Ceramic IF filters

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.

Reply to
John Wilkinson
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I read in sci.electronics.design that John Wilkinson wrote (in ) about 'Ceramic IF filters', on Sat, 8 Oct 2005:

Yes, it's quite normal.

Murata used to supply the transformer or give you a manufacturer's part number and a diagram of how to connect it.

That depends on what you are making, doesn't it.

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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Reply to
John Woodgate

Thanks for the reply John. Do you have an example of such a setup, with the filter connected to an IFT? I would just like to get an idea.

Reply to
John Wilkinson

Hi, As this is the second IF, the first having a 10KHz BW using a crystal filter at 45MHz. Will this aliviate the need for an additional IFT after the ceramic filter?

Thanks.

Reply to
John Wilkinson

Pretty much, though it's better to put the IFT before the filter than after. One of the biggest problems with the ceramic filters is stray coupling to/from surrounding circuitry. The plastic cased filters are really bad about this. If you can afford the metal cased models, the spurious responses are much lower. If not, then try adding your own shielding around the filters. If you're cascading ceramic filters, orientation is very important too. Don't put them side-by-side, but rather in a straight line.

I don't know about Collins, but the plastic case ceramic filter spurs may be only 15dB down from the center of the bandpass, or less depending on circuit design and parts layout. If your receiver specs and operating envireonment can tolerate this, then you can do without the IFT. Midland had a pocket scanner that used a 10.7 MHz xtal filter into a mixer then straight into a CFU-455F, resulting in relatively poor spurious rejection, about 40db. But the product was relatively complaint free.

Reply to
LR

I read in sci.electronics.design that John Wilkinson wrote (in ) about 'Ceramic IF filters', on Sat, 8 Oct 2005:

Murphy rules, I'm afraid. I found one receiver circuit, but I don't believe it; it has two coils in series between the signal and ... nothing. And I can't see that it's just a printing error; it was drawn wrongly.

The other source is a Toko catalogue, which shows the filters and transformer(s) all in one can, but no internal circuit details.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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Reply to
John Woodgate

I read in sci.electronics.design that John Wilkinson wrote (in ) about 'Ceramic IF filters', on Sat, 8 Oct 2005:

It's got to be a definite 'maybe'. The ceramic filters have a host of resonances and any one of those responses could give you a problem.

These filters are usually quite low impedance. You can get more IF gain by matching the impedance, using IFTs, to something more appropriate for whatever you are using to feed it and to follow it.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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Reply to
John Woodgate

Thanks again John for looking into this. I will probably end up using an IFT to step down the impedance from around

2K to 100 Ohms.

Best regards, John.

Reply to
John Wilkinson

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