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[SOLVED] RF signal of SPDT and Bandpass filter

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yuyee

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Hi,
I was confused on SP2T switch(0-8ghz) in RF circuit. And my frequency signal is from 2ghz-3ghz.
Would like to know whether a bandpass filter is needed to remove the unwanted frequency ?
Will there be other factor arise if BPF is implemented?

Pls advise. Thanks

Regards,
yuyee
 

0-8GHz is the Permissible Operating Bandwidth of the switch.There is no relationship between your frequency band and that.
If your system produces harmonics, a BPF will be convenient to suppress unwanted signals to radiate ( or whatsoever...) otherwise SP2T does not produce any harmonic since its admissible power level is not exceeded.
 

Well-understood, thanks Big Boss.
Initially, my thinking was the wide freq SPDT will allow more undesired freq signal to also pass through the switch.
Therefore, the BPF is to filter away the unwanted freq.
Should a BPF be connected after the amplifier?

Regards,
yuyee
 

Filters in "RF circuits" (you didn't tell anything about your application except the frequency range) can be useful or even required for different reasons. E.g. low-pass filters to comply with regulations for harmonic emissions. Or band-pass filters to block strong out-of-band interferers of a receiver. But I don't see a particular motivation related to your SPDT switch. Was is its purpose? RX/TX, antenna diversity?
 

Yes, the input signal comes from the antenna and goes into the SPDT to switch between Rx/Tx.
 

Hi all,

From the attached picture, it shows a measurement done using VNA, Im confuse where should the 50ohms terminator be attached to Port 2 or 3 ?
post.JPG

Regards,
Yuyee

- - - Updated - - -

sorry please ignore the previous attached. Here is the latest
post.JPG
 

yuyee said:
From the attached picture, it shows a measurement done using VNA, Im confuse where should the 50ohms terminator be attached to Port 2 or 3 ?
Click to expand...

The measurement is done with all ports terminated. The VNA ports are internally terminated with 50 ohm, so the extra 50 ohm termination is connected to the remaining port of the device under test.
 

yes but there is a particular port...not sure is port 2 or port 3
 

yuyee said:
yes but there is a particular port...not sure is port 2 or port 3
Click to expand...

I don't understand what your problem/question is? It seems that port 1 is the common port, and the SPDT switches that to port 2 (for high band) or port 3 (for low band). When the VNA is connected for measurement of path 1->2, you put the 50 ohm load at port 3. When the VNA is connected for measurement of path 1->3, you put the 50 ohm load at port 2. This is just to ensure that the "off" path is also terminated.
 

Looking at the VNA spectrum, which port is attached with the 50ohm terminator?
 

yuyee said:
Looking at the VNA spectrum, which port is attached with the 50ohm terminator?
Click to expand...

Looking at the curve and the frequency band labels in the diagram, this measurement is for the high band from port 1 to port 2. The 50 ohm termination is then at port 3 (low band output).
 

It's not clear at all what's shown in the VNA measurement. How are the VNA ports connected? What's causing the high pass behavior?

Nothing's been said yet about a band switching configuration.
 

FvM said:
It's not clear at all what's shown in the VNA measurement. How are the VNA ports connected? What's causing the high pass behavior?
Nothing's been said yet about a band switching configuration.
Click to expand...

Based on the diagram next to the measurement, the 3-port black box has two outputs for two different frequency bands. I was assuming that we indeed have a switch plus some filters. But you are right, the black box might also represent a diplexer. The answer regarding the 50 ohm load would be the same: connect at "the other output" that isn't connected to the VNA.
 

Thanks I didn't notice the frequency band annotation. The setup is still unclear, the tread is about a SPDT switch, not a diplexer.
 

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