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