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I built this single transistor RF amplifier about a year ago and it seems to work quite well but I wanted to try and characterize it more officially.

I was performing measurements on this amplifier with my oscilloscope (Siglent 1202X-E 200Mhz) but I wanted to be sure I was getting a somewhat accurate result so I ordered a TinySA, knowing that it could effectively measure dBm , that way I'd know for sure how much power this little amplifier puts out.

I'm using a Si5351 breakout board to generate the signal going into the amplifier, with the signal running at 144.690Mhz.

The single transistor amplifier is based around a 2N5179 and as you can see the circuit is built Manhattan style.

The issue I have is, the power that the TinySA is measuring versus what I calculate if I look at the amplifier output waveform on my oscilloscope, don't seem to agree.

The amplifier circuit is using a small toroid transformer on its output to match 50 ohms best as possible. I'm running the oscilloscope with a 10x probe, set in 10x mode. It should not load the circuit or change the behavior when measuring the circuit's output. In fact I can prove it does not change if you continue reading.

The TinySA indicates that the amplifier is putting out 13.4dBm or around there, or about 22 milliwatts. I have 35db of attenuation on the input of the TinySA and I've set up the TinySA to account for the attenuators (it's an option in the menu). In fact previously I was only using 20db of attenuation and I was getting the same exact result. I don't believe it's an issue with having a TinySA setting incorrect.

TinySA measurement

However, when I measure the amplifier output on the oscilloscope, I actually get a pretty clean sine wave and the scope says my RMS voltage is 1.5 volts. That would calculate to a value of 45 milliwatts, NOT 22 milliwatts. The scope is telling me I have 16.5dBm! The TinySA says 13.4dBm.

I measure the output like this:

scope probe using short spring ground

The scope readout:

scope indicates 1.5 volts rms at 144.690mhz

This is driving me crazy, I'm not understanding this at all.

I have tried a few things to check my equipment or setup:

  • I check the output of the amplifier also while the TinySA is connected. The values do not change on the scope or the TinySA, which to me means the scope is not loading down the circuit.
  • I thought perhaps the SMA cable has reflections (for example perhaps I am not exactly impedance matched) and since my scope was measuring at the SMA connector of the amp and the TinySA was at the other end of an SMA cable maybe my scope probe was at some high impedance point.. so I connected the TinySA directly to the SMA connector with a butt joint SMA connector. The TinySA measurement was exactly the same and did not increase much at all (certainly not by 3db)

Here's the strange thing. When I have the SMA cable connected, if I measure directly on the amplifier output I get 44milliwatts. If I have the SMA cable connected on the other end to a 50ohm load and I then measure the load on the other end with my Oscilloscope it shows only 1volt RMS. So my signal across the cable dropped? Could this be mismatch reflections in the cable then? I tried different cables and get the result. Or perhaps my oscilloscope is affecting the measurement in some way after all?

It's intersting that I removed the cable and put the TinySA directly on the output of the amp and it shows pretty much the same measurement but if I move the scope from the amplifier to the end of an SMA cable the measurements are different. It's quite odd that the difference is also close to exactly 2x

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Um ok...I think I might know what is happening?

I think the scope IS affecting the measurement. If I connect the SMA cable to a 50 ohm load and then measure across the 50ohm load, the scope tells me I get 1volt RMS.

If I then put on a longer SMA cable to the 50 ohm load and measure with the scope, now the scope says 1.4 volts RMS.

The voltage that I measure on the scope changes with the length of the SMA cable.

but if I put the TinySA on the cable, no matter the length, the value the TinySA reports is pretty much always the same.

So that must mean that yes the scope is affecting the measurement, perhaps its creating reflections too.

Guess I really need to look into getting a high frequency active probe for doing this type of work.

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You haven't mentioned the scope part number so we don't know the bandwidth. Neither of the instruments you are using are what I would consider laboratory-grade, with the tinySA farthest from lab grade. I'm not saying they are not very useful, it's just that I would expect them to be more useful for relative measurements than absolute measurements. You also need to expect cable lengths of a foot or so to have a significant impact UNLESS you have set things up to be impedance-matched.

You could put a 50 ohm termination at the oscilloscope input. Then alternately connecting the tinySA and the oscilloscope to the same source you should be able to determine the relative sensitivities. I'm not sure how different I expect them to be but- if the scope BW is substantially greater than 150 MHz- I wouldn't be surprised by several dB of difference.

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  • \$\begingroup\$ Scope is Siglent 1202X-E 200Mhz bandwidth \$\endgroup\$ Commented Jul 17 at 23:50

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