FUNCTION CORRELOGRAMPSD,X,Y,LAG,T,NF,R
;+
; PSD=CORRELOGRAMPSD(X,Y,LAG,T,NF)
; computes the power spectrum density by
; the Blackman-Tukey correlogram method.
;
; INPUTS
; X		- A Sequence of length NX
; Y		- A Sequence of lengty NY
; LAG	- The maximum lag to be used.
; T		- The interval between samples
; NF	- The number of frequency points
;
; OUTPUT
; PSD = Power Spectral Density estimate at NF points
;
; Notes:
; The length of the correlation sequence is
; N=NX<NY. The minimum is used.
;
; H. Rhody
; May, 1998
;
; REFERENCE
;	S. L. Marple,
;	Digital Spectral Analysis with Applications,
;	Prentice Hall, 1987
; 	Appendix 5.B
;-

lags=FINDGEN(LAG)+1
lags=[lags-LAG-1,0,lags]
w=0.538+0.462*cos(!pi*lags/LAG)

NX=N_ELEMENTS(X)
NY=N_ELEMENTS(Y)
N=NX<NY
R=C_CORRELATE(X[0:N-1],Y[0:N-1],lags,/COVARIANCE)
R=R*N/(N-ABS(lags))
R=R*w

S=[R[LAG:2*LAG],FLTARR(NF-2*LAG-1),R[0:LAG-1]]
PSD=ABS(FFT(S))
RETURN,SHIFT(PSD,NF/2)
END