I think you're probably worrying too much here. If you have just graduated you will be level with everyone else who has just graduated in terms of quantitity of knowledge. If you know more about filtering and less about software then in many ways you are better off than graduates who are the other way around - everyone knows about software so that's not really a selling point, whereas knowing a bit more about the filtering and signal processing side is far less common (and so far more interesting to employers).
But actually, employers don't expect recent graduates to know much about anything! What is expected is you have learnt how to learn, that you have a very general appreciation of what engineering is about, and that you know where to look to find a solution to a problem. Your real technical learning is about to start. It is extrememly common (in fact normal) for engineers to find that a few years after they graduate they are using completely different skills to their strongest degree areas: as an example I concentrated totally on digital electronics in my degree, and yet I have spent my whole career working in analogue design!
Personally, what I have seen work best for new engineers is to get a job in a field that interests them with an employer who is prepared to invest in training them in their specific area: if you're lucky you may then find yourself an employer who will pay for you to do a part time MSc/MEng which will have the big advantages that a) you're not paying for it and more importantly b) you will be able to relate it to the work you are doing. Which begs the question: how do you get that job? Remember again, potential employers are NOT expecting you to start engineering on day one, like any profession it takes a few years post graduate experience to become an engineer. What they are looking for from CVs and at interview is that you know how to find and use new information, you know how to work in a team, you know how to ask for help without wasting people's time, you're willing to get stuck in and have a go, and you can build a small electronic circuit without it blowing up (too much) and write a small piece of software without it crashing (too much).
But if you don't feel you could present those skills to a potential employer the right Master's degree may help you get that experience and confidence. Just be sure that it is honing your employability skills, not just your academic research skills (if that's the way you want to go).
Andy Millar CEng MIET CMgr MCMIhttp://www.linkedin.com/in/millarandy
"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert