I've got a pair of Marti STL-10 / R10 systems here around 949 MHz. Making 10 watts into a good dummy load.
I connected the RF output to a 50w 30dB coaxial attenuator followed by a 2w 20dB coaxial attenuator, to the input of my spectrum analyzer. The SA claims the 948 MHz carrier is at -9.75dBm, which is close enough for me. 10w is +40dBm and I've got 50dB of attenuation so as to not overload the SA. -10.0dBm would be the target.
These units generate and amplify the signal at 1/2 the desired output frequency. They then run it through a varactor doubler to come up with the 950 MHz signals.
There's a sub-harmonic at 474 MHz that's at -66dBm, which is about -56dBc. The Marti specs call for -60dBc. I also checked up around 1422 MHz (3 times 474 MHz) and found a similar -66dBm spike. My SA is only good to 1.5 GHz so I can't check any higher.
Both units have the same levels at the output frequency and +/- 1/2 the output frequency. Either they're both tuned wrong or that's just how they are.
Is this normal? Suggestions to minimize the out-of-band signals besides some sort of bandpass filter? Does anyone have anything to compare this to?
Gotta watch those Fresnel zones!
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