Kaben’s Bluetooth Transmitter
A more detailed look at Kaben’s Polar Loop Bluetooth transmitter architecture is provided in the diagram below. Here the complex (baseband) signal to be transmitted is first converted from Cartesian representation (I & Q) to Polar representation (Magnitude and Phase). The Phase component is fed through a Delta-Sigma Modulator to the (digital) Phase Detector of a Frequency Synthesizer loop. The Phase component of the modulating signal is thus superimposed onto the Voltage Controlled Oscillator output. Meanwhile, the Magnitude component of the modulating signal is held in memory to match the latency of the Phase component traveling through the synthesizer loop, converted to a (baseband) analog signal, and applied as an Automatic Gain Control signal to the output Power Amplifier. This Polar Modulation architecture embraces the frequency agility of a synthesizer loop, while enjoying the well known ultra-low phase-noise and spur-level of Kaben’s frequency synthesizers.
For the transmitter, phase noise and jitter are not the only impairments to the desired transmit signal. Gain and phase variation across the signal bandwidth are important, as are AM / AM and AM / PM distortion arising from the nonlinear Power Amplifier and from the non-ideal polar decomposition. Further, any relative delay between the phase and amplitude paths and quantization noise from the two Digital-to-Analog Converters are important.
The Bluetooth Standard spectral mask limits, and the output spectrum from an ideal, 4 dBm output power Polar Loop transmitter are shown in the top diagram below. As can be seen, the out-of-band emissions are always more than 10 dB below the required level. The bottom diagram shows the spectral mask limits and the output spectrum from a 4 dBm output power Polar Loop transmitter having significant impairments: a relative delay of +/- 3 samples and a quantization error of 6 bits. As can be seen, the spectral mask limits are still met.
The ideal transmit constellation for differential 8PSK modulation is shown in the top diagram. Here, the effect of finite symbol duration manifests itself as eight clusters of output phases / amplitudes, rather than eight distinct points. Including the above relative delay and quantization error, the 8PSK transmit constellation becomes that in the bottom diagram. As can be seen, the amplitude quantization has the most visible effect of stratifying the eight clusters of points.
The receiver constellations for the ideal and the impaired transmitter constellations are shown next. As can be seen, the eight clusters of output phases / amplitudes from the ideal transmitter are reduced to eight distinct points. Further, the stratified clusters from the impaired transmitter (having a relative delay of +/- 3 samples and a quantization error of 6 bits) collapses into eight tightly grouped clusters. These tight clusters are equivalent to a Differential Error Vector Magnitude value of -23.5 dB below the signal power.
