Yea, I understand where you’re at regarding the noise floor. The thing is that you’re just continually bouncing waves in the chamber, which will build up (you’re in a microwave oven) and increase the noise floor.
Think of the difference in background noise in a small library or church that’s all tile, granite and glass. You can hear everyone doing everything, and there’s a small “din” as all the sound bounces around with nowhere to go. You’re noise floor is higher. Take that same building with the same people, hang some tapestries, put that carpet down and it will be tens of dB quieter and without that “din”. You don’t need to upgrade the whole chamber, but some basic foam like I said can help a lot.
For the LNA, you don’t need anything fancy. I don’t know your frequency range and necessary specs, but we typically would use something like this, maybe even two of them depending upon line loss and the gain budget:
Put it as close to the receiving antenna as you can (right on the connector if possible), so it drives the signal down the cable and into the BB60. This should increase your SNR.
If you’re already near max input power level, the LNA could cause some issues there. You may want to toss a filter in in front of the LNA to knock down whatever else is causing the issue. But some red warning light in my head is going off because what’s being described isn’t totally matching with what I’ve seen in the field. The BB60C should have more than enough dynamic range for nearly any test. If you’re driving it near IF overload, but you’re still having sensitivity problems as evidenced by the attenuator test, then it feels like something else is going on. Have you checked for other interfering signals, zooming way out on the Spec An? Maybe your chamber isn’t as tight as you thought, or there’s some injection leakage. Is your test equipment *outside* the chamber, and just the antennas inside? Is the chamber appropriately sized for your lower frequencies (too small and it will act more like a waveguide)? Or maybe the chamber is just bouncing the signals around too much and you’re getting to much of that echo bouncing around nailing you. Have you tried it outside the chamber with the power tuned down some?
A box of anechoic foam (4 pieces) isn’t too expensive, and clever placing of it can help a lot. We don’t always use a chamber (in fact we often don’t), like I said slap a couple of pieces on plywood and make something that looks like an open box laid sideways that the antenna fits in and you’ll knock out the vast majority of interference. Add in a height differential or a piece of foam at the halfway point on the ground, and you’re pretty good to go.
So, try the LNA, that will definitely give you more sensitivity for better measurements. But my guess is that there is another gremlin or two out there that you’ll need to slay to get good results. It probably will be something you would never have thought of. Keep at it, RF test is hard!
edit: You also could always have a bad device; but that’s one of the gremlins to slay that I was talking about. Get some of the other stuff figured out, and see where it leads you.
Bypassing open air and just connecting Tx (or an appropriate surrogate) to Rx with an appropriately large attenuator (start with lots for safety, then dial it back) should give you the best signal characteristics and can help determine if you have a bad device or not. Could also be a bad Tx that’s putting out broadband noise also… Then you can also add in components walking the chain to see where things change. And antenna / atmosphere is a component in your link budget, so eventually you add that back in also 🙂
