LOW INSERTION BPF Help
- Passband 243-270Mhz
- IL 0.9 db max.
- VSWR 1.5:1
- stopband rejection (0-218Mhz) and 295-400Mhz)= greater than 90dB
Please suggest a suitable practical topology and also the value of Q for inductor to realize this filter/..........
Hello,
You need to use higher order lumped element topology...
Try with Nuhertz Filter Solutions Filter Synthesis tool to generate the topology based on your specifications, where you can get different topologies, & even you can tune & see Q effects....
Passive Filters by Nuhertz Technologies
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---manju---
Is this an image reject bpf for a receiver with a low IF?
If so, could you design with an image reject mixer? Otherwise this filter will be very difficult (impossible?) to realise with real components. Eg a 9th order design with inductor Q >350 will still give about 3 to 4dB insertion loss. Also, it will be difficult to realise the 90dB stopband unless you take great care with the PCB design and screening.
@Manjunatha_hv
Well the problem in this specification is 1st it very difficult to get such high rejection in stopband and 2ndly as the order of the filter increases insertion loss also increases due to finite Q.
@GOHZU
well yes i m making this filter for a Rx.
can u elaborate on image rejection mixer and i also wants to know what is the maximum value of Q for inductor is available in this frequency range because i m able to find only 120-140 Q inductor till now....
I'm assuming your IF is about 25MHz. Can you redesign for a higher IF?
Or, can you use a low side LO for the bottom half of your tuning range and a high side LO for the top side? This will ease the shape factor requirements for your filter but it does mean your LO will have to cover a wider range.
Also, you could use an image reject mixer but this will mean you will need more space and it may be difficult to get it to work over the wider LO range outlined above.
90dB is a very strict spec. Do you really need to meet this? Also, even with the above changes I doubt the loss of the revised BPF will be less than 2dB.
To get a high Q at 250MHz I think the inductors will have to be wound by hand. Eg 7 turns of 0.7mm wire on a 4.5mm former with the turns slightly spaced should give around 105nH and a high Q. (300?)
However, this Q will degrade if you screen each inductor too tightly. Also your high order BPF will need a LOT of PCB space if you built it with a string of these inductors.
I really don't think you will be able to meet your insertion loss and stopband requirements. Maybe you need to think again about the frequency plan. Can you have a higher IF?
You could use a tuned (or switched) BPF to get more selectivity but this will have a higher insertion loss and limited signal handling.
You can realistically get another 20 to 30dB rejection of the image with an image reject mixer.
You can best find out about image reject mixers with google. eg google the words below
image reject mixer tutorial
@GOHZU
well i m making this filter for diplexer so i can use same antenna for both of my receivers......
and i want to keep it passive only.... if you got any topology for that please tell the other pass band is 291-318MHz for diplexer...
actually problem is i m not getting 90dB rejection with fewer inductors....
hello
Try Quartz crystal band pass filters, specifically Half lattice crystal filter...
"A two pole filter (i.e. one with two crystals) is not normally adequate to meet many requirements. The shape factor can be greatly improved by adding further sections. Typically ultimate rejections of 70 dB and more are required in a receiver. As a rough guide a two pole filter will generally give a rejection of around 20 dB; a four pole filter, 50 dB; a six pole filter, 70 dB; and an eight pole one 90 dB."
Quartz crystal band pass filters :: Radio-Electronics.Com
http://whites.sdsmt.edu/classes/ee32...2Lecture12.pdf
---manju---
To my knowledge 90dB stopband supression is provided with SAW filters only..
To implement this kind of filter is almost impossible wit lumped elements..
Because there will always be some EM couplings between sections and filter components...
It's hard to do..
It is extremely difficult to realize this filter with LC design because of limited Q of coils. But if you have enough space, you can make it with cavity filter design.