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FM-quality sound achieved by audio Tx LSI

Posted: 16 Aug 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Norihiro Satsukawa? EE Times? THine Electronics? THG4649? FM transmitter LSI?

THine Electronics said its new chip will allow portable audio players to be enjoyed in more ways.

The chipmaker has developed THG4649, an FM transmitter LSI for audio, which claims to have the best sound quality in the industry. THine said the SNR of the FM modulating signal is at 70dB, which is said to be comparable to that of FM radio stations. The transmitter is targeted at portable audio players and cellphones.

THine expects applications to include wireless transmission of audio content, which are typically saved to portable devices and cellphones through automotive or home stereo equipment. However, portable devices equipped with FM transmission functions are already available in the market, thus preempting the arrival of the THine chip. Furthermore, several IC vendors and module manufacturers already supply FM transmitter LSIs and small-sized modules for this type of device.

According to the company, the trouble lies in the fact that in existing devices that use these parts, "sound quality deteriorates during FM transmission. We could only obtain sound quality that was even worse than what we get when listening with earphones or headphones." Few people actually used the kind of applications THine had anticipated. For this reason, THine decided to focus on sound quality. Its new FM transmitter also boasts high reproducibility of both high and low frequencies (treble and bass), although this does not appear in any specification values other than the SNR.

SiGe BiCMOS tech
The two-channel stereo analog audio signal is FM-modulated and output at frequencies used by FM radios. SiGe BiCMOS technology has been used in the hope of achieving high-quality sound. All circuits that affect sound quality have been constructed using bipolar technology. The only ones that have been constructed using CMOS are the logic circuits for control. Furthermore, the use of SiGe technology has allowed the company to improve the PLL circuits and PA characteristics of the wireless section.

The chip can also be easily embedded into the design of the device. Specifically, it has few external attachments and gives designers greater flexibility in choosing the antenna and wiring pattern for high-frequency signals.

According to THine, the only external attachments required are nine bypass capacitors. There is no need for external mounting of voltage control oscillators or PAs. Even without an externally mounted PA, high output power can be attained at a maximum of 118dBV (~ 12.6mW). A crystal oscillator is neither required if a clock signal of 10-44MHz is entered. Only the system-clock signal used by the device's other circuits needs to be supplied. However, a crystal oscillator of 10-44MHz output or a crystal resonator of 10-20MHz may be mounted externally if the clock signal cannot be supplied.

There is greater flexibility in the choice of antenna and wiring pattern because the output power can be controlled very precisely using the digital controls from the host microcomputer. Settings can be configured for 16 levels in increments of 1.5dB within the range 95.5-118dBV (~ 0.071-12.6mW). Accordingly, the output power can be adjusted to the desired value, regardless of whether the antenna properties or wiring-pattern characteristics differ from device to device.

The current consumption is 18mA (when output power is 95.5dBV) when transmitting and 10A when on standby. Power voltage is 2.7-3.45V. Three models have been supplied to account for the different transmission frequencies of FM radio bands in different regions. THG4649L is designed for Japanese FM frequencies of 76-90MHz, while THG4649H is for countries in Europe, North America and Asia, where FM frequencies range from 87.5MHz to 108MHz. Finally, THG4649M is targeted at intermediate frequencies between 80MHz and 100MHz.

- Norihiro Satsukawa
EE Times




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