Ericsson RBS 6601 RRUS 12 analysis(kind of)

[RickNitro]

Dear guys,
I've got myself an utilized, as not working, remote radio unit from multistandart ericsson base station. As being a bit into wireless communication tech, i' ve got intersted on how does it work.
At first, it seemed like a black magic, but as i was diving into it, it started to become more and more interesting.
I suppose, that I've caught some of the basic principles of Doherty circuit realization here. But there are couple of things I still cannot get or find any information:
1. The feedback and VSWR circuitry.
On the first photo, there is RF connector, possibly leading to a duplexer, where, I think, VSWR measured are done. Yet, I cannot get the point of amplifier circuit here, what is its purpose?
Neighboring circuit measures reflected wave, the one with amp direct wave, am I right? How the overall measuresement is done? Does it work as a some kind of feedback or feed-forward, or even active noise cancellation?
2. The purpose of the many DACs on the board.
Especially, there are three DACs right before the power amp. I can understand the purpose of two of them (some realization of Digital Doherty), but why the third one is here? From the datasheets, they have: 1.2 Gsps two bigger ones, 625 - little one. Is it some for some kind of envelope control? But if it is,I cannot see the feedback or advances dc controller.
3. What are these RORs for? I wasn't able to find any information about it...

Thank you in advance for your help!

[Kirby4life]

Wow, it's a very interesting topic. I'll wait with passion for the discussion.

[Da_Stier]

Hi and welcome to the forum 

Here are some of my thoughts on your points:

1. You want to be able to detect very small changes in VSWR and expect it to be in a somewhat reasonable range (like 2 max.).
If you have a VSWR close to 1 I could imagine that the amplifiers are used to get the reflected signal into the dynamic range of whatever they use to monitor it.
Another possibility would be the actual monitoring of the signal. A lot of RRUs have a complete IQ receiving path at the diplexer output to be able to set all the preemphesis etc. to get a good signal.
This is also kind of important on another question you asked about the feedback for setting the operating point of the PA.

2. There are quite a few DACs in these things doing all kind of calibration, preemphesis, operation point setting, etc.
I guess you might need to do a bit more tracing of what goes where (atleast roughly) to make a better guess.

3. The ROR things are kind of a RF / mixed signal "IC" type of thing.
Ericsson uses these a lot to get complex building blocks as a single reusable module.
The smaller ones tend to be VCOs, however these are probably more complex.
You can go ahead and desolder the lid with a hot air soldering station.
If you are careful you won't even damage the circuit below.


EDIT:
I think on your board the VCOs are the four panasonic marked metal lid devices.


Greetings,
Michael