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SA636 narrow band. Can the crystal be the second filter?

Question asked by Matt findlow on May 19, 2017
Latest reply on May 30, 2017 by Matt findlow

Hi,  

 

I am making a narrow band single conversion VHF receiver with 10.7Mhz IF using SA636.  Aviation band has 25khz channel spacing so I will be using a crystal filter.  The SA636 is optimised for ceramic filters with an impedance of 330 Ohms.  My 5th order crystal has an impedance of 3k||2pf.  For stability and a linear RSSI there needs to be additional insertion loss between the two amplifier stages, which means that the inter amplifier filter needs some level of impedance mismatch.  Considering that the crystal filter is already mismatched, it would be convenient for me to place the crystal filter as the second filter i.e. between the amplifier stages, and use the broader band ceramic filter as the first filter i.e. between the mixer and IF amplifier.  AN1993 shows a narrow band 10.7Mhz IF receiver with the crystal filter between the mixer and first IF amplifier, and then a ceramic filter between the amplifier stages.  What difference would it make to use the broader band ceramic filter as the first filter and the crystal as the second filter?

 

Cheers

 

Matt.

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