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

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

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mattfindlow
Contributor II

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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  • RF

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LPP
NXP Employee
NXP Employee

General practice is to use narrow-band filter at the input stages to reduce amount of extraneous signals on subsequent stages. If you use broadband filter at the input, the jamming signal may enter the input stages and it may cause harmonic products within the target bandwidth. Narrow band filter at the input increases receiver's immunity to the jamming and reduces total THD of the receiver.  

The impedance of the crystal filter can be tuned by using L type LC network
https://www.youtube.com/watch?v=94mAcmJ5J7w

The required attenuation can be provided by using series resistor. Note that 6dBV insertion loss must be provided between first and second IF stages. Attenuation is not required between the mixer and IF first stage.

My suggestion is to place crystal filter after the mixer and provide L-type impedance matching networks at both ports of the filter. You can use LPF, HPF or BPF configuration.

Impedance matching.jpg

Ceramic filter can be used between first and second IF stages. Typical attenuation of the ceramic filters is about 6dB and no additional attenuation is required. If attenuation is less than 6dB, for additional attenuation use a series resistor or resistor divider. For example, if the loss of the filter is 3dB, divider of series resistor 150 and shunt resistor 1300 Ohms can be used.


Have a great day,
Pavel
NXP TIC

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Note: If this post answers your question, please click the Correct Answer button. Thank you!
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LPP
NXP Employee
NXP Employee

General practice is to use narrow-band filter at the input stages to reduce amount of extraneous signals on subsequent stages. If you use broadband filter at the input, the jamming signal may enter the input stages and it may cause harmonic products within the target bandwidth. Narrow band filter at the input increases receiver's immunity to the jamming and reduces total THD of the receiver.  

The impedance of the crystal filter can be tuned by using L type LC network
https://www.youtube.com/watch?v=94mAcmJ5J7w

The required attenuation can be provided by using series resistor. Note that 6dBV insertion loss must be provided between first and second IF stages. Attenuation is not required between the mixer and IF first stage.

My suggestion is to place crystal filter after the mixer and provide L-type impedance matching networks at both ports of the filter. You can use LPF, HPF or BPF configuration.

Impedance matching.jpg

Ceramic filter can be used between first and second IF stages. Typical attenuation of the ceramic filters is about 6dB and no additional attenuation is required. If attenuation is less than 6dB, for additional attenuation use a series resistor or resistor divider. For example, if the loss of the filter is 3dB, divider of series resistor 150 and shunt resistor 1300 Ohms can be used.


Have a great day,
Pavel
NXP TIC

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------

861 Views
mattfindlow
Contributor II

Hi Pavel

Thanks for taking the time to reply.  I really appreciate it.  

Cheers

Matt

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