Optimizing SISO System with BPF vs Cavity Filter for Interference Protection

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Discussion Overview

The discussion revolves around optimizing a Single Input Single Output (SISO) system using either a Band Pass Filter (BPF) or a Cavity Filter for interference protection in a microwave communication setup. Participants explore issues related to signal quality, bit error rate (BER), and the performance of different filter types in the presence of interference from nearby WiFi signals.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports a degradation in BER after replacing a BPF with a cavity filter, despite the cavity filter having a narrower bandwidth.
  • Another participant suggests checking the signal with a spectrum analyzer to ensure the cavity filter is correctly tuned and has adequate bandwidth.
  • Concerns are raised about potential incorrect input or output matching of the cavity filter, with one participant questioning the labeling of the ports.
  • Discussion includes the presence of unwanted mixer products at the output of the mixer, specifically at 2462 MHz and 2522 MHz, and how these might affect performance.
  • Participants discuss the insertion losses of both the BPF and the cavity filter and suggest using a directional coupler to assess reflection issues.
  • One participant expresses confusion about the input and output ports of the cavity filter and seeks clarification on their differences.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the cause of the performance issues, with multiple competing views on potential problems such as matching, tuning, and the effects of unwanted mixer products remaining unresolved.

Contextual Notes

Limitations include unclear labeling of filter ports, potential mismatches in input/output connections, and the need for further testing with network analyzers to assess reflected signals.

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[/URL]Hi to all,

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!:cry::confused: i have 10-3 maybe 10-4 BER Can you please tell me some ideas to fix this problem?
 
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Have a look with a spectrum analyzer at your 2462 signal before and after the cavity filter to be sure the filter is correctly tuned and has adequate bandwidth.
 
Not much that can be said other than
1. Probably you have incorrect input or output matching
2. Maybe you are "using it wrong".
 
With the original setup you will have both mixer products at the output of the mixer - 2462 and 2522. Generally the LO frequency is such that it puts the unwanted mixer product outside the input bandpass.

What are the insertion losses of the BPF and the cavity? The Electrician has a good suggestion. Compare the output level of the BPF with the output of the cavity with a spectrum analyzer. If the output of the cavity is lower, you might try using a directional coupler between the LNA and the cavity to see how much reflection you're getting from the cavity. As meBigGuy suggests, it may be a matching problem.
 
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The Electrician said:
Have a look with a spectrum analyzer at your 2462 signal before and after the cavity filter to be sure the filter is correctly tuned and has adequate bandwidth.

Yes it is tuned in the right no prob with that.
 
meBigGuy said:
Not much that can be said other than
1. Probably you have incorrect input or output matching
2. Maybe you are "using it wrong".

Thanks for the reply.

1.First i don't understand what is the input and what is the output port!There is no label that explain me what is each port (!). Is there any difference? About the matching i will check it with
a network analyser better, maybe?

2. What do you mean is this way?What can it be wrong?
 
skeptic2 said:
With the original setup you will have both mixer products at the output of the mixer - 2462 and 2522. Generally the LO frequency is such that it puts the unwanted mixer product outside the input bandpass.

What are the insertion losses of the BPF and the cavity? The Electrician has a good suggestion. Compare the output level of the BPF with the output of the cavity with a spectrum analyzer. If the output of the cavity is lower, you might try using a directional coupler between the LNA and the cavity to see how much reflection you're getting from the cavity. As meBigGuy suggests, it may be a matching problem.

Yes you have right.The 2462 and 2522 (image freq) are going into my filter (from the TX_). The image freq is about 20dB Lower that the desired in my case. But in both senarios this image freq exists.

i don't have a directional coupler in my lab, but i have a network analyser, so i can see the reflected.

Also the cavity has two ports. I don't understand which one is the input and which one is output?Is there any difference with that?
 

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