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Transmission Line Tuned Differential Voltage Controlled Oscillator

IP.com Disclosure Number: IPCOM000107679D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 120K

Publishing Venue

IBM

Related People

Coughlin, TC: AUTHOR [+2]

Abstract

This article shows a design implementation of a high frequency Voltage- Controlled Oscillator (VCO) that will not run over 10% nominal frequency over the entire control voltage input to the VCO. This is important in systems where the clock recovery frequency upper limit could cause system metastability problems by running too fast.

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Transmission Line Tuned Differential Voltage Controlled Oscillator

       This article shows a design implementation of a high
frequency Voltage- Controlled Oscillator (VCO) that will not run over
10% nominal frequency over the entire control voltage input to the
VCO.  This is important in systems where the clock recovery frequency
upper limit could cause system metastability problems by running too
fast.

      Previous implementations of multi-vibrator VCO circuits use an
external trim to set the nominal frequency (necessary to remove
circuit tolerances and process variations) and the Phase-Locked Loop
(PLL) circuitry would supply a feedback control voltage necessary to
maintain this frequency phase lock to an incoming data stream.  This
type of design would allow greater than 10% frequency variation
with control voltage.  This frequency variation potential was
required because of large circuit variations due to temperature and
voltage.  The resulting circuit has a large amount of gain
(GHz/volt).  In a noisy environment, a higher gain at the control
voltage input results in a large amount of phase jitter and the
additional exposure of the phase-locked loop pulling out of lock.

      The block diagram shows the disclosed implementation. The VCO
frequency is set by adjusting the length of the differential
transmission lines.  The transmission lines are designed long enough
such that the VCO frequency is slower than required.  The
transmission lines are trimmed (shortened), causing a decrease in
propagation delay, resulting in an increase in the VCO frequency.
The control voltage is held half way between the active range during
the trim operation until the set point is reached.  The use of two
transmission lines provides us with the following advantages:
1.   A differential amplifier is used at the receiving end of the
transmission lines.  This configuration has the important feature of
being able to cancel unwanted common mode signals (noise) that may
appear on the lines.
2.   A twisted pair or parallel substrate transmission lines will
contain equal but opposite currents that will tend to cancel out each
others electromagnetic fields, reducing EMI.
3.   Because the two transmission lines do not need to be the exact
same length, more resolution can be attained in a two-step trim
operation.  The frequency is increased by trimming both lines until
we are very close to the set point, then just one line can be
trimmed, yielding twice the resolution upon reaching the set point.

      The biggest advantage in using this configuration over previ...