none of the professors in my department are active in those areas, so
it's tricky to get specific info about top researchers, top
departments, top journals, etc. in those areas.
I suggest trying to talk to faculty in your department anyway; at the very least, they should be able to point you in the right direction, or where to start looking.
Even in the (very unlikely) event that the faculty at your school aren't very helpful with your direct task of identifying researchers in your areas of interest, they should be able to help you start using Google (or whatever your favorite search engine is) to identify key papers to read, and, more importantly, to track down where the research that you are interested in doing is being done.
If you have several faculty in your department to choose from, talk to the ones that are "research active": they will know how to help you perfect your search skills.
To supplement any useful tidbits that come out of discussions with faculty at your school, let me also add:
As an undergraduate student, you may not have had a chance to start reading papers in your areas of interest; that's OK. If that is the case, then you can start looking for papers by searching some key words related to your interests on Google Scholar, for example.
From there, you can get an idea of who the lead researchers in your subfield are, and which researchers are citing certain papers you find interesting. Keep track of the author names: read the affiliation information in the paper or use Google to figure out where these folks are.
You can also try searching the web for research groups that are doing the kinds of research you find interesting. For example, if you are in the states (where institutions are on the .edu domain), you could do a Google search for
"high-speed PLL design" research site:edu
and you would obviously change the "high-speed PLL design" search term to something that interests you.
Using variants of the above Google search strategy, I have found a lot of research groups (and their PIs, postdocs, etc.) that are doing the kinds of research that I've wanted to come up to speed on.