Oscillator Control & GPS

The VNGBOX micro needs to produce a very accurate, high resolution and very noise-free DC control voltage to steer the reference oscillator phase accurately. Any noise on this signal will result in noise on the reference; any non-linearity, especially any unexpected steps in the digital to analog conversion, will result in stability problems. The
Oscillator Control & GPS - schematic

circuit is able to generate a very accurate and linear 12-bit DC voltage from 0V to 5V (4096 steps of about 1mV). The micro operates two parallel 6-bit PWM converters, which operate synchronously. Each converter uses six bits of the 12-bit control word. The most significant bits are output on signal EFC6, and the least significant on EFC0. Since the most significant bits have 64 times the significance of the least significant bits, the two signals need to be combined with this gain ratio. A PWM D-A converter usually operates with a low-pass filter, and this design is no different, except the two converters sum together and share a single filter capacitor. Input EFC6 has a 5k resistance (two 10k resistors R14 and R15 in parallel), while input EFC0 has 320k (two 640k resistors, R16 and R17). The weighting of the sum at C7 gives a very linear control voltage with very little ripple. The advantage of using two six-bit converters instead of one 12-bit converter, is that the PWM ripple frequency (clock frequency divided by two to the power of the number of bits) is much higher, making the ripple much easier to remove in the low pass filter. The PWM operates at 2kHz, so the ripple frequency is 31. 25Hz instead of 0. 44Hz. Most oscillators (V-OCXO or V-TCXO) have a voltage control range of 0 - 5V or 0 - 12V. These can be controlled directly by connecting the VCO control input directly to R18/C7, which forms a further 0. 1Hz low...

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