Narrow Bandpass Filter (NBP) - Manufacturer & Supplier

Narrow Bandpass Filter (NBP) - Manufacturer & Supplier

If you are in search of a solution to selectively filter light in your optical system, you've landed in the right spot. The narrow bandpass filter is a crucial component for any optical setup that requires the passage of specific wavelength ranges, while effectively blocking all other wavelengths outside that specified range.

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A narrow bandpass filter serves as an optical barrier that permits only a defined range of wavelengths to traverse, blocking all other wavelengths that fall outside this range. These filters can be tailored to either very narrow or broader wavelength ranges, depending on their intended application.

Utilizing a combination of thin films and coatings, optical narrow bandpass filters selectively reflect and transmit light of predetermined wavelengths. The design of these filters can vary, providing either a high or low Q factor, a critical metric that indicates the selectivity of the filter in allowing the desired wavelengths to pass through.

Creating a 10 kHz bandwidth filter at 10 MHz presents a challenge for a R-L-C filter design. Even if a high-order filter is successfully constructed, it may fail to work effectively due to errors arising from part tolerances.

The only truly passive method that might yield reliable results is by employing a 10 MHz crystal. It is advisable to precede this with an L-C filter to eliminate any frequencies that could induce the crystal to resonate at harmonics. This L-C pre-filter will assist in diminishing the power of the signals that the crystal must eliminate.

There exists an alternative approach, albeit an active and more intricate one, which utilizes the technique of heterodyning. This method involves shifting the original frequency to a lower range where the desired bandwidth constitutes a larger fraction of it, and then shifting the output back to the original frequency. Operating at a relatively wider bandwidth at the lower frequency makes it easier to manage. Older AM radios implemented this technique without shifting back since they only required amplitude, which they could retrieve from the altered frequency.

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Historically, 450 kHz was a commonly used Intermediate Frequency (IF) for AM radios meant to capture the commercial AM band ranging from 550 kHz to 1.7 MHz. The tuning knob adjusted the local oscillator, which was required to be 450 kHz less than the reception frequency. The resultant signal would go through a 450 kHz narrow band filter and amplifier, necessitating about 20 kHz bandwidth, approximately 4.4% of 450 kHz. This was feasible with a few carefully calibrated factory-tuned components. In “super heterodyne” radios, the tuning knob also adjusted an L-C filter to selectively choose the RF frequency of interest. Notably, because of product modulation (the method of mixing the local oscillator with the filtered RF), there are effectively two RF frequencies that lead to the 450 kHz IF: the local oscillator plus 450 kHz (the desired RF frequency) and the local oscillator minus 450 kHz, identified as the “image” frequency. The original L-C filter for the RF needed to be sufficiently narrow to eliminate the image frequency prior to the heterodyning process.

Additionally, consider what functionality you expect from the final narrow band signal. If you aim to simply AM detect it, alternative approaches might be available, rather than starting with a very narrow band filter. It is unwise to pursue this without a clearer understanding of what exactly you intend to achieve, including the source of the 10 MHz signal, the type of modulation to be detected, the extent of out-of-band noise inherent in the input signal, and more.

Olin's observations hold true. Constructing such a high-Q filter with L-C passives at 10 MHz is impractical. A heterodyne-type system provides a viable solution, and it may actually be more accessible for DIY enthusiasts than assembling a crystal-lattice type of filter.

The challenge surrounding homebrewed crystal filters is the necessity of having a sufficient stock of crystals available to select those with desired characteristics. Each crystal must resonate at specific frequencies that are located to either side of your center frequency to yield the preferred passband and shape factors. Commercial crystal filter manufacturers typically have either internal crystal grinding and tuning capabilities or possess the sufficient sales volume to justify outsourcing it to specialized crystal producers.

Building a heterodyne-type of BPF is relatively straightforward, given the availability of user-friendly ICs such as the SA602 Gilbert Cell mixers that have been on the market for around 30 years. While a detailed schematic may be complex to provide, the general principle follows this layout:

10MHz Input-----SA602 #1----your 10kHz LC BPF-----SA602 #2-----LC LPF----BPF 10MHz Output | | | | -----------Local Oscillator--------

It's worth noting that heterodyned filtering is an established technique, hence there is ample information available on its workings (which are fairly straightforward but would require extensive detail to elucidate here). The additional low-pass filter at the output is crucial, given that frequency mixers generate a set of frequencies through both summation and difference, and one wants to retain only the components centered at 10 MHz, the intended BPF center frequency. This LPF successfully rejects the unwanted LO+10MHz component.

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