## Copyright (C) 2018 Jens Ahrensfeld
## 
## This program is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or
## (at your option) any later version.
## 
## This program is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
## GNU General Public License for more details.
## 
## You should have received a copy of the GNU General Public License
## along with this program.  If not, see <http://www.gnu.org/licenses/>.

## -*- texinfo -*- 
## @deftypefn {Function File} {@var{retval} =} tunable_bandpass (@var{input1}, @var{input2})
##
## @seealso{}
## @end deftypefn

## Author: Jens Ahrensfeld <ahrensfeld@w2ess001vm>
## Created: 2018-11-26
## taken from: https://www.mwrf.com/components/bandpass-filters-feature-wide-tuning-ranges

function tuneable_bandpass(varargin)

params = struct ('f_low_Hz', 40e6, 'f_high_Hz', 800e6, 'f_step_Hz', 1e6, 'B_geo_Hz', 10e6, 'R_geo', 500, 'R_0', 50, 'k_gamma', 1.0);
units = struct ('f_low_Hz', 'Hz', 'f_high_Hz', 'Hz', 'f_step_Hz', 'Hz', 'B_geo_Hz', 'Hz', 'R_geo', 'O', 'R_0', 'O', 'k_gamma', '');

% Parse parameters
names = fieldnames(params);
for k=1:nargin-1,
	for n=1:length(names),
		if strcmpi(varargin{k}, names{n})
			params.(names{n}) = varargin{k+1};
		end
	end
end

print_parameters(params, units)

f_low_Hz = params.f_low_Hz;		% Minimum tunable filter center frequency (Hz)
f_high_Hz = params.f_high_Hz;	% Maximum tunable filter center frequency (Hz)

B_geo_Hz = params.B_geo_Hz;		% -3 dB RF filter bandwidth (in Hz) at the geometric center frequency
R_geo = params.R_geo; 			% Internal filter impedance (in O) at the geometric center frequency of the filter
R_0 = params.R_0;				% Port impedance, usually taken as 50 O

k_gamma = params.k_gamma;		% Constant value. A value of ? = 1.0 results in a constant-Q filter design,
								% while a value of ? = 2.0 results in a constant-bandwidth filter design.
								% A value between one and two will yield a blend of the two filter characteristics.
								% Inductor Q values will be more critical as the value of ? is increased. 

f_step_Hz = params.f_step_Hz;					
f_Hz = f_low_Hz:f_step_Hz:f_high_Hz;
omega = 2*pi*f_Hz;

f_geo_Hz = sqrt(f_low_Hz*f_high_Hz)			% Hz
omega_geo = 2*pi*f_geo_Hz				 	% rad/sec

% Design I.
R_t = R_geo*(omega/omega_geo).^k_gamma;		% Equ. (1)
L_c = R_geo/omega_geo						% Equ. (2)

C_tgeo = 1./(pi*sqrt(2)*R_geo*B_geo_Hz)		% Equ. (3)
L_eff = 1./(omega_geo^2*C_tgeo)			% Equ. (4)
L_r = L_eff * L_c / (L_c - L_eff)			% Equ. (5)

C_t = (omega_geo./omega).^2 * C_tgeo;		% Equ. (6)

% Tuning capacitances C1 and C2 are given by Eqs. 7 and 8
C_1 = (omega*R_0).^(-1) .* sqrt((omega_geo./omega).^k_gamma * (R_0/R_geo) + omega.^2.*C_t.^2*R_0*R_geo.*(omega/omega_geo).^k_gamma - 1);			% Equ. (7)
C_2 = ((omega.^2.*C_t)./((omega_geo./omega).^(2*k_gamma) .* (1/R_geo)^2 + (omega.*C_t).^2) - (omega.^2*R_0^2.*C_1)./(1+(omega*R_0.*C_1).^2)).^(-1); % Equ. (8)

C_1_geo = (omega_geo*R_0).^(-1) .* sqrt((omega_geo./omega_geo).^k_gamma * (R_0/R_geo) + omega_geo.^2.*C_tgeo.^2*R_0*R_geo.*(omega_geo/omega_geo).^k_gamma - 1)			% Equ. (7)
C_2_geo = ((omega_geo.^2.*C_tgeo)./((omega_geo./omega_geo).^(2*k_gamma) .* (1/R_geo)^2 + (omega_geo.*C_tgeo).^2) - (omega_geo.^2*R_0^2.*C_1_geo)./(1+(omega_geo*R_0.*C_1_geo).^2)).^(-1) % Equ. (8)

subplot (2,1,1)
plot(f_Hz*1e-6, C_1*1e12); grid; xlabel("f/MHz"), ylabel("C_1 [pF]");
subplot (2,1,2)
plot(f_Hz*1e-6, C_2*1e12); grid; xlabel("f/MHz"), ylabel("C_2 [pF]");

% Design II.
L_c = R_geo/omega_geo;						% Equ. (9)
L_eff = R_geo./(pi*sqrt(2)*B_geo_Hz);			% Equ. (10)
L_r = L_eff - L_c;							% Equ. (11)
C_tgeo = 1./(L_eff*omega_geo^2);			% Equ. (12)
C_t = (omega_geo./omega).^2 * C_tgeo;		% Equ. (13)
a = -omega.^2*R_0.*R_t;						% Equ. (14)
b = 1./C_t;									% ...
d = R_t/R_0;								% Equ. (14)
alpha = (d-b.^2/a)./(1+b.^2/a - d);			% Equ. (15)
beta = (b./a)./(1 + b.^2/a - d);			% Equ. (16)
C_2 = alpha.*C_1 + beta;						% Equ. (17)

endfunction