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45MHz crystal filters
Hi,
I am using a couple of 45MHz crystal filters in my first IF amp. Has anyone any experience of using these, and give me any advice on problems they encountered. I think I have a problem with screening, as they require 700 Ohms termination resisistance. When the manufacturers spec a termination resistance, do the filters themselves look like say 700 Ohms, or do I need to set this externally with a resistance? Best regards, John. |
45MHz crystal filters
On Wed, 22 Mar 2006 10:29:56 +0000, John Wilkinson
wrote: Hi, I am using a couple of 45MHz crystal filters in my first IF amp. Has anyone any experience of using these, and give me any advice on problems they encountered. I think I have a problem with screening, as they require 700 Ohms termination resisistance. When the manufacturers spec a termination resistance, do the filters themselves look like say 700 Ohms, or do I need to set this externally with a resistance? The filters -do not- look like 700 ohm, at least not in the stopband. Unless a filter has a dissipative loss, the only way it can attenuate is by reflecting out-of-band signal(s). To do this, it must be highly mismatched. The termination impedance is what the filter needs to see to perform as specified. You must set this either with a resistance and/or an amplifier designed to present this impedance to the filter. |
45MHz crystal filters
wrote:
On Wed, 22 Mar 2006 08:25:24 -0800, Tim Wescott wrote: wrote: On Wed, 22 Mar 2006 10:29:56 +0000, John Wilkinson wrote: Hi, I am using a couple of 45MHz crystal filters in my first IF amp. Has anyone any experience of using these, and give me any advice on problems they encountered. I think I have a problem with screening, as they require 700 Ohms termination resisistance. When the manufacturers spec a termination resistance, do the filters themselves look like say 700 Ohms, or do I need to set this externally with a resistance? Best regards, John. It means looking from the filter into the next stage it should see a 700ohm resistive load. How you get from 700 to the optimum for the stage is up to you but FETs are higher input impedence for noise and gain and transistors (BJT) are generally lower. Devices like NE602, MC1350 and similar differential inputs are around 1.5k. Allison It means that the source impedance of the amplifier driving the filter, _as well as_ the input impedance of the amplifier following need to be 700 ohms, resistive. Get this wrong and the filter shape will be off. Ah, I've done a few filters in life. While your right your way to fast to audit. It means looking from the filter into the next stage it should see a 700ohm resistive load. Maybe that should have been for the pendantic. It means looking from the filter into the adjacent stages it should see a 700ohm resistive load. There the use of "adjacent" rather than "next" should satisfy. Allison Yes, I think you have more experience than me, and I suppose I was being pedantic -- but they do want good termination on both ends. I forgot to add the part that would always happen in my lab: make sure to do a few sweeps, because no matter what my design looks like on paper I always get it wrong the first few times. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ |
45MHz crystal filters
Hi John:
terminations are an integral part of a filter design. there are a few usual or widespread impedances in use. actually the impedance has a marked effect on the filter's bandwidth, more than on shape. the narrower the filter, the lower the impedance. much more critical is the screening problem: the filters's cases must be very well grounded! any leakage here goes around the filter.it's not enough to solder to ground the relevant lead: you have to add a strap from the case to ground. Saandy 4Z5KS John Wilkinson wrote: Hi, I am using a couple of 45MHz crystal filters in my first IF amp. Has anyone any experience of using these, and give me any advice on problems they encountered. I think I have a problem with screening, as they require 700 Ohms termination resisistance. When the manufacturers spec a termination resistance, do the filters themselves look like say 700 Ohms, or do I need to set this externally with a resistance? Best regards, John. |
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