Using microcontrollers in CATV applications

Using microcontrollers in CATV applications--Page 2.
The mixer and IF subsystem receive the 38.9MHz IF input and provide RSSI (Receive Signal Strength Indicator) output (Voltage output) based on the signal strength of the IF input. For example, 40dBµV of signal input gives an RSSI of 1VDC while 60dBµV of signal input gives an RSSI of 2VDC, and so on. The RSSI is a linear voltage with respect to the input signal strength. The microcontroller detects the RSSI voltage through an external ADC and displays the strength of the RF signal on the LCD display.

An FM IF demodulator block demodulates the audio signal from the IF output and provides the audio signal to audio amplifier.

Microcontroller Block:

Typically, an 8- or 16-bit microcontroller is used for this application. External EEPROM on the board stores the last channel number as well as favorite channel numbers. The microcontroller displays the signal strength of the RF channel on the LCD or graphical display.

The user can tune any single TV channel using the microcontroller and display the strength of the RF signal on the LCD display or the signal strength of all the channels can be displayed on a bar graph using a graphical LCD.  The user can measure the different parameter of RF inputs (C/N ratio, TILT/SLOPE ratio) using the microcontroller and an external ADC, as well as measure the strength of audio carriers by re-tuning the tuner at the appropriate audio frequency. The user can hear the sound through the loudspeaker (1W) of any channel by tuning the tuner to the audio frequency of the particular channel. The user can hear the audible level tone in proportion to the level measured for hands-free operations. User can select any two channels above and below to discover undesired signal levels quickly.

The microcontroller controls the volume of the speaker through an external 8-bit DAC (Digital to Analog converter). It will increase, decrease or mute the volume of the speaker through this external DAC and audio amplifier.


Power Supply Block:
The power supply block consists of a 9V/12V rechargeable battery as a power source.  It uses an onboard LDO (Low dropout) regulator to step down the DC voltages. The microcontroller detects the battery voltage and provides status information (LOW BAT and CHARGING) using the front panel LEDs.

Microcontroller in Optical signal measurement uses below blocks:

Figure 3. Microcontroller in Optical signal measurement application.

Optical Input Section:

The optical input section receives the optical input through a shielded optical cable (ST, FC, SC type). The optical signal input range is -94dBm to -27dBm. The photodiode supports wavelengths of 1300 nm – 1550nm, 2.5 GHz. The optical signal level meter measures optical power at 1310nm and 1550nm wavelengths, with the selection of 1310nm and 1550nm done by the microcontroller.

The photodetector receives the optical signals at 1310nm and 1550nm wavelengths and convert them in to the RF signal; i.e., the photodetector converts the optical signal into a current output. This current output is converted into a voltage by external Transimpedance amplifiers (TIA). The microcontroller reads this voltage through external 8-/16-/20-bit ADCs.


Microcontroller Block:

Typically, an 8-/16-bit microcontroller is used for this application. External EEPROM on the board store reference power levels for each wavelength. The microcontroller displays the signal strength of the optical signal (dBm, dB, µW, and mW) on the LCD or graphical display.

Users can measure the different parameter of optical inputs (dBm, dB, µW, and mW) at 1310nm and 1550nm wavelengths, using an external TIA and ADC.  The microcontroller controls the tone of an external buzzer using an internal PWM. Users can access optical signal information thorough a UART interface. The microcontroller detects the battery voltage and provides its status (LOW BAT and CHARGING) on the front panel LEDs.