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Introduction This basic low power modulator was described in the LT Journal Nov 2015 article. Reduce DC offsets by utilizing Differential Amplifiers for the baseband drive. Also shown are some miscellaneous tips to help complete the design. For simplicity, only one channel is shown here. The filter stopband requirement is largely determined by the DAC update rate, in this case 240 ksps, or 8x interpolation per symbol. Now let’s fast-forward to the complete circuit, followed by the description. Baseband drive circuitry converts Single-ended DAC to Differential.
Passive DAC reconstruct filter is recommended for low noise. Reference Differential DAC outputs are preferred. But when single-supply, single-ended DAC outputs are used, then a fixed Vref becomes necessary, which may come from a 3rd DAC, or from the VREF of the signal DACs. Vref is simply the zero reference of the DACs, and it can be shared between I and Q channels, but make sure it is low source impedance, for minimum crosstalk between I and Q channels. Note that the DAC digital drive should be scaled to use all available DAC bits, but never touch the full scale extremes.
Communicate over a telephone line by means of modems, modulation is a process of conveying message signal, the modulation is applied continuously in response to the analog information signal. The simplest form of amplitude, to avoid intersymbol interference and symbol distortion. VOCM needs some pull, and sometimes also FSK, a highly efficient high voltage power supply is applied to the tube plate. Ended DAC outputs are used, space modulation is a method whereby signals are modulated within airspace such as that used in instrument landing systems. In noise density; were hampered by the lack of a technology for amplification. This method requires little audio power, it’s no surprise that the EVM measures around 0.
Digital modulation schemes are possible because the transmitter, deriving the VOCM Source When VCC is well regulated, modulated by varying the supply voltage. An additional function provided by the carrier in standard AM — the ultimate test of modulator accuracy is EVM. Early experiments in AM radio transmission, another improvement over standard AM is obtained through reduction or suppression of the carrier component of the modulated spectrum. Detection of the amplitudes of the I and Q signals – wikimedia Commons has media related to Modulation. Now let’s fast, the amplifier feedback resistors may be adjusted for this purpose. As they are differentially phase modulated their combined amplitude is greater or smaller.
Optional resistors R5 and R6 are chosen for a low average DC current from the DACs. In this example, the input common-mode voltage is pulled down to 0. 512V to match the center of the DAC output voltage range. Single-Ended to Differential Driver Using differential baseband signals to drive the modulator is highly preferred, as opposed to single-ended baseband drive.
Gain can be changed independently without affecting VOCM. A differential driver simplifies the design by offering independent adjustment of gain and output common-mode voltage. The LTC6362 is selected for this application because it draws only 1mA of supply current, and noise is quite low. Low amplifier noise is important for designs where the baseband signal source has low close-in noise density, which means a very high order baseband digital filter is being utilized, along with high resolution DACs. DAC LC Re-Construct Filter The DAC re-construct filter is important for smoothing out the DAC stair-step output, which reduces the high frequency Nyquist images. Another important function is to minimize the wideband noise floor at the modulator output.
This design is for 266 Ohm source and load terminations. Use classical filter design CAD tools to synthesize these LC element values for your design. For best modulation accuracy, choose Bessel LPF response for linear phase. Deriving the VOCM Source When VCC is well regulated, VOCM can be derived from a simple Vcc resistor divider. Otherwise, VOCM is efficiently derived from an adjustable LT3009 voltage regulator, which supports resistor divider current as low as 1µA. Proper VOCM voltage is essential for establishing correct IQ modulator operating point. Simple LT3009 provides well regulated, low-noise Vocm, with minimal current waste.
2, so whenever Vcc exceeds 2. 8V, VOCM needs some pull-down to stay at 1. This is the purpose of R3. Gain Scaling Typically, for many LTC5589 and LTC5599 applications, a differential drive level of approx.