On Tue, Jan 21, 2025 at 01:21 PM, <lschafer@...> wrote:
- I selected the cal output of 30MHz.
- connected the cal output with the SA input with an oscilloscope in between (50 ohm termination assumed to be provided by the input).
Not using a probe going in directly with a BNC T and a 50ohm terminator when open ended. Scope input is 1M|15pF. Again, initially I did not expect that after the requisite calibration this TinySA instrument is off by -20dBm which is a lot.
Problem 1: The scope shows about 40mV RMS which translates to -15dBm, but the TinySA shows the main peak at -35.5dBm on the fundamental 30MHz tone. Why is this?
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You cannot connect this way and expect to get good results.? When you connect a coax cable to the T you are loading down with an impedance that will depend on the length of the coax and the scope termination impedance at the other end.? The impedance will be very low if the length of cable is close to 1/4 wavelength.? You might think that a 10X probe would do the job if placed on the T instead.? That will place 15 pF or so across the tinySA input which is a reactance of 354 ohms at 30 MHz.? This will result in a slight drop in the measure voltage and the display on the tinySA. However the signal on your scope will be fairly noisy . The reason is that -35 dBm is 4 mV RMS and a 10x probe will reduce this to 400 microvolts into the scope input.?
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Forget the T. The proper way to measure is to use a direct connection from CAL to RF input on the tinySA.? Then use a quality coax cable from the CAL output of the tinySA to the scope input? which is terminated with 50 ohms. make sure your scope is set to 1X or you will read the wrong voltage on the scope.? Also set your scope to read RMS voltage or divide the peak-peak by 2.83.
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Problem 2: The signal generator inside the TinySA claims to output -18dBm, but when measured with a scope on a 50 ohm load I measure around 285mV which corresponds to +2dBm. Why?
-18 dBm is 28.2 mV.? You measured 10x this so I suspect your scope is set to 10X
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Problem 3: Using Nano VNA sine generator @140MHz with a 10dBm attenuator (because the NanoVNA outputs about +10dBm when set to 0dB!), measured with scope around 220mV RMS showing the tone in the TinySA of around -20dBm when it should show 0dBm. What is the problem here?
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Many questions need to be answered.? Your attenuator might not be 10 dBm at 140 MHz.? Measure it with the NanoVNA.?? What is your scope bandwidth?? What type of coax and length did you use (dB loss in cable??).? Did you terminate in 50 ohms and not use a T?
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Roger
Roger Need