Oscillators – Frequency stabilization
Reexamination Certificate
2000-09-29
2003-02-04
Kinkead, Arnold (Department: 2817)
Oscillators
Frequency stabilization
C331S057000, C331S017000, C331S034000, C331S185000, C327S156000
Reexamination Certificate
active
06515552
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to voltage controlled oscillator (VCO) circuits, and in particular, to VCO circuits using circuit parameter clamping techniques for stabilizing free run frequencies.
2. Description of the Related Art
Referring to
FIG. 1
, a typical phase lock loop (PLL) circuit
10
includes an integrated circuit
12
having a phase detector
14
, VCO
16
and feedback divider
18
integrated therein and interconnected substantially as shown. In accordance with well-known conventional PLL principles, a referenced clock source
20
provides a reference clock signal
21
. The phase detector
14
compares this signal
21
with a feedback signal
19
which is generally a frequency-divided version of the output signal
17
from the VCO. The feedback divider
18
is typically a counter that is programmable as to the divisor by which the frequency of the VCO signal
17
is divided.
The phase detector output signal
15
includes a DC component representing the phase difference between the two input signals
21
,
19
, plus numerous higher frequencies components as well. These higher frequency components are filtered out using a low pass filter
22
which is external to the integrated circuit
12
. It is the low pass filtered signal
23
, which is essentially a virtual DC signal, that is used to drive the VCO
16
and thereby control the frequency of its output signal
17
.
Frequently, this type of circuit
10
is used in a free running, or “unmodulated,” mode in which the output signal
15
from the phase detector
14
, which is a “modulation” signal inasmuch as it is used to control, e.g., “modulate,” the frequency of the VCO
16
, is disabled or otherwise disconnected. This is done so as to allow the VCO
16
to run in its free run, or unmodulated, mode. Ideally, it is generally desirable that the frequency of the VCO signal remain substantially constant during this mode of operation. However, due to a number of factors, this is often not the case. For example, as is well known, the semiconductor devices used to implement the VCO
16
have a number of operating characteristics (e.g., threshold voltage, saturation voltage and current, channel resistivity, etc.) that can be affected by many things, including variations in ambient operating temperature and variations in the fabrication processes used to fabricate the semiconductor devices. As a result, the frequency of the VCO
17
can have variations in its frequency introduced.
Referring to
FIG. 2
, at least two conventional techniques have been developed to address this problem. (As indicated, a common implementation of a VCO
16
includes a voltage-to-current (V:I) conversion circuit
40
that converts the input control voltage
23
to one or more control currents
41
a
,
41
b
. which, in turn, control the operating frequency of a current controlled oscillator (ICO)
42
.) One technique involves the use of a DC voltage source
30
to apply a clamping voltage at the input
23
of the VCO
16
. This is achieved by using an operational amplifier A
1
with a diode D
1
in its feedback circuit to buffer an input DC voltage and apply it to the input
23
of the VCO
16
. As a solution, however, this technique is only partially successful. For example, variations in the operating characteristics of the semiconductor devices within the VCO
16
, e.g., as caused by variations in operating temperature or fabrication processes, will cause the frequency of the VCO signal
17
to vary, notwithstanding the constant DC voltage at the input
23
.
Another technique involves the use of current clamping by mixing static currents
11
,
12
with the individual output currents
41
a
,
41
b
of the V:I converter
40
within the VCO
16
. However, this approach also is only partially successful. The VCO
16
will often still have poor phase jitter performance due to the difficulty in generating a low noise static current, i.e., using current sources
52
and
54
, in those applications involving mixed signal circuits or high-speed large signal analog amplifier circuits.
Accordingly, it would be desirable to have a technique for clamping a voltage or current controlled oscillator in such a manner as to significantly reduce any frequency variations or phase jitter introduced by variations in semiconductor device operating characteristics or on-chip noise.
SUMMARY OF THE INVENTION
An integrated circuit with a voltage-controlled oscillator that provides an oscillation signal with an unmodulated frequency that remains constant in response to reception of a bias current having a constant magnitude and with a modulated frequency that varies in response to reception of a modulation voltage, in accordance with one embodiment of the present invention, includes a current-to-voltage conversion circuit, a voltage-to-current conversion circuit and a current-controlled oscillator circuit. The current-to-voltage conversion circuit includes a portion of a plurality of integrated, like semiconductor devices with a plurality of operating characteristics and generates, in response to reception of a bias current, a control voltage with a magnitude corresponding to a magnitude of the bias current. The voltage-to-current conversion circuit includes another portion of the plurality of integrated, like semiconductor devices, is coupled to the current-to-voltage conversion circuit and generates, in response to reception of the control voltage and a modulation voltage, a control current with a magnitude that corresponds to a magnitude of the modulation voltage during reception of the modulation voltage; and corresponds to the control voltage magnitude otherwise The current-controlled oscillator circuit includes still another portion of the plurality of integrated, like semiconductor devices, is coupled to the voltage-to-current conversion circuit and generates, in response to reception of the control current, an oscillation signal with an unmodulated frequency corresponding to the bias current magnitude and with a modulated frequency corresponding to the modulation voltage. The unmodulated frequency is defined by the bias current magnitude substantially irrespective of variations in respective ones of the plurality of operating characteristics caused by variations in operating temperatures and processes of fabrication of the plurality of integrated, like semiconductor devices.
REFERENCES:
patent: 5629650 (1997-05-01), Gersbach et al.
patent: 5740213 (1998-04-01), Dreyer
Chiu Hon Kin
Hojabri Peyman
Kinkead Arnold
National Semiconductor Corporation
Wildman Harrold Allen & Dixon
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