RF filter design is a piece of cake these days thanks to computer design and simulation tools. But actually realizing the simulated filter response in the real world can be a completely different matter! This video provides an introduction to practical RF filter design by building, testing, and tweaking a 137MHz bandpass filter suitable for NOAA APT satellite reception.

References and additional reading:

Description | Reference |
---|---|

Designing a high-Q VHF bandpass filter | A VHF Bandpass Filter for the QST Spectrum Analyzer, Wes Hayward, W7ZOI |

Enameled wire capacitors | Capacitance of a Wire Above a Foil, Wes Hayward, W7ZOI |

Tutorial on double tuned bandpass filters | The Double-Tuned Filter: An Experimenter’s Tutorial, Wes Hayward, W7ZOI |

Air core inductor Q | The Elusive Q of Single-Layer Air-Core Coils, George Murphey |

Craig,

You are a good teacher. It is hard to find one who can explain RF techniques in simple terms.

Your videos need to reach a larger audience. Every maker space in America should show your video’s.

Good tutorial. My thoughts:

1. Self-supporting solenoidal inductors like the ones in your video can allow you to eliminate the variable capacitors. Many FM radios and VHF TV tuners use the aforementioned inductors in conjunction with fixed capacitors in order to minimize cost. The filters are then tuned by either compressing or expanding the inductors.

2. In commercial designs at VHF and above, it is a standard procedure to account for component parasitics and Q factor in the simulation. At microwaves, modeling the parasitics is essential whether commercial or for fun. See the article below:

http://www.cel.com/pdf/press/parasitics.pdf

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Hi Craig, I really enjoy a lot your excellent hands-on tutorials on RF design, and yor Jim Williams analog IQ tests. Please keep new videos coming !

sir how to change the center frequency by tweaking the capacitor , is it by both of them or by one

Both the tuned circuits have to be tuned for the desired center frequency, so you’d need to tweak both of the capacitors.

Hi, I would like to know how do you design de filter circuit? If it was on the Matlab itself or books, because I need to build a bandpass filter based on your filter design, but with center frequency at 140 MHz.

Thank you ðŸ˜€

I used a commercial design tool, but I’d recommend a free filter design tool like Elsie: http://www.tonnesoftware.com/elsie.html

This is a 137MHz Top C coupled shunt resonator Butterworth filter. CF is 137Mhz, 3dB BW 8MHz. The design impedance is about 2000 ohm, with source /termination matching element to match to 50 ohm. It can be easily designed with PCFILT, a window based Public domain software (Author is a professional Filter designer). The order of filter is 2. Using higher design impedance will yield a better Inductor Value (100nh) , which has better Q at this frequency.

To tune the actual design is where the real fun starts.

This filter is a 2nd order Top C coupled shunt resonator Butterworth filter. The CF frequency is 137MHz, 3dB BW 8MHz. The design impedance is about 2000 ohm, to get a good inductor value (100nH) at this frequency. There is a free domain PCFILT software (the author had kindly made it free open public domain, cheers to him) from ALK Engineering. You can design this filter using this software. It even helps to match the 2000 design impedance to 50 ohm, with matching sections.

You can get professional design at no cost. Check the freeware

thank you so much for your help.

you gave a wonderful understanding on all topics.

thanks again

i’m trying to design a rf filter, with high impedance at specific frequency, can anybody point me to which filter will do the job?