FM1246

The FM1246 front-end is designed to receive both TV and FM stereo signals in the PC Multimedia environment. The units are available with separate 75 W inputs for TV and FM broadcast reception. The input connectors available are either standard phono or IEC type (female for TV, male for FM radio). The tuning, bandswitching and antenna selection are made through the I2C-bus.

The front-end covers the TV bands from 45.75 MHz to 855.25 MHz. The FM band covers the standard 87.50 MHz to 108.00 MHz. An RDS decoder can be connected to the AF-MPX output. The FM1246 meets the input immunity and radiation requirements of CENELEC.

The FM1246 consists of a TV tuner, an FM radio tuner and an IF section, all on a single PCB. The front-end is assembled in a metal housing made of a rectangular tin-plated steel frame, with front and rear covers, which have soldered contacts to the frame. The two phono or IEC antenna connectors (female for TV, male for FM radio) are mounted on one side of the frame for the TV/FM signal inputs. All other connections are made via pins at the bottom.

The TV IF section uses an intercarrier SAW filter, followed by a true-synchronous vision IF demodulator (PLL) IC. The analog AFC voltage is fed to the 5-level A/D converter in the PLL tuning IC, so that the AFC status can be read via the I²C-bus.

In the FM radio mode, the level detector and AFT functions are provide for the auto-search routines. The level detector and AFT status can be read from the A/D bits in the status-byte. The mute function is also provided to suppress the audio output signal, if required.

The tuner AGC for both TV and FM radio operation, is generated with a novel AGC detector which measures the IF signal level directly at the tuner IF output pins. As opposed to the  conventional AGC detector, this new circuit allows a higher take-over level and offers superior immunity against tuner overload.

The demodulation of the TV sound and FM radio IF(10.70 MHz) is done with a PLL demodulation circuit contained in the IF IC. In the FM radio mode, the multiplexed AF signal is then channelled to a stereo decoder IC which extracts the left/right analog signals. The multiplexed audio signal is also available at pin 25 to support RDS implementations. The 2nd IF TV sound output is provided to connect external (analog 2 carrier or digital) stereo processing.