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ML2712EH Datasheet(PDF) 9 Page - Micro Linear Corporation |
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ML2712EH Datasheet(HTML) 9 Page - Micro Linear Corporation |
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9 / 22 page  ML2712 9 January, 2000 PRELIMINARY PRELIMINARY DATASHEET Any modulation on SMOD is duplicated at the final frequency, provided is it inside the control loop bandwidth. The spectrum of SMOD and the final output frequency are shown in Figure 4. PSD F1LO 1IF SMOD FREQUENCY 1IF TRANSMIT SIGNAL Figure 4. Simplified Spectrum of Directly Modulated VCO Transmitter The PLL loop bandwidth is at least four times that of the modulation rate. Meeting the IEEE802.11 FHSS specification for 1Msymbol/sec requires a PLL control loop bandwidth greater than 4MHz. To achieve the Wideband PLL dynamics and the RF channel spacing requirements, the Transmit VCO is down converted with the 1LO signal (at Frequency F1LO). The RF channel spacing can be achieved by stepping the 1LO VCO. This allows the same VCOs and PLLs to be used in both Transmit and Receive Modes. For typical WLAN operation the receive frequency and transmit frequency are the same (between frequency hops). Therefore the 1LO (and 2LO) frequencies do not require re-tuning when switching from transmit to receive. As a result the transmit to receive turn- round time is very rapid. It is determined by the power on, settling, and lock up times of the Wideband PLL. For 802.11 FH systems the requirement is less than 2 msec. TRANSMITTER PLL The transmitter wideband PLL is shown in Figure 5. An external VCO provides a nominal 2.45GHz signal which is coupled to the Transmit RF mixer using a directional coupler or similar external circuit. The transmiter RF mixer is used to down-convert the transmiter VCO signal using the frequency doubled 1LO VCO to ML2713 ML2712 ANTI ALIAS FILTER SAW IF TX RF MIXER COUPLER RX 1LO VCO AND PLL 2LO VCO AND PLL VCO AND LOOP FILTER TO TX POWER AMPLIFIER TANK TANK DIGITALLY GENERATED MODULATED TONE Figure 5. Wideband Phase Locked The Transmit RF input is a single ended design with a nominal 50 ohms input impedance. The Transmit RF down- converter and Wideband PLL are only enabled in TRANSMIT Mode. The Wideband PLL is designed to ensure the Transmit VCO is within the pull-in range of the Wideband PLL (so it will lock when transmitting), to achieve required lock up time, and to set the loop bandwidth. The Wideband PLL loop bandwidth requirement is 4 times the symbol rate at a minimum. The ML2712 is capable of loop bandwidth greater than 5MHz. Since the bandwidth of the Wideband PLL can be >4MHz, the lock up time for 802.11 applications is typically less than 2 msec. The maximum frequency pull-in range for the transmit VCO is 500MHz to ensure that the down-converted signal passes through the low pass filter. Design of the ML2712 enables the large pull-in frequency in the Wideband PLL. When TS (Pin 47)is asserted the Wideband charge pump output is first clamped to mid- rail. A PFD detector is then used to ensure frequency lock. Finally the output switches to an XOR detector for accurate phase tracking. After TS is asserted WBCP is clamped to a nominal voltage of mid rail of VQWB (the charge pump supply voltage) for 0.25 msec for 16MHz and 32MHz at the reference input, or 0.2 msec for 20MHz or 40MHz. This pulls the VCO to mid range. When the clamp is disabled a phase frequency detector (PFD) pulls the VCO to frequency lock for 1 msec with 16 MHz and 32MHz reference input, or 0.8 msec with 20MHz or 40MHz reference input. This rapidly pulls the VCO to frequency lock. If the Wideband bit (Control Word B b11) is set low, the PFD is then disabled and an XOR phase detector is used until the TS signal is de-asserted. A factor of four difference between the PFD and XOR charge pump currents keeps KD, the phase detector gain, constant. The PFD charge pump current is nominally 2mA and the XOR charge pump current is nominally 0.25mA. A lock detect output from the Wideband PLL indicates Frequency Lock. An indication of the transmitter not locked is required by some regulatory authorities. produce a 260MHz (nominal) signal. This signal is fed via a low pass filter to the Wideband Phase Comparator where is it compared to the Transmit IF reference signal, supplied by the ML2713. The Wideband Phase Comparator output controls the transmit VCO via an external loop filter. The bandwidth of the Wideband Phase Comparator is high enough to enable a relatively low tolerance Transmit VCO to be used. MODES OF OPERATION (CONTINUED) |
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