You will need a bit more attenuation than that. And still more if you want the most accurate measurements possible.

The math is simplest if you do the calculations in dBm (dB referenced to 1 milliwatt). 1mW = 0 dBm. 1W =?+30 dBm. 10W =?+40 dBm. For HTs and QRP rigs, perhaps the most important number is that 5W =?+37 dBm.

The maximum permissible input to the tinySA Ultra is?+6 dBm (4 mW). If you are applying a?+37 dBm signal, you need 31 dB of attenuation to decrease that to?+6 dBm, more than the 25 dB external attenuator that you proposed, and a tad more than the 30 dB attenuators that are popular surplus items. And you want a bit more in case your HT produces more than 5W; I'd recommend starting with at least 34 dB, which would make you safe for up to 10W.

The internal attenuator in the tinySA Ultra is an active device, and as such it causes some distortion. That will make the device under test (DUT) look a bit worse than it actually is. For best measurement accuracy, you want the internal attenuator to be inactive, which means you need an additional 30 dB of attenuation to get down to the range where it's not used. That theoretically comes at -24 dBm, but the auto mode of the tinySA is a bit more conservative about using the attenuator; you'll get down close to -30 dBm before it turns off.

For my measurements, I use a 30 dB fixed attenuator with a 20W rating, plus a step attenuator. For testing higher power radios I also have a 20 dB fixed attenuator with a rating of 150W that I can add to the chain. It's often difficult to find the rating of surplus step attenuators; it's safest to treat them as having a maximum input rating of?+17 dBm (50 mW). Some fixed attenuators specify an input end and an output end; make sure to heed those, or else you can burn out your attenuator. If you buy a used attenuator, you should test it for accuracy, which you can do with the tinySA itself using its signal generator as the source signal (procedure at ) or with another instrument such as a NanoVNA. The NanoVNA is nice because it can easily produce a display of attenuation over a range of frequencies.

For testing an HT, I would start with the 30 dB fixed attenuator plus a setting of 40 dB on the step attenuator. That would reduce the expected +37 dBm input signal from the HT to -33 dBm. Then I would reduce the amount of attenuation in steps until the internal attenuator turns on, then back off from that until it turns off again. That procedure gets maximum dynamic range and minimum distortion from the tinySA. The actual output of "5W" handhelds varies a bit, but I'd typically end up with about 35 dB on the step attenuator. Once you have dialed in the needed amount attenuation, you can set that in the tinySA so that it will directly display the actual signal levels.

ALWAYS start with at least as much attenuation as you need, adding a bit more for safety if possible. You can then reduce it to the optimum point. If you start with an inadequate amount you can destroy the front end of the tinySA, and you don't want that.

On Mon, Jan 20, 2025 at 12:32?PM Joe Tomasone via <joe=[email protected]> wrote:

I just received the TinySA Ultra.? ?As a first practical use, I want to measure spurious emissions from my various transmitters.? ?However, I understand that there is a maximum of -25dbm on the low input, and a maximum 30dbm internal attenuator.? By my calculations, this falls far short of even a 1w transmitter.??

?

Am I correct in assuming that connecting a 1w transmitter (an HT, say) to the TinySA via SMA cable to the low input will be definition require a minimum of a 25dbm attenuator plus the internal 30dbm attenuator to get a 1w signal under the -25dbm low input threshold?

?

?

?

?

I just got a new TinySA Ultra (SeeSii store, Aliexpress) and get doubtful measurement results.
- 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).

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?

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?

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?

?

I also did the linearity measurement which reveals no significant deviations (a straight line) across the attenuator steps at 120MHz and also at 200KHz and at 2GHz (at the latter frequency the peak showed up at -23dBm and "reduced linearity" warning showed up). All in all, seems no problem with the attenuator.

?

Now to the strange noise floor behavior. Input was terminated and the noise floor started at 600KHz with around -85dBm and transitioned in weird steps towards -120dBm at around 1MHz. Does this indicate a problem?

?

I do have historic background in RF engineering but I am relatively new to these new small devices and appreciate any feedback on what I identified as problems.

?

Thank you in advance

Lutz

?