i have a SISO system with microwaves devices. The 16-QAM information is about 20KHz , upconverted at 30 MHz and then at 2462MHz.At the receiver the same procedure, downconverting. After the first LNA i have a BPF fc=2430MHz which has large Bandwidth so it does not protect me from interferense signals. It has 200MHz BW. Although, i have a BER about 10-4 maybe 10-5 if the channels is clear (more often at evening).I did this setup in a Laboratory and behind us there are a lot of with WiFis transmitting all day etc…
i did a purchase of Cavity Filters with center freq at 2462MHz and BW=25MHz. i replace the BPF with the cavity and…the result is worse!!! :think: :neutral: i have 10-3 maybe 10-4 BER Can you please tell me some ideas to fix this problem?
waltr:
Did you ensure a good impedance match into and out of the Filter? If the match is not good then there will be reflections and a poor through put.
Do you have a network analyzer available to sweep the system?
Is the filter actually at the center frequency? Or does it need tuning?
Dear waltr,
a long time to hear news from you. Remember me? I had a lot questions about RF Design, also in the past months…
The filter is in the center frequency. I did the purchase and i told to the company to align the filter. When i took it in my hands i also check it in the spectrum analyser at tracking generator. I din’t ensure a good match into and out to the filter :?
Compute group delay and resultant distortion, esp. with 16 QAM. Should use constant amplitude modulation like QPSK at such low modulation orders.
Maybe you have right but i don’t have any problem at the time with the crystal filter. the problem is with the cavity.any help about this?maybe some pads in the input/output?
Yep, I remember you and figured this was for the same project. Looks like the project is progressing.
That filter is symmetrical so either port is an input or output.
They don’t spec the impedance but since they use N connectors it is either 50 or 75Ohm. I’m guessing 50Ohm and this can be determine by measuring the diameter on the N connectors center pin and comparing to a know 50Ohm N connector.
What is the impedance of the LNA output and the mixer input?
Also keep in mind that mixers can have lots of rf coming out of their input and not at a constant impedance. So they tend to need a passive pad or circular to maintain an impedance match to a filter.
This filter does have a 3dB loss in band. Is this the same as the crystal filter you were using?
waltr:
Yep, I remember you and figured this was for the same project. Looks like the project is progressing.
That filter is symmetrical so either port is an input or output.
They don’t spec the impedance but since they use N connectors it is either 50 or 75Ohm. I’m guessing 50Ohm and this can be determine by measuring the diameter on the N connectors center pin and comparing to a know 50Ohm N connector.
What is the impedance of the LNA output and the mixer input?
Also keep in mind that mixers can have lots of rf coming out of their input and not at a constant impedance. So they tend to need a passive pad or circular to maintain an impedance match to a filter.
This filter does have a 3dB loss in band. Is this the same as the crystal filter you were using?
Yes the project is in process. Yes, about the ports i did the purchase told them to put SMA ports 50 Ohm.
The mixer and LNa are also 50 OHM devices. Xm…so maybe a pad need between the cavity and the mixer. Yes all the filters has about 3dB loss.
Dear waltr, maybe also the group delay of the cavity filter is greater that the correspoding οf the classical BPF that i have in the figure? what are the type of cavity design ?maybe Butterworth , Chebyshev?
:neutral: 
i have 10-3 maybe 10-4 BER Can you please tell me some ideas to fix this problem?