
Philips Semiconductors
Application note
AN2003
SA1630 IF transceiver demonstration board
1999 Jan 05
28
Solving Equation (92) for phase error yields:
Θ
= (K * 360) /
π
where K = 10
(SBS/20)
(93)
The datasheet minimum sideband suppression specification is –35 dBc. Inserting this into Equation (93) shows that the maximum amount of
quadrature phase error that can be tolerated while still meeting datasheet specifications and assuming there are no other errors in the system is
Θ
= 2.0 degrees
(94)
As shown previously, amplitude mismatch errors also contribute to decreasing sideband suppression performance, and therefore the actual
maximum amount of phase error that can be tolerated should be somewhat less than this.
Offset error only
In the discussions above, Equations (40) and (41) show the sideband frequency content as being independent of the offset error terms I(0) and
Q(0) and the offset error terms affect the carrier suppression performance only. So, Equation (39) need only be considered when evaluating the
effect of offset errors while assuming ideal quadrature phase and amplitude matching conditions. With this in mind, let’s make the following
assumptions:
G
e
= 1 (channel mixer gains and baseband input signal amplitudes are perfectly matched)
Θ
= 0 (there is no quadrature phase error)
(95)
(96)
I(0) not = Q(0) not = 0 (input offset error)
(97)
I = Q = M (the baseband input signal amplitudes are equal)
(98)
Substituting Equations (95) and (96) into Equations (42) and (43) gives:
A =
1
/
2
G * 1 * cos 0 =
1
/
2
G
B =
1
/
2
G * 1 * sin 0 = 0
(99)
(100)
Substituting Equations (97), (98), (99), and (100) into Equation (39) yields:
C = 2
1
/
2
G I(0) sin
ω
c
t + 0 + G Q(0) cos
ω
c
t = G I(0) sin
ω
c
t + G Q(0) cos
ω
c
t
(101)
Carrier Suppression(CS)
20log
mag(C)
mag(LSB)
(102)
The expression for LSB under ideal quadrature phase and amplitude matching conditions is shown in Equation (54).
From Equations (54) and (101):
mag(C)
G I(0)
2
Q(0)
2
(103)
mag (LSB) = G M
(104)
Substituting Equations (103) and (104) into Equation (102) gives:
CS
20log
G I(0)
2
Q(0)
2
GM
20log
I(0)
2
Q(0)
2
M
(105)
Let’s compare this equation to the SA1630 datasheet specifications for carrier suppression and differential peak-to-peak baseband input level.
If we assume that the I(0) and Q(0) are equal:
X(0) = I(0) = Q(0)
(106)
Substitute Equation (106) into Equation (105) and solve for an expression relating the maximum offset error as a function of carrier suppression
and input signal amplitude yields:
X(0)
KM
2
where K = 10
(CS/20)
(107)
Relating the RMS voltage M to an equivalent differential peak-to-peak baseband input level gives:
M
V
p
p
2
1
2
(108)
Substituting Equation (108) into Equation (107) gives:
X(0)
KV
p
p
4
where again K = 10
(CS/20)
(109)