Electronic Design

Upstream Cable Modem Line Driver Targets Tight DOCSIS Specifications

This class-A programmable-gain amplifier with a low-power disable mode exceeds DOCSIS 1.1 requirements for output power, noise, and distortion.

Since the rollout of proprietary cable-modem systems in the early nineties, the use of data transport over coaxial cables has expanded substantially. Today, market analysts predict that the count for cable-modem users far exceeds one million. Plus, with standards in place, these high-speed data transceivers have acquired further momentum. Their speed and power capabilities are exploited in emerging applications like IP telephony and voice-over-IP (VoIP) systems, in addition to a variety of fiber-optic distribution hubs and home-networking applications.

As these modems proliferate across a broad range of applications, their output power de- mands are getting more stringent while the noise and distortion requirements are getting tighter. All this means that the output amplifier stage of the cable modem must generate the requisite high-quality power with good linearity in order to drive the cable with minimal distortion at higher symbol rates. Furthermore, the amplifier must generate this output drive power without dissipating more so that the thermal management of the system will be eased.

With these latest needs of cable-modem designers in mind, Texas Instruments Inc. (TI) has developed a class-A programmable-gain amplifier (PGA). It's crafted to be compliant with the data-over-cable system interface specifications (DOCSIS) version 1.1, recently released by the Multimedia Cable Network System Partners (MCNS), a co-alition of cable-TV operators. Spe-cifically designed for the upstream data path, the THS6101 is one of the first amplifiers to be fully compliant with the DOCSIS specifications, claims TI. In fact, according to TI, the THS6101's output power, noise, and distortion performance far exceeds the DOCSIS 1.1 requirements. And, it achieves this compliance without requiring an external switch.

Interestingly, the THS6101 also is TI's first offering in the upstream communications market. According to Marco Corsi, senior member of the technical staff at the company, this mixed-signal amplifier is made possible by an advanced complementary bipolar process, which also enables the capability of integrating submicron CMOS logic on the same monolithic die. The company calls it BiCOM II. This process is specifically developed for a new generation of high-performance analog and mixed-signal ICs that require much faster transistors. The BiCOM II combines faster 3- to 15-V bipolar transistors with 0.7-µm CMOS logic to dramatically boost the integration density. At the same time, it keeps power consumption to a new low (Fig. 1).

Results reveal that the cutoff frequencies for the bipolar transistors are much higher. Additionally, there's a much higher product of beta (ß) and Early voltage. The pnp transistor offers an fT of 4.5 GHz and a ß of 90 with an Early voltage of 50. For the npn type, the fT is 6 GHz with 100 for ß and 150 for the Early voltage. The high Early voltage ensures a high intrinsic gain for the transistor.

Besides speed, the BiCOM II process features extremely low-noise performance. It also provides Schottky diodes, precision capacitors and resistors, fuses, and laser-trimmable metal links.

"Because digital logic and control is a key part of this proprietary multistage architecture, a low-power biCMOS process like BiCOM II becomes crucial for such a design," notes Corsi. It enables power-efficient analog and CMOS logic/control to reside side by side on the same compact chip without draining the battery. "With an earlier-generation complementary-bipolar process, such an integration was unthinkable," asserts Corsi. "In effect, the BiCOM II allows us to design an amplifier with good linearity and noise performance, but with insignificant consumption of power," he adds.

Because the cable modem is turned off most of the time, the THS6101 is built to draw a very small amount of quiescent current in the "sleep" mode when it's disabled. The upstream driver is disabled between bursts to save power and prevent other modems from polluting the channel.

Preliminary data sheets indicate that typical disable-mode quiescent current for this part is merely 2 mA, resulting in 10 mW of power consumption. Depending on the gain setting, the quiescent current could range anywhere from 11 mA to 175 mA when in the transmit mode. This enables the THS6101 to minimize power consumption for a given output.

At maximum gain setting, for instance, the amplifier is capable of providing +62 dBmV at its output. This accounts for nearly 4-dB losses encountered in the output transformer and the tuner to ensure a +58-dBmV voltage level on the line in compliance with the DOCSIS specifications. At a 10% transmit duty cycle, the amplifier's average power consumption at maximum gain is 98 mW and 16 mW at the minimum gain (Fig. 2). In comparison to other upstream drivers on the market, TI claims that the THS6101 offers the lowest average power consumption.

The chip operates from a single 5-V supply. This is an attractive feature for applications like lifeline telephone services in VoIP cable networks, wherein the device must be powered off the cable when there's a power outage. Furthermore, the low-power solution also will enable cable-service providers to minimize their battery backup costs.

To meet the wide output range of the DOCSIS standard, this PGA allows a 54-dB gain range. It's programmable in 6-dB steps via a compatible serial interface. Consequently, the gain of the amplifier can be set from +32 dB down to -22 dB via the serial interface with a 4-bit setting (see the table). The amplifier uses an 8-bit word to control both gain and power optimization. The last four bits of the 8-bit code represent the THS6101's gain. The most significant four bits represent the bias code that varies the power consumption ±25% within the nominal value.

Thus, the THS6101 provides nine coarse-gain steps of 6 dB for a full gain range of 54 dB. Fine-gain control required by DOCSIS is provided by the built-in variable attenuator of the upstream modulator of the cable modem. To produce the correct output power levels, though, the PGA needs nominal input voltage levels of +30 dBmV to +25 dBmV.

Dynamic Performance
This novel multistage design results in very low-noise and distortion characteristics for the amplifier too. This is especially the case at the low end of the output power level, where the specifications get tougher. The THS6101 exceeds the DOCSIS needs of a -53-dBc in-band noise level at a 160-ksymbol/s data rate. At minimum gain and a 160-ksymbol/s data rate, the cable line driver is rated to produce less than 10 nV/Ö—Hz of output noise, which translates to a level of -47 dBmV. That's significantly lower than the DOCSIS requirement of -45 dBmV.

In the disable mode, the noise performance is even better. The disable mode voltage noise is 2.0 nV/‾Hz, which translates to a -61 dBmV noise level. When set at maximum gain and minimum data rate, the noise level of the amplifier soars to 95 nV/ ‾Hz. Here again, the unit outperforms the DOCSIS standard for output noise level at maximum gain.

The distortion performance also is superior. The THS6101 architecture results in a very low third-order-harmonic distortion of -65 dBc at a 29.3-MHz input frequency and an output of +56 dBmV. Signal linearity is maintained at maximum output levels too, providing a third-harmonic distortion of -56 dBc at a 29.3-MHz input and a 62-dBmV output. For the two-tone intermodulation effects, the device exhibits a third-order-intercept point (IP3) of +85 dBmV at a frequency of 65 MHz and an output power level of +60 dBmV.

Because cable-modem transmission is burst-mode in nature, the output voltage transient is controlled when switching between transmit and disable modes. The unit ensures that when the modem is set to transmit at +55 dBmV, the glitch voltage is less than 100 mV. It decreases by a factor of two for each 6-dB reduction in the output-voltage level.

To accomplish such an upstream power amplifier with output power, noise, and distortion surpassing DOCSIS 1.1 specifications, TI's designers have developed a novel three-stage architecture. In essence, this scheme uses 10 parallel gain circuits, with each gain circuit comprising three stages (Fig. 3). The three-stage architecture involves a transconductance input stage, which converts differential input voltage into current output. This is followed by the two current-gain stages. Varying the input transconductance and subsequent current-gain blocks provides the gain range that the DOCSIS standard demands.

Each gain circuit is controlled individually to obtain the overall gain as well as the output power level. The gain control interface is connected to the first two stages. The open-collector output stage is externally matched to the 75-Ω cable through a 2:1 balun transformer. This transformer also converts the THS6101's differential output to a single-ended one for driving the cable (Fig. 4). The line is properly terminated with an external 300-Ω resistor.

Price & Availability
Available now, the THS6101 is slated to go into volume production by the end of this year. Operating from a single 5-V supply, it comes in a 24-pin PowerPAD TSSOP. In 1000-piece quantities, the THS6101 is priced at $3.55 each.

Texas Instruments Inc., Semiconductor Group, SC-00011, Literature Response Center, P.O. Box 954, Santa Clara, CA 91380; (800) 477-8924, ext. 4500; www.ti.com.

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