I'm new to this site so sorry if this is not in the right place. I need to design a narrow band 10 MHz bandpass filter for an RF application. The -3 dB bandwidth should be about 10 kHz or narrower. Do you think it is possible to design and build one that is passive, maybe of seventh order, like a Chebychev, I have been trying to design one using a tonne software program called Elsie. Most of you probably wouldn't have heard of it. The problem is that it only goes up to seventh order and I think I need more than a seventh order passive filter to do this, could someone please inform me whether they think it's possible to do this using a seventh order passive filter. With inductors over course that are of reasonable Q values, not with Q's of infinity, etc. if not could some please suggest a good program for designing filters that goes to more than 7th order. Also I would like to do this passively. A this is what I have been trying to do for a while but would it be better it I tried to make an active solution?? I would prefer a passive one however. Any help would be greatly appreciated. Thanks
2 Answers
10 kHz bandwidth at 10 MHz is very tight for a R-L-C filter. Even if you could put a high enough order filter together, it would be useless due to part tolerance errors.
The only passive way to do this that has any chance of working is to use a 10 MHz crystal. You should still preceed it with a L-C filter to eliminate frequencies that can make the crystal resonate at overtones (harmonics). The L-C pre-filter will also help reduce the power of the signals the crystal has to get rid of.
There is another way, but it is definitely active and more complex, and uses the technique of hetrodyning. The basic concept is to shift the original frequency to a lower value where the desired bandwidth is a much larger fraction of the frequency, then shift the result back. The relatively wider bandwidth at the lower frequency makes a filter more tractable. Old AM radios used this technique, but didn't bother shifting back since they only wanted the amplitude and could get that from the shifted frequency.
450 kHz was a common IF (intermediated frequency) for AM radios intended to receive the commercial AM band from about 550 kHz to 1.7 MHz. The tuning knob would adjust the local oscillator, which needed to be 450 kHz less than the reception frequency. The result would go thru a 450 kHz narrow band filter and amplifier. This needed about 20 kHz bandwidth, which is 4.4% of 450 kHz. That was doable with a few carefully factory-tuned parts. In "super hetrodyne" radios, the tuning knob also adjusted a L-C filter to roughly select the RF frequency of interest. Note that due to how product modulation works (which is how the local oscillator was "mixed" with the filtered RF), there are actually two RF frequencies that result in the 450 kHz IF. These are the local oscillator plus 450 kHz (the desired RF frequency), and the local oscillator minus 450 kHz, called the "image" frequency. The original L-C filter on the RF needed to be tight enough to eliminate the image frequency before the hetrodyning.
You should also consider what you want to do with the final narrow band signal. If you just want to AM detect it, for example, then there may be other ways than starting with a very narrow band filter. It's not worth going into this without more information about what exactly you are trying to do, where this 10 MHz signal is coming from, what kind of modulation you want to detect, how much out of band noise the input signal contains, etc.
Olin's correct. It's not practical to build such a high-Q filter with L-C passives at 10MHz. A heterodyne-type system is one way to achieve what you're trying to do this and may actually be better for a do-it-yourselfer than trying to build a crystal-lattice type of filter.
The problem with homebrew crystal filters is that you need to have a sizable supply of crystals on hand to select the ones with the requisite characteristics; each needs to be resonant at specific frequencies to the "left and right" of your center frequency to give you the passband and shape-factors you desire. Commercial crystal filter manufacturers either have in-house crystal grinding/tuning facilities or they have enough sales volume to warrant farming such work out to places who specialize in crystal manufacture.
A heterodyne-type of BPF isn't all that difficult to create since easy-to-use IC's such as the SA602 Gilbert Cell mixers have been around (for maybe 30 years by now). It's difficult to draw a detailed schematic of what you do but here's the general idea:
10MHz Input-----SA602 #1----your 10kHz LC BPF-----SA602 #2-----LC LPF----BPF 10MHz Output
| |
| |
-----------Local Oscillator--------
Heterodyned filtering is very old technique, incidentally, so there's lots of info on how it works (not at all difficult to understand but too much detail needed for me to cover that here). You need the extra low-pass filter at the output since all frequency mixers produce a sum-and-difference set of frequency products and you only want to retain that which comes out centered at 10MHz, your BPF center frequency of interest. That LPF rejects the LO+10MHz component you don't want.
I happen to have a student copy of Tonne Software ELSIE and it was easy to produce a practical filter that'd meet your spec with a 3rd-order Chebyshev, nodal-capacitor-coupled bandpass filter with 0.5dB of passband ripple. You can also use a program like AADE.com's freeware to do the same. The LPF is even simpler than the BPF to make. You can get the SA602's from Mouser or DigiKey here in the US or Farrels in the EU, as well as the other parts you'll need to build this. Some of these filter programs produce oddball LC values but some also have features that let you use standard-value components to create the filter response you want. Good Luck!
