Implementing Image Calibration; Image Frequency Mechanism; Image Calibration Overview - Analog Devices AFDF7021 Application Note

Application Note

IMPLEMENTING IMAGE CALIBRATION

IMAGE FREQUENCY MECHANISM

Figure 2 shows two signals applied to the input of a quadrature
receiver.
cos (ω ) + cos (ω
SIG IMG
Figure 2. Quadrature Receiver
On the ADF7021 series of parts, the intermediate frequency,
�� − �� = �� = 100 kHz
ω , is set at 100 kHz. Equation1 describes this mechanism.
IF
������ ���� ����
If another signal is present at ω
is also be generated at −ω which interferes with the wanted
IF
signal at +ω . This is the image frequency. Equation 2 describes
IF
�� − �� = −�� = −100 kHz
this mechanism.
������ ���� ����
Figure 3 shows the relative location of these frequencies.
In theory, the image frequency component can be infinitely
rejected if the gain balance and phase orthogonality between
the I and Q paths are perfectly aligned. The image calibration
procedure identifies gain and phase settings which optimally
match the quadrature paths.
I
cos (ω
)
LO
)
Q
sin (ω
)
LO
, 200 kHz below ω , a signal
IMG LO
ω – ω
IMG LO
–100kHz DC
Figure 3. Frequencies for the

IMAGE CALIBRATION OVERVIEW

Image calibration is carried out by applying a tone at the image
frequency, monitoring the amplitude, and attenuating using the
gain and phase adjust register settings.
Figure 1 gives an overview of the image calibration functional
block on the ADF7021
(1)
the 7-bit on-chip ADC. The gain and phase are adjusted with a
digital register write to Register 5 and the RSSI readback is
repeated.
This process is repeated until gain and phase values are
identified which result in a minimum RSSI value.
(2)
A tone can be applied at the image frequency using an external
signal source or an internally generated tone. The method
for generating an internal tone is explained in detail in this
application note.
Identifying the optimum gain and phase values can be achieved
using a brute force method or the gradient descent algorithm.
The brute force method is simply an exhaustive sweep of all
possible gain and phase values. The gradient descent algorithm
is a tested algorithm for finding the RSSI minimum in fewer
operations than a brute force sweep. This algorithm is explained
in detail in this application note.
Rev. 0 | Page 3 of 12
ω – ω
SIG LO
100kHz
ω ω
100kHz
IMG
(ω
)
IF
ADF7021 Series of Parts
series of parts. The RSSI is read back via
AN-1258
100kHz
ω
LO SIG