Simple scalar network analyzer


rheslip blogged about his simple scalar network analyzer project:

The principle is to stimulate an electrical network with a sinewave and measure the magnitude of the response. If we sweep over a range of frequencies and measure the power transmitted through the network we can determine its frequency response.  Transfer from the input port (1) to the output port (2) is called the network’s S12 S-parameter response. By using a return loss bridge or coupler we can measure the reflected power – the S11 response. A Vector Network Analyzer is a much more complex piece of gear that measures the phase response of the network as well.
In practical terms, a Scalar Network Analyzer allows you to test and characterize crystal filters, attenuators, highpass/lowpass/bandpass filters, cable losses, and antennas among other things. Its also useful as a signal generator and the power detector can be used on its own for power measurements.

Source code is on github.  Project info at rheslip’s blog.

Check out the video after the break.

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  1. >Transfer from the input port (1) to the output port (2) is called the network’s S12 S-parameter response.

    That should really be S21 (the ratio of the voltage ‘wave’ exiting from port 2 over the voltage ‘wave’ entering at port 1)

  2. “Transfer from the input port (1) to the output port (2) is called the network’s S12 S-parameter response.” Umm, I don’t think so: S21 (not S12) is the forward voltage gain. Look up “Scattering Parameters” on Wikipedia. Also, in a strict sense S-Parms are vector quantities, not scalar.

    1. Drone, think what you will. But be more careful when referring to Wikipedia articles. They might actually contradict what you are thinking. (Hint: Look at the Wikipedia article you mentioned again. Note the definition of S12 being the relationship of b2/ba. That should be enough for you to figure out why what you think is wrong wheras rheslips statment is not.I love it when people refer to Wikipedia without actually reading and understanding what is written there…

      1. To clarify, with “being wong” i meant assuming that “forward voltage gain” is implying a transfer from port 1 to port 2…

      2. And i am stupid for completely f*ckung up the definiton of S12 i have given. I would love an edit button…

        S12 is ofcourse the relationship b1/a2

      3. I’ll try to be brief, but clear – refer to Wikipedia if you want a comprehensive answer:
        You have a device, you apply a signal across 2 pins [port 1].
        Your signal IN is: “a1”
        [Reflected] signal coming back OUT is “b1”
        S11 is the power at port 1, coming FROM port 1, and is: b1 / a1.

        You device has a second pair of pins [port 2]
        Signal coming OUT is: “b2”
        [Reflected] signal going back IN from the load you have on this port is: “a2”
        S21 is the power out of port 2, coming from port 1, and is: b2 / a1.

        Finishing off:
        S22 [power at port 2, from port 2] is the portion of signal, reflected from the load back to the device, that gets reflected AGAIN and goes back out to the load: b2 / a2

        S12 [power at port 1, from port 2] is the portion of signal, reflected from the load back to the device, that passes THROUGH the device and appears at the input port: b1 / a2

        Sorry for the long answer, just wanted to be avoid the need to read Wikipedia.

  3. Ooh I want one of these.
    Maybe a later version will use AD8302 [measures phase, but only to -60dBm vs AD8307 -80dBm]

  4. As I stated, gain (or loss) from Port-1 to Port-2 is S21, not S12. Thank you for acknowledging this. As for a/b notation: For a two-port network matrix. The incident voltage at each port is denoted by “a”, while the voltage leaving a port is denoted by “b”. So S21=b2/a1 where b2 is the voltage leaving Port-2 and a1 is voltage incident on Port-1. S21 is a vector quantity (as in a Vector Network Analyzer or VNA) while |S21| is a scalar magnitude value (as in a Scalar Network Analyzer or SNA). The Wikipedia Scattering-Parameters page I cited is correct (for today anyway). There’s another reference at www(dot)microwaves101(dot)com – search the Encyclopedia pages for “S-Parameters”. And if you need more on this subject there are plenty of other resources out there including some nice App-Notes from the likes of by HP (once Agilent, now Keysight, and next-week – who knows). Now; wait until you try to turn this nice little SNA project into a VNA – have fun trying to resolve your phases properly! Have fun… David in Jakarta

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