Bluetooth devices operate in the ISM band and communicate with each other via Gaussian Frequency Shift Keying (GFSK) digital frequency modulation technology. Time division multiplexing (TDD) is used between devices, and an extremely fast frequency hopping scheme is used to facilitate congestion. Improve link reliability in the band. For Bluetooth devices, the RF part is one of the main test content.
The Bluetooth RF design uses a variety of system architectures, ranging from traditional analog modulation based on IF systems to digital IQ modulator/demodulator configurations, but no matter which design configuration is used, it must be developed during product development. Solve the following problem:
·Regular requirements worldwide
·Bluetooth certification ·Simple and efficient manufacturing test ·Good compatibility with other manufacturers' products Bluetooth RF technologyBluetooth devices operate in the ISM band and typically operate on 79 channels between 2.402 GHz and 2.48 GHz. It uses a digital frequency modulation technique called 0.5BT Gaussian Frequency Shift Keying (GFSK) to communicate with each other. That is to say, shifting the carrier up by 157 kHz means "1", moving down 157 kHz means "0", the rate is 1 million symbols (or bits) / sec, and then setting the -3 dB bandwidth of the data filter to 500 kHz with "0.5". This limits the spectrum occupied by the radio.
The two devices communicate by time division multiplexing (TDD), and the transmitter and receiver alternately transmit in separate time intervals, that is, one next to the other, and an extremely fast frequency hopping scheme (1,600) is also used. Jump/second) to improve link reliability in crowded bands. The US Federal Communications Commission expects that band utilization will continue to increase, so reliability is the most basic requirement.
In the Bluetooth architecture shown in Figure 1, the receiver uses only one down-conversion. This type of design uses a simple local oscillator, the output is multiplied, and switches between the receiver and the transmitter. FSK allows direct VCO modulation, baseband data through a fixed time delay and no overshoot Gaussian filter, while pulse shaping is only used in the transmitter, phase-locked loop (PLL) can be used in the baseband with a sample-and-hold circuit or phase modulator Phase modulation. Usually the intermediate frequency is quite high to limit the physical size of the filter components, keeping the intermediate frequency away from the LO frequency, ensuring adequate image rejection. If the level is too high and the receiver input is overloaded, the antenna switch should be used.
Test itemsHere are some tests that apply to the RF portion of Bluetooth devices.
Power—The output amplifier is an option that undoubtedly increases the output power of a Class I (+20dBm) output amplifier. Although the level accuracy index is not required, excessive power output should be avoided to avoid unnecessary battery power consumption.
Regardless of whether the design provides power of +20dBm or lower, the receiver needs to have a received signal strength indication. The RSSI information allows different power devices to communicate with each other. The power slope in such designs can be achieved by controlling the bias current of the amplifier.
Unlike other TDMA systems such as DECT or GSM, Bluetooth spectrum testing is not limited to separate power control and modulation error tests, and its measurement interval must be long enough to capture the effects of slope and modulation. In practice this does not affect certification, and time gating measurements are of great value because they can quickly identify defects. Some designs use unspecified cycles before modulation begins, which is typically used for receiver preparation.
Frequency Error—All frequency measurements in the Bluetooth specification use a shorter 4 microsecond or 10 microsecond strobe cycle, which can cause uncertainty in the measurement results and can be understood from different angles. First, a narrow time opening means that the measurement bandwidth cutoff frequency is high, and various types of noise are introduced into the measurement. Secondly, the error mechanism should be considered. For example, in short interval measurement, the quantization noise from the measuring device or the oscillator sideband noise will account for A larger percentage, and these noise effects are averaged out over a longer measurement interval. Therefore, the design scope should consider this factor, which should exceed the static error generated by the reference crystal.
Frequency Drift—The drift measurement combines a short 10-bit adjacent data set with longer drift results across the pulse. This error can occur if a sample-and-hold design is used in the transmitter design. For other types of designs, a harmful 4kHz to 100kHz modulation component or noise like ripple can be observed on the waveform, indicating that it can be used as another method to ensure good decoupling of the power supply.
Modulation—In the transmit path, the VCO in Figure 1 is directly modulated. To avoid modulating components within the PLL stripping bandwidth, the transmission device can be opened or phase error corrected (two-point modulation). The sample-and-hold technique should be effective, but care must be taken to avoid frequency drift. Unless the digital technique is used to adjust the synthesizer's division ratio, the phase modulator should be calibrated to avoid the problem of low response flatness for different data pattern modulations.
The Bluetooth RF specification checks the peak frequency offsets of the two different patterns 11110000 and 10101010. The output of the GMSK modulation filter reaches the maximum value after 2.5 bits. The first code can check this, the cutoff point of the GMSK filter and The shape is checked by the second code. In the ideal case, the 1010 code peak offset is 88% of 11110000, and some designs transmit a higher ratio than 0.5BT Gaussian filtering. The highest basic modulation frequency is 500 kHz, and the bit rate at this time is 1 million symbols/second.
Insulated Power Cable,Bimetallic Crimp Lugs Cable,Pvc Copper Cable,Cable With Copper Tube Terminal
Taixing Longyi Terminals Co.,Ltd. , https://www.longyicopperlugs.com