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Cascading SAW filters [hlp]

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elec350

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cascaded saw filters

Hi
i want to cascade two RF SAW filters. Is it a true idea?
The datasheet of SAW filter:
 

biff44

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Well, as with any filter, you need some isolation between the two filters, such as a 3 dB pad or an amplifier stage. Also, you really have to pay attention to packaging issues with regards to leakage. If one saw filter gives you 50 dB of rejection, putting two together will not give you 100 dB of rejection unless you put them into two different metal housing shields!
 

    elec350

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elec350

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hello
biff44:
Well, as with any filter, you need some isolation between the two filters, such as a 3 dB pad or an amplifier stage. Also, you really have to pay attention to packaging issues with regards to leakage. If one saw filter gives you 50 dB of rejection, putting two together will not give you 100 dB of rejection unless you put them into two different metal housing shields!
pls explain about 3dB pad and the reason for different metal housing shields.
 

radiohead

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To be honest, I'm not so sure about the answer but I'll give you my opinion. The reason why an attenuator is required between two RF filters is that they are designed for 50 ohm (or something else) and won't operate well any more is the load impedance is difference. At the passband edges the reflection coefficient is typically lower let's say -6 dB. The 3 dB attenuator would then increase the return loss to -12 dB.

I think the same goes for acoustic filters, the only difference is that there is an energy conveyer from acoustic to electric and back. But an incorrect non-resistive load would also result in different acoustic reflections at the SAW filter output.

So in short my answer is yes you need some attenuation but I'm not 100% sure. Are there some real SAW experts on the forum?

The reason for a different metal housing is simple: This maximizes isolation (which are vibrations here) between both filtering structures and bigger suppression can be reached.
 

biff44

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Use some common sense.

Why don't you get 10 saw filters and connect them in series. That way you can get 500 dB of out of band rejection!
 

    elec350

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