The block diagram below provides a detailed view of the Bluetooth transmitter circuits being developed this semester.  This transmitter is designed as a companion to the Bluetooth receiver designed and fabricated during the prior year.  Details of the prior year's design can be found here.
 
 

Transmitter Architecture

The transmitter employs a heterodyne approach to create the 2.4 GHz output signal, reusing and modifying several components from the receiver design.  In particular, the low frequency VCO (126 MHz nominal) is a modified version of that used in the previous class's receiver, with a modulation input added.  The synthesizer loop elements surrounding this VCO are also modified to allow open-loop modulation to be performed.

The VCO is modulated by data supplied at the TX Data input.  This data is converted to differential form and then filtered with an approximation of a Gaussian waveshape to meet Bluetooth spectrum masks.  A GainCal input is provided as a one-time-trim on the modulation index during manufacture/test.

The FM-modulated IF signal produced by the 126 MHz VCO is then up-converted to 2.4 GHz using an SSB upconversion mixer.  This design is similar to, but not identical to, the image reject mixers used previously in the receiver design.  Modifications include a revision of the architecture to address the difference between SSB and image-reject requirements and use of poly-phase shifting networks to achieve high attenuation of the undesired product.

The up-converted signal is delivered to a revised version of the Q-enhanced filtering circuits used in the receiver.  The filter is used in this case to increase rejection of the unwanted mixing product, and (in conjunction with the matching network) to achieve the necessary attenuation of harmonics.  A transmit/receive (T/R) switch is added to decouple the transmit filter/amplifier circuit from the receive circuits when the transmitter is not active, providing the best possible receive noise figure.  The high-frequency VCO (2.57 GHz nominal) which is used by the mixer is reused from the receiver circuits, as are surrounding synthesis components.

Finally, an ALC loop is added to level the output amplitude from the tranmitter to the desired 0 dBm (class-3) Bluetooth requirement.  The ALC loop feeds back to the Q-control (amplitude-control) input on the low-frequency VCO, providing the added benefit of keeping this VCO operating in its linear range for good frequency-control, power consumption, and phase-noise performance.
 
 

Last Update:  08 May 2001