AD8326 datasheet(13/24 Pages) AD

13 / 24 page

REV. 0

AD8326

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Distortion, Adjacent Channel Power, and DOCSIS

In order to deliver +58 dBmV of high fidelity output power

required by DOCSIS, the PA is required to deliver up to

+67 dBmV. This added power is required to compensate for

losses associated with the transformer, diplexer, directional

coupler, and splitters that may be included in the upstream

path of the cable telephony. It should be noted that the AD8326

was characterized with the TOKO 617DB-A0070 transformer.

TPC 7, TPC 8, TPC 13, and TPC 14 show the AD8326 second

and third harmonic distortion performance versus fundamental

frequency for various output power levels. These figures are

useful for determining the in band harmonic levels from 5 MHz

to 65 MHz. Harmonics higher in frequency will be sharply

attenuated by the low-pass filter function of the diplexer.

Another measure of signal integrity is adjacent channel power,

commonly referred to as ACP. DOCSIS section 4.2.10.1.1

states, “Spurious emissions from a transmitted carrier may

occur in an adjacent channel that could be occupied by a carrier

of the same or different symbol rates.” TPC 9 shows the mea-

sured ACP for a +67 dBmV 16 QAM signal taken at the output

of the AD8326 evaluation board, through a 75

Ω to 50 Ω

matching pad (5.7 dB of loss). The transmit channel width

and adjacent channel width in TPC 9 correspond to symbol

rates of 160 KSYM/SEC. Table I shows the ACP results for

the AD8326 for all conditions in DOCSIS Table 4-7 “Adjacent

Channel Spurious Emissions.”

Table I. Adjacent Channel Power

Adjacent Channel Symbol Rate

Transmit

160K/s

320K/s

640K/s

1280K/s 2560K/s

Symbol

ACP

Rate

(dBc)

160K/s

–57

–59

–62

–63

–64

320K/s

–57

–58

–60

–62

–64

640K/s

–55

–58

–60

–62

1280K/s

–55

–57

–58

–60

2560K/s

–53

–56

–57

Noise and DOCSIS

At minimum gain, the AD8326 output noise spectral density is

13.3 nV/

√Hz measured at 10 MHz. DOCSIS Table 4-8, “Spuri-

ous Emissions in 5 MHz to 42 MHz,” specifies the output noise

for various symbol rates. The calculated noise power in dBmV

for 160 KSYM/SECOND is:

Comparing the computed noise power of –45.5 dBmV to the

+8 dBmV signal yields –53.5 dBc, which meets the required level

set forth in DOCSIS Table 4-8. As the AD8326 gain is increased

from this minimum value, the output signal increases at a faster

rate than the noise, resulting in a signal to noise ratio that improves

with gain. In transmit disable mode, the output noise spectral

density is 1.4 nV/

√Hz, which results in –65 dBmV when computed

over 160KSYM/SECOND.

The noise power was measured directly at the output of the

transformer. In a typical cable telephony application there will

be a 6 dB pad, or splitter, which will further attenuate the noise

by 6 dB.

Evaluation Board Features and Operation

The AD8326 evaluation boards (Part # AD8326ARE-EVAL

and AD8326ARP-EVAL) and control software can be used to

control the AD8326 upstream cable driver via the parallel port

of a PC. A standard printer cable connected between the paral-

lel port and the evaluation board is used to feed all the necessary

data to the AD8326 by means of the Windows 9X-based control

software. This package provides a means of evaluating the amplifier

by providing a convenient way to program the gain/attenuation

as well as offering easy control of the asynchronous TXEN and

SLEEP pins. With this evaluation kit, the AD8326 can be evalu-

ated in either a single-ended or differential input configuration.

The amplifier can also be evaluated with or without the PULSE

diplexer in the output signal path. A schematic of the evaluation

board is provided in Figure 11.

Output Transformer and Diplexer

A 1:1 transformer is needed to couple the differential outputs of

the AD8326 to the cable while maintaining a proper impedance

match. The specified transformer is available from TOKO (Part

# 617DB-A0070); however, M/A-COM part # ETC-1-1T may

also be used. The evaluation board is equipped with the TOKO

transformer, but is also designed to accept the M/A-COM trans-

former. The PULSE diplexer included on the evaluation board

provides a high-order low-pass filter function, typically used in the

upstream path. To remove the diplexer from the signal path,

remove the 0

Ω chip resistors at R7 and R19, and install a 0 Ω

chip resistor at R6 so the output signal is directed away from

the diplexer and toward the CABLE port of the evaluation

board (Figure 11). The ability of the PULSE diplexer to achieve

DOCSIS compliance is neither expressed nor implied by Analog

Devices Inc. Data on the diplexer should be obtained from

PULSE. When using the diplexer, be sure to properly terminate

the cable port (75

Ω) so that the AD8326 draws minimal current.

Overshoot on PC Printer Ports

The data lines on some PC parallel printer ports have excessive

overshoot that may cause communications problems when pre-

sented to the CLK pin of the AD8326. The evaluation board

was designed to accommodate a series resistor and shunt capaci-

tor (R2 and C2 in Figure 11) to filter the CLK signal if required.

Installing Visual Basic Control Software

Install the “CabDrive_26” software by running “setup.exe” on

disk one of the AD8326 Evaluation Software. Follow on-screen

directions and insert disk two when prompted. Choose instal-

lation directory, and then select the icon in the upper left to

complete installation.

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