Unexpected Power Reading With 2.4GHz 2W PA, 4th 20dB Attenuator Not Reducing Signal

I'm doing an RF test where the output of a 2.4GHz 2W PA is connected to a SA through a few attenuators. The signal is QPSK modulated data.

When I put 3x 30dB attenuators in series, I get the expected power measurement of -57dBm (33-90) at the SA. But when I put a 4th attenuator of 20dB, I still get the same power level of -57dB at the SA rather than -77dBm. The noise floor of the SA is -93dBm with the selected RBW. What could be the reason for this strange observation? Some kind of leakage?

Hello
There are only two reasons why the signal is not attenuated:
- the attenuator is broken or faulty;
- the attenuator has been designed for other frequencies.
Greetings
Giovanni

I don’t necessarily agree with Gio’s assessment. I don’t think that-much attenuation in a single chain is necessarily workable.

The problem itself is interesting though.

I would try some experiments:
Remove one 30 dB attenuator (so now we just have 60dB attenuation). Does the spectrum analyser still read correctly?

Now add in the 20dB. Does the value drop 20dB?

Are there any adapters not mentioned?

I think it is possible that there is leakage across the whole chain of attenuators, limiting the maximum attenuation possible. You don’t mention the series type. Some types have better earth (ground) connections than others.

If you have already figured out the solution, it would be nice to post it here.

First, Thanks for this great information. This post is very helpful to all users.
Thanks a for sharing this awesome article.

There could be several reasons why you are not seeing the expected attenuation in your measurement setup. Here are a few possibilities:

1. The attenuators you are using may not be accurately calibrated, or there may be variations in their insertion loss. It's possible that the 20 dB attenuator is not providing the expected amount of attenuation.

2. There could be reflections or standing waves present in the transmission line, which could cause some of the signal power to be reflected back towards the PA, resulting in a higher power measurement at the SA. This could be due to mismatched impedance between the PA and the transmission line, or discontinuities in the line (such as connectors or bends).

3. There could be other sources of interference or noise present in the measurement environment that are contributing to the power measurement at the SA. This could include other RF sources, electronic devices, or even ambient sources such as electrical power lines.

4. There could be an issue with the SA itself, such as a malfunctioning input amplifier or incorrect calibration.

To troubleshoot this issue, it would be helpful to carefully examine each component in the measurement setup and ensure that they are operating correctly. It may also be helpful to use additional test equipment, such as a spectrum analyzer or a network analyzer, to more accurately measure the power levels and identify any issues in the transmission line.