45MHz Group Delay Filter Design!

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Hi,All!

Are there any RF guys here?

I'm developing a NOISE BLANKER for an HF-FM Radio.
The first IF of the radio is 45MHz, and I think to blank the RF on that stage. I finished the IF amplifier (90db with AGC) and the NB gate, but I need about 1us delay for the main IF path. I use Ansoft Designer and Serenade 8.0, there are a pritty good filter design tool, but I can't reach 1us of group delay with that. The best solution I've reached was with Bessel Aproximation and GD=400ns with BW=2MHz. It is good to have a wider bandwidth and flat GD response for easy tuning of the filter.

OK...,so my question is there any experienced person with RF filter design on that forum, to help me to find some solution on that problem.

Daniel Nikolov,
Bulgaria

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Appears the only way to get more group delay is to use a wider filter. Product I have work on has about 800-900ns at 45MHz, but with a BW of 4MHz. Our latest product does the time delay at the RF frequency (66-88MHz), so 1us is easier to achieve.

Maybe, you could consider going to a even higher IF, i.e. 75-100MHz.

Good luck,
davef

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That seems like a LOT of group delay. Why do you need so much? 1us is equivalent to over 100m of transmission line!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I guess the group delay is needed to allow the nosie blanker time enough to decide on blank no blank switching

Let me conslt my filter synthesis bible...( by Anatoli Zverev ) an all pass network may be what you need.

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ignoramus wrote:
I guess the group delay is needed to allow the nosie blanker time enough to decide on blank no blank switching

Yes, that's right!

I need a delay about 20% greater than the time needed to amplify, detect and compare a video pulse (4-10us filled with 45MHz), and my prediction from the simulation is about 500-600ns, I'm not realized the IF amplifier yet indeed, only the detector, comparator and NB gate circuits, but soon I'll have the exact time - now I develop the demo board. I think to tune the blanking pulse moment with multivibrator and 1us delay in main IF path is pritty good decision for me.
I will still try to design a LC filter, but It will be difficult to tune a 5th order filter indeed.
The another decision that tald me a friend of mine is to use a SAW filter, and now I'm searching for that one.
I have a service manuals for KENWOOD and YAESU radios with NOISE BLANKER, and they all are realized with coupled tanks filter circuits as a delay line. I will work on that also.

Any other ides will be appreciated!

Thanks all, and will wait for other posts!

Daniel

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Bratko,

While we are looking at Surface Acoustic Wave ( SAW) delay lines...

have you considered PAL delay line.. an inexpensive solution ( for a one off... i understandand Bulgaria runs PAL system) and if not PAL then SECAM.. both use a similar delay line.