function ValDB = db(X, Y, Z )
% Convert a value to decibel.
% Arguments:
%
% X ... the ratio to be converted to decibel
% Y ... second argument, either a mode string ('Power', 'power', 'Voltage', 'voltage') or a numeric resistor value
% Z ... third argument, numeric resistor value

% Constant strings that select the mode of operation
VOLTAGE_NAMES   = {'Voltage', 'voltage'};
POWER_NAMES = {'Power', 'power'};
USAGE       = 'Placeholder for a very verbose desciption.';

if(nargin == 1)
    U='voltage';
    R = 1;
elseif(nargin == 2)

    if(ischar(Y))
        U = Y;
        R = 1;

    elseif(isnumeric(Y))
        U = 'voltage';
        R = Y;
    else
        disp USAGE
        return
    end
elseif(nargin == 3)

    if( ischar(Y) && isnumeric(Z) )
        U = Y;
        R = Z;
    else
        disp USAGE
        return
    end
else
    disp USAGE
    return
end

switch U
    case VOLTAGE_NAMES
        Factor = 20;
    case POWER_NAMES
        Factor = 10;
        R = 1;      % Ignore any value for R
    otherwise
        disp( USAGE );
        return
end

% Convert ratio to decibel
ValDB = Factor * log10( X ) - 10 * log10(R);

end