REV. 0

AD9870

–16–

Table IX. SPI Registers Associated with AGC

A

Bit

Default

(Hex)

Breakdown

Width

Value

Name

0x03

(7)

1

0

ATTEN

(6:0)

7

0x3F

AGCG(14:8)

0x04

(7:0)

8

0xFF

AGCG(7:0)

0x05

(7:4)

4

0

AGCA

(3:0)

4

0

AGCD

0x06

(7:4)

4

0

AGCO

(3:0)

4

0

AGCD

(2:0)

3

0

AGCR

System Noise Figure (NF) vs. VGA (or AGC) Control

The AD9870’s system NF is a strong function of the gain set-

ting of the VGA. The noise present at the output of the VGA

and input of the ADC is relatively large and independent of the

VGA setting. Under small signal conditions in which the VGA is

set to its maximum gain, this noise referred back to the input of

the LNA’s input has less of an effect on raising the AD9870’s

system NF. However, under large signal conditions in which the

gain of the VGA must be reduced to prevent ADC clipping, this

noise quickly becomes a significant contributor in determining

the AD9870’s overall NF. Figure 11 shows how the NF of the

AD9870 in AGC mode remains relatively constant as an inter-

ferer signal input power is increased until its power reaches a

programmed reference level (i.e., –3 dB) at which point the NF

degrades almost 1 dB per dB as the interferer signal is increased

beyond this point, forcing the VGA gain to decrease. As a result, it

is recommended that the AGC referenced level be set to 3 (i.e.,

AGCR = 1) to maintain the best possible NF over the widest

input signal range.

INTERFERER AMPLITUDE – dBm

45

24

–85

–20

–80 –75 –70 –65 –60 –55 –50 –45

–40 –35 –30 –25

42

27

21

18

36

30

39

33

15

12

9

NOISE FIGURE

MEAN AGC VALUE

ADC CLIPS

AT –24 dBm

150

100

50

0

–50

–100

–150

Figure 11. Noise Figure vs. Interferer Signal Level with an

IF = 73.35 MHz and CLK = 18 MSPS and AGCR = 1

DECIMATION FILTER

cascade of three linear phase FIR filters: DEC1, DEC2, and DEC3

as shown in Figure 12. DEC1 downsamples by a factor of 20

using a fourth-order comb filter. DEC2 also uses a fourth-

order comb filter, but its decimation factor is set by the M

control register. DEC3 is a decimate-by-3 FIR filter.

M

DEC2

FILTER

M + 1

DEC3

FIR

FILTER

3

I

Q

COMPLEX

DATA TO

SSI PORT

COS

SIN

DATA FROM

MODULATOR

DEC1

FILTER

20

Figure 12. Decimation Filter Architecture

Figure 13a shows the response of the complete decimation filter

on a linear frequency axis for frequencies up to the third alias.

As this figure shows, the alias with the least attenuation is

located at the lower end of the third alias band and has an

attenuation of 83 dB.

FREQUENCY – kHz

0

–40

–100

030

10

20

–20

–60

–80

70

40

60

50

FOLDING

POINT

5kHz PASSBAND

88dB ATTENUATION

(MIN)

83dB ATTENUATION

(MIN)

>100dB ATTENUATION

Showing the First Three Alias Bands

Figure 13b shows the full response of the decimation filter with

the decimation factor set to 60 on a logarithmic frequency

scale, while Figure 13c shows the folded frequency response

on a linear frequency scale and Figure 13d shows a blowup of

the passband. The location of the cutoff frequency shown in Figure

13b is inversely proportional to the decimation factor. However,

since both DEC1 and DEC2 are fourth-order comb filters, their

combination is also a fourth-order comb filter and thus the

shapes of the frequency responses shown in Figures 13c and

13d are independent of the decimation factor.