Homepage - RF Cafe
Webmaster: Kirt Blattenberger | KB3UON | Sitemap | ©1996-2014     Visit RF Cafe on
      Menu below is just a small sample of what is here!          Visit RF Cafe on Facebook Visit RF Cafe on Twitter Visit RF Cafe on LinkedIn
Custom Search
More than 10,000 searchable pages indexed.
•−•  ••−•    −•−•  •−  ••−•  •
RF Cafe Morse Code >Hear It<

Crystal Filter - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


SDO
Post subject: Crystal Filter Posted: Fri Oct 31, 2008 7:42 am

Captain


Joined: Tue Aug 26, 2008 5:18 am
Posts: 22
Location: UK
Hi all,
I am new to low frequency design and presently designing channel selective module. I am using crystal filter and a gain stage in my design along with other components. Gain stage is matched to 50 ohm. Crystal filters are high impedance (around 900 ohm). IF frequency is 21MHz. What is normal practice to match crystal filter to 50ohm matched gain block?. I've tried lumped element matching net work and it seems to be working.
Thanks.
SDO


Top

markapexwireless
Post subject: Posted: Fri Oct 31, 2008 5:53 pm

Captain

Joined: Tue Jul 08, 2008 7:15 pm
Posts: 11
Location: Boulder Colorado
Hi,

One common method is to use "tapped capacitor" transformer which is essentially an inductor and two series caps in parallel with inductor. Amp output goes to tap between caps and filter goes to inductor. Cap ratio is square root of impedance transformation which in your case is 4.2. Composite L/C is tuned to 21.4 MHz. This network will provide additional BP filtering particularly away from 21.4 MHz IF.

Solid State Radio Engineering by Krauss, Bostian and Raab does a great job on these types of networks, but you can probably find detailed design guide on the internet.

Good Luck!!

- Mark -


Top

darcyrandall2004
Post subject: Posted: Mon Nov 03, 2008 1:10 am

Colonel


Joined: Tue Feb 27, 2007 6:16 am
Posts: 46
Hello,

I have some Matlab code that calculates the component values required for a Tapped Cap Match. Perhaps this will be of some help. Always check the results. Cheers

function [L,C1,C2]=Tapped_cap_match(r1x,r2x,Q,fo)
format long
%r1x and r2x are the source and load impedance respectively.
%no need to enter the conjugate values for max power transfer. Calcs automatically.
%r1x > r2x
%r1x-|-------------
% | | |C1
% |L| ---
% | | ---
% | |--------r2x
% | C2---
% | ---
% | |

%change r1x to a parallel combination of reactance and resistor
%NO VALUES SHOULD BE NEGATIVE!
wo=2*pi*fo;

Rs=real(r1x);
Xs=imag(r1x);

if Xs<0 %capacitive
C=abs(1/(Xs*wo));
Rin=(1+wo^2*C^2*Rs^2)/(wo^2*C^2*Rs);
Xc=(Rin*Rs-Rs^2)^.5;
Cp=C/(1+(Rs^2/Xc^2));
elseif Xs>0 %inductive
Ls=Xs/wo;
Rin=(Rs^2+wo^2*Ls^2)/(Rs);
Lp=(Rs^2+wo^2*Ls^2)/(wo^2*Ls);
else Rin=real(r1x)
end

Rs=real(r2x);
X2s=imag(r2x);

if X2s<0 %capacitive
C=abs(1/(X2s*wo));
R2=(1+wo^2*C^2*Rs^2)/(wo^2*C^2*Rs);
Xc=(R2*Rs-Rs^2)^.5;
C2p=C/(1+(Rs^2/Xc^2));
elseif X2s>0 %inductive
Ls=X2s/wo;
R2=(Rs^2+wo^2*Ls^2)/(Rs);
L2p=(Rs^2+wo^2*Ls^2)/(wo^2*Ls);
else R2=real(r2x)
end


L=Rin/(wo*Q); %calculate the inductance

%R2s= R2/(Q2^2+1)=Rin/(Q^2+1)

Q2=sqrt( (R2/Rin)*(Q^2+1)-1 );

C2=Q2/(wo*R2);

C1=C2*(Q2^2+1)/(Q*Q2-Q2^2);

%account for reactance of source and load impedances.
if Xs<0 %capacitive
L=1/(wo*j)*(1/(wo*L*j)-wo*Cp*j)^-1;
elseif Xs>0 %inductive
L=1/(wo*j)*(1/(wo*L*j)-1/(wo*Lp*j))^-1;
end


if X2s<0 %capacitive
C2=C2-C2p;
elseif X2s>0 %inductive
C2=(C2*wo*j-1/(wo*L2p*j))/(wo*j);
end


a=1/r1x+1/(wo*L*j)
b=1/a+1/(wo*C1*j)
c=1/b+wo*C2*j
d=1/c
r2x

_________________
Regards, Darcy Randall, Perth, Western Australia


Top

SDO
Post subject: Thank youPosted: Mon Nov 03, 2008 6:46 am

Captain


Joined: Tue Aug 26, 2008 5:18 am
Posts: 22
Location: UK
Thank you gentle men for your help.
Much appreciated.


Top

nubbage
Post subject: Posted: Mon Nov 10, 2008 5:58 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Darcy
To design a transformer, I need to translate the routine you wrote, because I do not have Matlab. I use Fortran, CalcEd and Mathcad for complex math.
What are the a, b, c and d variables at the end?
The Fortran equivalent compiled OK, but on running the *.exe file it throws up an "arithmatic error". I was running a 50MHz calc, with a Q of 5, with input Z of (75+j15) and an output Z of (25-j10).
Trev









Posted  11/12/2012
A Disruptive Web Presence

Custom Search
Over 10,000 pages indexed! (none duped or pirated)

Read About RF Cafe
Webmaster: Kirt Blattenberger
    KB3UON

RF Cafe Software

RF Cascade Workbook
RF Cascade Workbook is a very extensive system cascaded component Excel workbook that includes the standard Gain, NF, IP2, IP3, Psat calculations, input & output VSWR, noise BW, min/max tolerance, DC power cauculations, graphing of all RF parameters, and has a graphical block diagram tool. An extensive User's Guide is also included. - Only $35.
RF system analysis including
frequency conversion & filters

RF & EE Symbols Word
RF Stencils for Visio

Product & Service Directory
Personally Selected Manufacturers
RF Cafe T-Shirts & Mugs

RF Cafe Software

Calculator Workbook
RF Workbench
Smith Chart™ for Visio
Smith Chart™ for Excel
Your RF Cafe Progenitor & Webmaster
Click here to read about RF CafeKirt Blattenberger... single-handedly redefining what an
                                 engineering website should be.

View the YouTube RF Cafe Intro Video Carpe Diem! (Seize the Day!)

5CCG (5th MOB): My USAF radar shop

Airplanes and Rockets: My personal hobby website

Equine Kingdom: My daughter Sally's horse riding website