Datasheet

[ ]
[ ]
( ) ( )
0 d 0 d
0
0 0
0
d n
1
Vi(t) sin t t sin t 22.5 t
16
4 1 4
LO(t) sin t sin t 22.5
16
2
Vo(t) cos t t
é ù
æ ö
= w + + w = w + ° + w
ê ú
ç ÷
ê ú
è ø
ë û
é ù
æ ö
= w + = w + °
ê ú
ç ÷
p p
ê ú
è ø
ë û
= w + w
p
f
f
f
1
T
16
2
16
p
1
T
16
10
2
20log
p
AFE5808
SLOS688C –SEPTEMBER 2010–REVISED APRIL 2012 www.ti.com
From the above equations, the 3rd and 5th order harmonics from the LO can interface with the 3rd and 5th order
harmonic signals in the Vi(t); or the noise around the 3rd and 5th order harmonics in the Vi(t). Therefore the
mixer’s performance is degraded. In order to eliminate this side effect due to the square-wave demodulation, a
proprietary harmonic suppression circuit is implemented in the AFE5808. The 3rd and 5th harmonic components
from the LO can be suppressed by over 12 dB. Thus the LNA output noise around the 3rd and 5th order
harmonic bands will not be down-converted to base band. Hence, better noise figure is achieved. The conversion
loss of the mixer is about -4 dB which is derived from
The mixed current outputs of the 8 channels are summed together internally. An internal low noise operational
amplifier is used to convert the summed current to a voltage output. The internal summing amplifier is designed
to accomplish low power consumption, low noise, and ease of use. CW outputs from multiple AFE5808s can be
further combined on system board to implement a CW beamformer with more than 8 channels. More detail
information can be found in the application information section.
Multiple clock options are supported in the AFE5808 CW path. Two CW clock inputs are required: N × ƒ
cw
clock
and 1 × ƒ
cw
clock, where ƒ
cw
is the CW transmitting frequency and N could be 16, 8, 4, or 1. Users have the
flexibility to select the most convenient system clock solution for the AFE5808. In the 16 × ƒ
cw
and 8×fcw modes,
the 3rd and 5th harmonic suppression feature can be supported. Thus the 16 × ƒ
cw
and 8 × ƒ
cw
modes achieves
better performance than the 4 × ƒ
cw
and 1 × ƒ
cw
modes
16 × ƒ
cw
Mode
The 16 × ƒ
cw
mode achieves the best phase accuracy compared to other modes. It is the default mode for CW
operation. In this mode, 16 × ƒ
cw
and 1 × ƒ
cw
clocks are required. 16 × fcw generates LO signals with 16
accurate phases. Multiple AFE5808s can be synchronized by the 1 × ƒ
cw
, i.e. LO signals in multiple AFEs can
have the same starting phase. The phase noise spec is critical only for 16X clock. 1X clock is for synchronization
only and doesn’t require low phase noise. Please see the phase noise requirement in the section of application
information.
The top level clock distribution diagram is shown in the below Figure 71. Each mixer's clock is distributed through
a 16 × 8 cross-point switch. The inputs of the cross-point switch are 16 different phases of the 1x clock. It is
recommended to align the rising edges of the 1 x ƒ
cw
and 16 x ƒ
cw
clocks.
The cross-point switch distributes the clocks with appropriate phase delay to each mixer. For example, Vi(t) is a
received signal with a delay of , a delayed LO(t) should be applied to the mixer in order to compensate for
the delay. Thus a 22.5⁰ delayed clock, i.e. , is selected for this channel. The mathematic calculation is
expressed in the following equations:
(3)
Vo(t) represents the demodulated Doppler signal of each channel. When the doppler signals from N channels are
summed, the signal to noise ratio improves.
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