Frequency selective power limiter
Hello! My DUT creates two signals: (1) ~4GHz, -30dBm and (2) ~8GHz, +15dBm. I need to pass the low-power signal (1), while blocking or at least trimming the highpower signal (2) down to 0dBm. Is there a way without resorting to a low-pass filter? Would a broadband power limiter also trim the weaker signal (1), while limiting the stronger one (2)? Thank you!
A limiter will add loss to the path when input signal is high enough. So yes, both signals will be attenuated. If no suitable filter is at hand, you can McGyver one;
make a 1/4 stub that kills the 8 GHz signal, A SMA T and a short piece of semirigid to be cut and grinded to suitable length, You need a network analyzer to get it right, or at least a spectrum analyzer + white noise source.
Some loss will occur at 4 GHz also, so the stub filter needs to be characterized. Also, your test object might respond poorly to total reflection of the 8 GHz signal, pad with 10 dB in front of the filter. Is it for measurements or for usage ? Lowpass filters are reasonable priced at mini-circuits.
Thank you for your response. This is indeed for measurements. The problem is that those frequencies change from device to device -- I was hoping to find a broad-band solution. Well, it seems I will have to buy a couple of filters to cover the variety of DUTs. The semirigid stub is a nice idea though. Thanks!
I think lowpass filters are the most economical way to handle this. You have to analyze the variety of DUTs and see that you get a filter for each flavour, taking in account not only that the lower freq is within the passband byt also that the higher freq gets attenuated enough.
There are tunable filters, quite expensive things. A bandpassfilter covering the low freq of all DUTs could be possible.
Another approach could be a diplexer if one is to be found that suit all types of DUTs. This would give the advantage of possibility to proper terminate the unwanted signal into 50 ohm, like this one:
I agree, you need a diplexer to split the signals, then operate on them independently
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