Using microcontrollers in CATV applications

Microcontrollers in current Cable Television (CATV) applications are mainly used for two functions: RF signal level measurement and optical signal level measurement. In the CATV industry, CATV operators use RF Transmitter (RF Modulator) and RF combiners to transmit the RF signal from operator end to the customer end. CATV operators use RF repeaters (RF amplifiers) to avoid RF signal losses. An RF signal Level meter is used to measure the strength of RF signal and different RF parameters at both the operator end and user end.

 

In CATV applications, the RF signal transmission can be replaced by an optical signal transmission. There will be loss in RF signal and Video and Audio signal quality while transmitting the RF signal over long distances. For this reason, CATV operators use an optical transmitter that converts the RF signal into an optical signal and transmits the RF information at 1310nm and 1550nm optical wavelengths. CATV operators use optical receivers at receiver end to convert the optical signal and the RF information it contains back into an RF signal that will go to the Television (TV) input. An optical signal level meter is used to measure the strength of the optical signal at 1310nm and 1550nm wavelengths.

 

For RF signal level measurement, a microcontroller measures the strength of the RF signal (dBµV/ dBmV) and Audio carrier signal (dBµV/ dBmV). It also measures different RF parameters like C/N ratio and TILT/SLOPE, controls the buzzer feature for audible level of tone in proportion to measured level for hands-free operations, measures RF AC line input voltage, measures audio quality to control audio speaker (sound) output, manages the front panel mechanical keypad, and displays the results on the LCD display. 

 

For Optical signal level measurement: a microcontroller measures the strength of the optical signal (dBm, dB, µW & mW) at different optical wavelengths (1310nm & 1550nm) as well as performs different optical signal measurements. 

 

This article will discuss the uses of microcontrollers in CATV applications, in addition to various design techniques and design challenges associated with their use.

 

Microcontroller in RF signal measurement uses below blocks:

Figure 1. Microcontroller in RF signal measurement application.

Cable Tuner Block:

The cable tuner block receives RF inputs through a shielded RF cable. The RF input frequency range is 48MHz to 860MHz. The input RF signal strength is approximately 30dBµV to 120dBµV with a line impedance of 75E. The RF input also goes to the switchable 0dBµV/ 40dBµV RF attenuator, which is controlled by the microcontroller. 40dBµV attenuation is switched ON to measure the noise level of the input signal.

RF input signal has an AC line input voltage of 0 to 100VAC, and an RF AC line voltage converted into the DC voltage by onboard rectifiers. The microcontroller measures the AC line input voltage and displays the voltage reading on the LCD display.  

The microcontroller communicates with the TV tuner through an I2C (SCL, SDA) interface and adjusts the Analog and Cable Tuner to the appropriate IF (Intermediate frequency) frequency, which is 38.9MHz for PAL (Phase alternate Line) region.

The tuner block provides amplification to the RF signal level. The user selects television channels using a front panel keypad. If the user selects Channel #1, for example, it is 48.25MHz, so the microcontroller tunes the local oscillator at 87.15MHz (48.25MHz + 38.9MHz = 87.15MHz) using the I2C interface. The output of the Mixer will be 38.9MHz (87.15MHz - 48.25MHz = 38.9MHz), which is the IF frequency. The tuner will provide the IF with Video and Audio information of Channel#1. The user can fine-tune any channel using remote control by adjusting the local oscillator frequency. The TV tuner has a step size of 31.25KHz, 62.5KHz, and 50KHz.  The microcontroller can increase or decrease the frequency of the local oscillator by ±31.25KHz, based on the fine-tuning by the user. When the user selects the channel search function, the microcontroller tunes the local oscillator to the respective frequencies.

Figure 2. Cable Tuner Block.

Mixer and IF Subsystem Block:

The output of the tuner is the IF frequency, and this output will pass through an IF Band pass Filter (BPF) which is used to fine-tune the bandwidth of IF frequency and also provide attenuation to the RF signal level. For example, if the tuner provides a gain of 40dBµV, then the filter section provides attenuation of 40dBµV so the input of the mixer and IF subsystem will be the same as the RF input.