ADL5961ACCZ-R2 datasheet(19/34 Pages) AD

19 / 34 page

Data Sheet

ADL5961

THEORY OF OPERATION

analog.com

Rev. 0 | 19 of 34

frequency offset of a few MHz between RF and LO up to 26.5 GHz

signal frequencies.

The frequency divider and frequency multipliers integrated into

the LO interface of the ADL5961 enable measurement sweeps

beyond the frequency range supported by the LO source itself,

typically a frequency synthesizer. When Bit 4 in Register 0x20 is

cleared (disabling bypass mode), the LOMODE bits (Register 0x20,

Bits[1:0]) can be used to program the LO multiplication factor, MLO

as shown in the following equation:

MLO=2LOMODE−1

(2)

That is, divide by 2, or multiply by 1, 2, or 4. To ensure that

the desired output signal downconverts at the desired IF output

frequency, fIF, the frequency supplied to the LO interface must meet

the following condition:

fLO= fRF±fIF/MLO

(3)

Note, as previously detailed in Table 1, that the divide-by-2 mode

can only be used for LO input frequencies up to 2.4 GHz, whereas

the doubler and quadrupler modes only operate from 2 GHz to

8 GHz and from 4 GHz to 8 GHz, respectively. The LO interface

also contains high-frequency filters that suppress the harmonics

and subharmonics in the multiplier outputs. The center frequency

of these filters can be programmed through Register 0x21 and

OFFSET FREQUENCY INTERFACE

Further simplification of the VNA configuration can be achieved

by employing the differential offset frequency interface. In this

configuration, a single swept source can be used to drive the RF

and LO interfaces at the same frequency (zero frequency offset),

while the IF output frequency is set by the signal applied to the

offset frequency interface. To enable the offset mixer, clear Bit 4

in Register 0x20 and program the OFMODE bits in Bits[3:2] in

contains a programmable divider with ratios of 1, 2, or 4. The

multiplication factor, MOF, is as shown in Table 7.

Table 7. Offset Input Configuration

Divide by

MOF

0.5

10 (Default)

0.25

Dividers off

Not applicable

The IF output signal frequency, including the offset mixer, is repre-

sented as follows:

fIF=fRF−MLO×fLO+MOF×fOF

(4)

For a true zero-offset sweep, the LO frequency, therefore, must

satisfy the following:

fLO=fRF/MLO

(5)

such that the first two terms of Equation 4 cancel out, and the IF

output frequency equals the following:

fIF=MOF×fOF

(6)

The MOF = 1/4 (OFMODE = 2) setting is particularly useful and

is the recommended OFMODE setting. When the offset frequency

interface is driven by the ADC sample clock frequency, fS, it centers

precisely the IF output signal in the first Nyquist zone of the

ADC, with 4× domain sample points per completed cycle of the IF

waveform. In this mode, the discrete time nature of the IF waveform

becomes evident due to the divide by 4 digital dividing of the offset

input.

IF SIGNAL PATH

The IF output signal of the mixers is passed through LPFs to

remove unwanted mixing products and noise. The bandwidth of

these filters is SPI-programmable through Register 0x25. The same

bandwidth setting is applied to both ADL5961 IF channels.

The IF amplifiers that follow the LPFs have individually SPI-pro-

grammable gain, adjustable in 6 dB steps. This programmable

gain enables optimal interfacing of both channels to the ADC input

dynamic range.

The IF output interfaces of the ADL5961 are suited to drive a wide

range of ADCs directly. To avoid aliasing of broadband noise, it

is recommended to insert a simple antialiasing filter as shown in

Figure 53.

Figure 53. Interfacing the ADL5961 to an ADC

Each of the differential IF output nodes are low source impedance.

For this reason, it is recommended to use series resistors when

necessary, such as for driving filters or highly capacitive loads or

cables, as shown on the test circuit (see Figure 72).

ADL5961ACCZ-R2 数据表(PDF) 19 Page - Analog Devices

ADL5961ACCZ-R2 数据表(HTML) 19 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

数据表下载