There are two kinds of situations where you might want to measure the input impedance of a product, system or circuit:
1. At a Low frequency like an audio amplifier where there is no real concern about the complex aspects of impedance.
2. At Higher frequencies like a receiver pre-amp where we are very much interested in the complex impedance.
I covered the first of these in the last video. If you missed that one, here is the LINK to it:
https://youtu.be/JCcHrXeIaYQ
In THIS video I will be measuring the input impedance of a VHF preamp that is part of my little 2 meter amplifier. I will see if we can do this with the nanoVNA.
Regardless of what VNA I use to make the measurement, the principles remain the same as to how to go about it. These principles can be applied to other amplifier where the nanoVNA can be used, so stay tuned regardless.
There are some rather surprising results to our efforts.
One of these is the actual SWR which is represented by the measured input impedance at 144 MHz. I kept puzzling at this. I verified it by at least two on-line calculators (Impedance-to-SWR calculators), by my Excel spreadsheet that I developed and by my manual calculation. They all came up with the same numbers!
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Time Markers for Your Convenience
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00:05 Introductory Comments
01:08 Planning Our Measurement
01:13 Concern #1: Input Requirements
03:41 Can the nanoVNA do this?
03:50 nanoVNA Setup
04:41 nanoVNA Results
06:30 Can the Tektronix VNA do this?
06:59 Tektronix VNA Performance
07:29 Concern #2: Output Loading
08:00 SETUP
08:00 Connections
09:37 Tektronix VNA Setup
09:39 Settings
10:33 Calibration (VERY QUICKLY!)
11:16 Adding the Port Extension
12:30 Making the Measurement
12:33 Getting the Amplifier ready
13:05 Measurement Results
14:35 Conclusions and Debriefing
15:25 Toodle-Oots