Upstream fiber links in a community antenna television (CATV) system are usually among the most difficult elements of the network to align properly. Set-top boxes and cable modems employ "long-loop" automatic gain control (AGC) (in other words, far-end power-control), which attempts to normalize the RF path end-to-end.
Alignment is very important because for every decibel of uncorrected optical loss, there's a 2-dB optical-modulation-index (OMI) variance at the transmitting node. This can easily lead to intermodulation problems when multiple services depend on simultaneous use of the link. The main alignment difficulty is providing an accurate test signal at the remote node so that the head-end receiver can be adjusted to account for the actual link-loss encountered. Compounding the problem is additional uncertainty in the OMI that's generated by the test signals.
One elegant solution is to treat the dc photocurrent in the receiver as a calibrated 50% OMI test signal and set the receiver RF gain accordingly. Historically, this has been difficult because the variable-gain or variable-loss element (for example, PIN diodes) was a bandpass structure and not able to simultaneously scale signal and dc together. However, this is possible using the LT1256 current-feedback op amp with voltage-controlled gain capability.
The op amp lends itself nicely to the upstream receiver function for several reasons. First, its frequency response is flat to over 30 MHz when used at unity gain (for the full gain setting). In addition, it offers a true high-impedance input to minimize loading of the photodetector. The LT1256 is connected, along with a dc loop-integrator, to form an OMI-stabilized AGC function. Locking the AGC to the "dc-pilot" signal ensures long-loop power controls will establish predictable payload OMIs, and with a greater degree of precision and uniformity than a manual adjustment process. A further benefit is rapid no-hassle system installation and low-downtime unit replacement procedures.
In the receiver circuit, a suitable optical detector (for example, JDS EPM705) delivers photocurrent into a nominal 1-kΩ load resistance and drives an LT1256 noninverting input (see the figure). The LT1256 is configured to provide a voltage gain range of 0.2 to 1.0 with a linear control voltage input of 0.5 to 2.5 V, respectively. The bandwidth of the amplifier is relatively invariant over this 14-dB gain range, providing dc to 30 MHz response with ±1-dB flatness. Harmonic distortion is generally below −50 dBc for 15% OMI signals, which is well under the system noise floor for modulation bandwidths typical of upstream signals.
A precision op amp filters and integrates the dc component of the amplifier output, forming the AGC control signal. The AGC action servos the dc output level of the LT1256 to 120 mV, which represents the signal magnitude corresponding to 50% OMI. In addition, the LT1256 output is ac-coupled to a 12-dB attenuator/matching network. This provides an appropriate 75-Ω RF output level that's related only to OMI, not the nature of any payloads (present or not) on the link.