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Direct-Coupled-Resonator-Bandpass.php   17030 Bytes    06-06-2016 22:02:28


Direct Coupled Resonator Bandpass Filter Designer




Direct Coupled Resonator Bandpass


REQUIREMENTS


Center Frequency [MHz]

Bandwidth [MHz]

   

Passband Ripple [dB]

  (0...6)

Number of Resonators

   

Impedance [Ω]

Inductance  




➤ If the Ripple is 0 dB, a Butterworth Characteristic results. If larger : Chebyshev.



DESIGN DATA FOR YOUR BANDPASS





Inspired by the paper "Direct-Coupled Resonator Filters" by Seymour B. Cohn, June 1956.

If you want to build a filter like this one, we highly recommend, that you simulate the amplitude response first ! Due to the capacitive coupling, the attenuation above the passband is rather poor if you choose the bandwidth too large. We suggest to start with a bandwidth not larger than 10 %.




Example from practice




For an Receiver, we want to design an IF Bandpass with the following key data : Center Frequency F = 270 MHz, Bandwidth B = 10 MHz, Ripple R = 0 dB (Butterworth), System Impedance R = 50 Ω, 2 resonators, allow the Calculator to choose inductors from E12 range. As we want to use standard values (capacitors and inductors), we do play a little a sophisticated guess and choose L = 27 nH. (Switch to "use my value"). The Designer finally suggests the following values :

Direct Coupled Resonator Bandpass Filter Designer
http://www.changpuak.ch/electronics/Direct-Coupled-Resonator-Bandpass.php
Javascript Version : 27. Jan 2014
-----------------------------------------------------------------------------
Design Data for a 2-Resonator Bandpass Filter.
Center Frequency : 270.0 MHz
Bandwidth : 12.0 MHz
Passband Ripple : 0 dB (Butterworth Characteristic)
System Impedance : 50 Ohm
-----------------------------------------------------------------------------
Coupling Capacitor : 2.22 pF
Resonator #1 C : 10.32 pF // L : 27.00 nH
Coupling Capacitor : 0.40 pF
Resonator #2 C : 10.32 pF // L : 27.00 nH
Coupling Capacitor : 2.22 pF
-----------------------------------------------------------------------------
Please verify by simulation that attenuation above passband is sufficient.
Negative capacitances indicate an unhappy inductance.


Approach 1   

Uses 2.2 pF SQCB low ESR capacitors from ACX RF, TOKO LL2012-FHL Chip Inductors, muRata 3-10 pF Trimmer and a 'wire' capacitor for coupling. Insertion Loss was 5.7 dB.

Direct Coupled Resonator Bandpass


Direct Coupled Resonator Bandpass

Approach 2   

Uses 2.2 pF SQCB low ESR capacitors from ACX RF, TOKO LLQ2012 Wire Wound Chip Inductors, muRata 3-10 pF Trimmer and a 2 x 1 pF in series to do the coupling, as the 'wired' capacitor was somehow tricky to handle.

Direct Coupled Resonator Bandpass

Direct Coupled Resonator Bandpass

Approach 3   

Uses 2.2 pF SQCB low ESR capacitors from ACX RF, muRata 3-10 pF Trimmer and a 2 x 1 pF in series to do the coupling.
DIY coils (n=4, d=4). Insertion Loss was 1.3 dB.

Direct Coupled Resonator Bandpass

Direct Coupled Resonator Bandpass


The insertion loss of the board was not calibrated out to get nicer measurements. The idea was to show, that the calculator delivers useful values :-)

And yes, the bandwidth is larger. This is due to the (slightly) different coupling-capacitor. We therefore introduced the bandwidth-tuning-buttons to adjust for a minimum value of e.g. 1 pF.

The larger bandwidth was also caused by an error in the javascript. We would like to thank Gregory for his valuable feedback on this. MUITO OBRIGADO !!!




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t1 = 3876 d

t2 = 539 ms

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