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Chinese duplexers
On 8/30/2011 6:01 AM, Rob wrote:
Geoffrey S. wrote: Rob wrote: No. There are two sides, one of them for the high frequency and the other for the low frequency. The filter response is such that it passes on one frequency and notches on the other. Ok thanks. Note that they are tunable anywhere in the 136-17? range. Assuming the pass side is set to pass 145.600 (the input) and the notch side is set to notch out 145.000 (the output) and they are connected in series, would that work? It is not a pass side and a notch side. Both sides are pass/notch. But on one side the pass freq is above the notch freq, and on the other side the pass freq is below the notch freq. On the Chinese duplexers you have found, the Q is probably not high enough to tune the pass and notch to frequencies 600 kHz apart. one might be able to cascade two of them, but Rob's point about Q is well taken. It depends on whether the problem is the "depth of rejection" or the "steepness of skirt". A single resonant unit has a narrow spike on top of gentle shoulders. Say one section has Q of 1000 (i.e. the 3dB bandwidth is 100kHz for 100MHz center frequency), but the rejection at 1 MHz away is only 20dB. I could cascade 3 sections, tuned exactly the same, and I'd have 9dB loss at 100kHz, and 60dB rejection at 1 MHz away. The 3dB bandwidth might be around 30kHz, or, you could stagger the tuning slightly to get a little broader flat top, but keep the 60dB (i.e. moving the center frequency of one cavity by 10kHz won't change the attenuation at 101 MHz) The design challenge is that you need to suppress the transmit signal (at, say, 50W, +47dBm) low enough so that the front end of the receiver can handle it without blocking (say, you want it down around -50dBm). With our not so hot cavities described above, you'd need to stack up 5 sections at 20dB per section to get the 100dB suppression. Well, that's not so great, because you probably now have a bunch of additional loss in the receive path, AND you've got a real tuning chore on your hands to make sure that all of them are tuned appropriately to get the required bandwidth. Enter the idea of a notch. Let's say our example is 101 MHz Tx and 100 MHz Rx. Rather than rely on the "far away" response of a resonator to suppress the transmit, I can put a 101 MHz notch filter on the input to my receiver. If I can get 30dB/section rejection, then 3 or 4 sections will knock the Tx power down low enough to not block the receiver's front end. But wait, there's more... that Tx isn't a narrow spike. It has phase noise sidebands that go out fairly far. Unfortunately, the run of the mill transmitter might only suppress "off channel spurious and noise" by, say, -50dBc (and a rig designed to operate half duplex is probably worse). That means that your 50W (+47dBm) transmitter is putting out -3dBm *at the receive frequency*, so no amount of filtering on the receive path will help. Again, enter the notch filter.. you put a 100 MHz notch on the output of the transmitter. Now.. as Rob points out, the Q might be kind of low. If the Q is, say, 100, then that 100MHz filter is 1MHz wide, and by the time you cascade enough to get the BW down, the loss will be huge. As you can see, this whole duplexer/diplexer/multicoupler design thing can get pretty complex, and there's not usually a simple cookbook answer. |
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