloscope (preferably digital) are required, plus the following test aids that we can supply. These should be studied carefully if you wish to substitute them with anything else.

1) Broadband Amplifier

2) RF Bridge Appendix B

3) 6 dB Splitter

4) Termination Box

 The measurements are made at a low frequency because of the difficulty of pulsing the RF signal fast enough to be able to switch the RF signal on within about 90° of the waveform. A PIN switch and pulse generator used as a substitute for pulse modulation of the signal generator would also require similar frequencies but may give better results as the generator used in the original measurement tended to have levelling difficulties, even with ALC turned off. The cables and connectors used for the measurement must be of high quality in order to ensure that there is minimum loss and the cable used for the stub filter also needs to be low loss and about 20 metres in length in order to reach the low frequencies required. The RG223 that was used for the original measurement is a relatively good quality cable but was only just adequate to demonstrate the principal of these measurements.

A test set up is made as per Figure 13 and the Signal Generator set to 2.5 MHz at 0 dBm.  The oscilloscope is set to 50Ω input and the Signal Generator level adjusted to give 2 Volts pp at the coaxial cable output on the oscilloscope.

Figure 13

[Set up for calibrating input voltage]

The test set up is changed as per Figure 14 and the Signal Generator frequency adjusted to give a minimum level on the oscilloscope. This sets the generator frequency for the stub filter operate at a quarter wave. 

Figure 14

[Set up for calibrating input frequency]

 

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