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Problem of two Series SAW Filter.

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mehdu

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Hi

When I series two SAW filter, why the attenuation less than sum both of them in stop band region. Each of them (in stop band) have 60dB attenuation but when series them, the attenuation is equal to 85dB.
What happening? Why didn't reach to 120dB?

Then, I put a 20dB attenuation between them ( SG --> SAW1 --> 20dB attenuation --> SAW2--> SA ), and the attenuation is equal to 90dB, this meaning only 5dB attenuate with 20dB attenuation.
What happening again?

However, the insertion loss is true when series. Each of the have 8dB insertion loss and when both of them is series the insertion loss equal to 16dB.

Thanks in advanced
 

Hi,

In simulaton?
In real world?
How did you come to the results?

Klaus
 

In real.
I test Each one separately. Give a single tone signal in stop band frequency of saw and measure attenuation with spectrum analyzer.
 

(1) Noise floor level of your Spectrum Analyzer
(2) Improper termination for stop band

I think (1) is dominant.
Show me input signal level of Signal Generator.
Show me noise floor level of your Spectrum Analyzer.
It depends on RBW, Sweep Time, Pre Amplifier, etc.
However dynamic range of it is 90~100dB at most.
 
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The noise floor of SA is about -136dBm. At the time of testing ( Res BW , Span , ...) is about -110dBm.(SA Model: N9918A)
I don't understand (2).
 

Show me input signal level.
Show me RBW of Spectrum Analyzer.

(2) is very common sense.
Can you understand proper termination for SAW ?
Attenuator can improve termination in cascade.

Your result using attenuator seems to be reasonable.

How do you connect two SAWs ?
On PCB ?
If so, 120dB is impossible.
 
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input level 17dBm, and after insertion loss is about 0dBm.
SA picture: https://ibb.co/ZJ8Fvcz
ok, got it. Each of them is a module with case and connected with male thru SMA.

- - - Updated - - -

- - - Updated - - -

This might be leakage that happens outside the SAW, for example grund currents that couple from input to output.

In first I thought it was leakage, so I put distance between them about 1 meter with cable.


85dB isolation is a lot ....

85dB is a lot but I need to achieve about 110dB. Why the sum of the two attenuators is not correct?
 

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Each of them is a module with case and connected with male thru SMA.
............
Why the sum of the two attenuators is not correct?
Show me result of adding 20dB attenuator between two SAWs.

In this case, output level is :
-20dBm for passband
-20-120=-140dBm, however it limits at most -100~-110dBm for stopband.
So suprresion level of two cascaded SAWs is 80~90dB.
Your result is reasonable limitation of your instrument.

20dB attenuator is too large for improving termination.
Try 3dB, 6dB, 10dB attenuators between two SAWs.
 
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Show me Spectrum of single SAW(input=17dBm).

Do you have network Analyzer ?
If you have it, show me {s11, s12, s21, s22} of single and cascaded SAW.
 
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Show me Spectrum of single SAW(input=17dBm).

Show me photos of your DUT of single and cascaded.
 

It is an isolation problem.
In order to achieve such high stop band attenuation, you have to pay extreme attention to isolation.
Go step by step, disconnect the DUT but do not OFF the RF signal (you may keep the power 10 dBm max), observe how much coupling occurs, change your test cable arrangement see whether it affects.
You may observe how much coupling occurs when you use single DUT with terminated output.
By the way, put all the screws and tight them.
Good luck.
 

Show me Spectrum of following case.

SG Output Level=+17dBm

Input of SAW is connected to SG.
Output of SAW is connected 50ohm load
Input of SA is cable of open.

I suspect leakage of Cable.
 
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