Browse Prior Art Database

Noise Insensitive Biasing for Filter to VCO Interface

IP.com Disclosure Number: IPCOM000108087D
Original Publication Date: 1992-Apr-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 2 page(s) / 74K

Publishing Venue

IBM

Related People

Baumgartner, SJ: AUTHOR [+4]

Abstract

The interface circuit to be described converts a differential input voltage to two equal bias currents which can be used to drive a voltage- controlled oscillator (VCO). The circuit is inherently high in noise rejection due to the differential circuitry used. The circuit also improves noise rejection by introducing noise roll-off capacitors in the interface biasing. These capacitors are used with operational amplifier (OP AMP) circuits which require only a few picofarads for low bandwidth roll-off. The low capacitance requires only on-chip (integrated) capacitors.

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Noise Insensitive Biasing for Filter to VCO Interface

       The interface circuit to be described converts a
differential input voltage to two equal bias currents which can be
used to drive a voltage- controlled oscillator (VCO). The circuit is
inherently high in noise rejection due to the differential circuitry
used.  The circuit also improves noise rejection by introducing noise
roll-off capacitors in the interface biasing.  These capacitors are
used with operational amplifier (OP AMP) circuits which require only
a few picofarads for low bandwidth roll-off.  The low capacitance
requires only on-chip (integrated) capacitors.

      The circuit presented in the figure provides equal currents IVP
and IVN to a voltage-controlled oscillator. The VCO can be of many
types and is most typically an emitter-coupled multivibrator.  The
frequency of the VCO increases as currents IVP and IVN increase.  The
IVP, IVN currents will increase when the input control voltage,
VFILT, becomes more positive, and will decrease when VFILT becomes
more negative.

      The IVP and IVN currents are provided from two operational
amplifiers, OP AMP 1 and OP AMP 2.  A stable reference, VBG, and
current source I1 bias diode D0 at about 0.8 volt.  Because of the
action of OP AMP 1, a voltage equal to the drop across diode D0
appears across resistor RG.  This voltage causes a current to flow in
the transistor Q0 collector which is equal to the diode drop divided
by resistance RG. ...