Hi,
I am trying to design a 455KHz IF stage. I would like to use a 455KHz 7KHz
BW ceramic filter. Looking at the Murata data, they suggest that an
additional IFT is used to reduce the spurious responses.
Has anyone used such a scheme?
Does it mean connecting the IFT directly to the output of the filter?

Do I really need to worry about the spurious resonses?
Collins mechanical filters seem to have the same problem, but do these also
usually have an IFT as well?

Thanks,
John.

In article ,
John Wilkinson wrote:

I am trying to design a 455KHz IF stage. I would like to use a 455KHz 7KHz
BW ceramic filter. Looking at the Murata data, they suggest that an
additional IFT is used to reduce the spurious responses.
Has anyone used such a scheme?
Does it mean connecting the IFT directly to the output of the filter?

Do I really need to worry about the spurious resonses?
Collins mechanical filters seem to have the same problem, but do these also
usually have an IFT as well?


Depends on how much rejection you want/need to have away from the
passband. Those little 455 kHz ceramic filters inherently have "flyback"
or spurious responses in the region 100~200 kHz above the passband,
IIRC. The attenuation at those peaks may be as little as ~15 dB. So it
is common to have more than one such filter in the IF to achieve the
desired overall selectivity and IF image rejection. Usually cascaded
filters/IFTs are separated by an amplifier stage to set the correct
termination impedance for each filter. Please tell us more about the
application and maybe we can assist further.

Regards,
Joe