Hello,
so I have done a few more tests to try to determine what may have gone belly up in the input RF module.
First, I have tested the spectrum analyzer, on each of the three input ports 1, 2 and 4. On all of these, signal path is OK, signal goes through. Just one note: There is decent crosstalk from port 4 to port 2. Is this normal? Applying 2GHz -10dBm from external source to port 4, analyzer listening on port 2 I get about -50dBm (just about 40dB attenuation). In the opposite direction, -10dBm 2GHz into port 2, analyzer listening on port 4, there is about -70dBm (60dB down). Is this normal?
By looking at the input section block diagram, I think this is OK, as obviously, the switch circled green is the only one providing port isolation in between port 2 and 4 in the receive path. In the opposite situation there are two switches providing the isolation (the other one below the circled one), hence I see double (60dB) the attenuation. Good, this makes sense.
Now on to the more interesting stuff:
Test with connectors open:
Analyzer listening on port 1. Internal generator output +13 dBm 350 MHz on port 3: Analyzer reading
-42 dBm.
Analyzer listening on port 1. Internal generator output +13 dBm 1000 MHz on port 3: Analyzer reading
-22 dBm.
Analyzer listening on port 2. Internal generator output +13 dBm 350 MHz on port 3: Analyzer reading -82 dBm.
Analyzer listening on port 2. Internal generator output +13 dBm 1000 MHz on port 3: Analyzer reading
-57 dBm.
Analyzer listening on port 4. Internal generator output +13 dBm 350 MHz on port 3: Analyzer reading -95 dBm (noisefloor).
Analyzer listening on port 4. Internal generator output +13 dBm 1000 MHz on port 3: Analyzer reading -83 dBm.
Red marked values look suspicious.
So lets test more, now always connect respective RF ports together with a cable:
Analyzer listening on port 1. Internal generator output +13 dBm 350 MHz on port 3: Analyzer reading
-42 dBm. Like without a cable!.
Analyzer listening on port 1. Internal generator output +13 dBm 1000 MHz on port 3: Analyzer reading
-22 dBm. Like without a cable!This certainly does not look right at all. Like no signal coming out. Connecting external spectrum analyzer to RF port 3 shows there is just -44 dBm signal at 350 MHz and a much stronger -35 dBm second harmonic.
So, we definitely have a dead RF out 3. Shiiiiit.
Trying to output the RF gen on port 2 and measuring it externally shows about the correct amplitude. So does when the generator's output is set to port 1.
Interesting things only happen when both the generator and analyzer are on the same RXTX port 1 or 2. It screams input overload and shows much higher input level then it is in reality. Not sure if it is due to the internal construction of the frontend or what.
Now I'll try to figure out what RF switch may be dead inside the frontend.