Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
1999-04-13
2001-04-10
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S205000, C327S018000
Reexamination Certificate
active
06215334
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of signal detection. More specifically, one embodiment of the invention provides an improved circuit for detecting a signal which is a pulse stream with at least some predetermined characteristics using those predetermined conditions to detect the pulse stream accurately through noise added to the signal.
Pulse detection is a well-known form of signal detection. Typically, a binary (i.e., comprising logical “0”'s or “1”'s) data stream is encoded as pulses in a pulse stream. The presence of a pulse in a certain time window encodes for a logical “1” and the absence of a pulse in the time window encodes for a logical “0”. In a variation of such a system, pulse widths carry the information, with a pulse's width representing a digital value. With this “pulse width encoding”, there are a limited number of valid widths a pulse can have. In yet another variation, information is encoded in the falling or rising edges of a signal. Regardless of how the information is encoded, accurate detection and decoding rely on precise detection of the beginnings and/or ends of pulses.
Pulse detection is needed in, for example, modems, bar-code readers, and optically-coupled transmitter/receiver pairs. The latter includes fiber optic systems and optocouplers.
In such systems, the information is clearly encoded and the pulses are transmitted with very sharp rising and falling edges. However, during transmission, pulse edges get distorted due to channel bandwidth limitations, detection circuit bandwidth limitations and noise.
A detection circuit normally amplifies an incoming signal and then applies the amplified signal to a decision circuit. If the level of the amplified signal is below a predetermined level (the “detection threshold”), the decision circuit outputs a logical “0” as its estimation of the digital value encoded in the signal. If the signal is above the detection threshold, then the decision circuit outputs a logical “1”.
An example of a known detection circuit
10
is shown in FIG.
1
. Detection circuit
10
is shown comprising an amplifier
12
, a peak detector
14
and ad comparator
16
. Waveforms at nodes
20
,
22
,
24
,
26
and
28
of detection circuit
10
are shown in
FIG. 2
as waveforms W
20
, W
22
, W
24
, W
26
and W
28
, respectively. The top portion of
FIG. 2
shows the original signal which, after transmission and amplification by amplifier
12
, is waveform W
20
. The signal at node
20
is applied to peak detector
14
, result in a positive peak signal (W
22
) and a negative peak signal (W
24
), which are averaged (W
26
) and used as the detection threshold, which comparator
16
compares with the amplified input signal from node
20
. Comparator
16
outputs a logical “1” at node
28
if node
20
is more positive than the sum signal at node
26
, otherwise it outputs a logical “0”. The output of comparator
16
changes when the signal at node
20
is about halfway between its positive and negative peaks.
Another known detection circuit
30
is shown in
FIG. 3
, with the waveforms shown in FIG.
4
. While detection circuit
30
is more complex than decision circuit
10
shown in
FIG. 1
, it has additional capabilities. For example, detection circuit
30
has peak detectors
31
that are resetable. Each peak detector
31
acquires an updated peak value after each positive or negative transition of the output signal. Each peak detector
31
has a comparator
34
with a small hysteresis to prevent oscillations near the switching point. As with detection circuit
10
, the input signal is amplified by an amplifier
32
, and the outputs of peak detectors
31
are averaged at node
56
and used as the threshold voltage for comparator
38
. The output of detection circuit
30
is at node
60
. That output is also used to reset the switches in peak detectors
31
, as the output signal at node
60
is fed to edge detectors
42
(one directly and one after being inverted by an inverter
40
) coupled to the switches.
Waveforms at nodes
50
,
52
,
54
,
56
,
58
,
60
, and
64
of detection circuit
30
are shown in
FIG. 4
as waveforms W
50
, W
52
, W
54
, W
56
, W
58
, W
60
, W
62
and W
64
, respectively.
Yet another detection circuit
70
is shown in
FIG. 5
, with associated waveforms shown in FIG.
6
. Detection circuit provides an output response with less delay than other detection circuits, and has better transition detection, but requires a noise-free environment. The increased noise sensitivity comes from a peaking circuit
82
, which is needed for the improved signal transition detection. Peaking circuit
82
amplifies noise and interference more than the signal. Consequently, at the output of the peaking circuit, the signal-to-noise ratio is much worse than at the input. This makes the circuit unreliable in noisy environments. When the noise is amplified, multiple transitions might be spuriously detected at transition points, such as t
1
-t
5
shown in
FIG. 6
, where only single transitions should have been detected.
From the above it is seen that an improved detection circuit is needed.
SUMMARY OF THE INVENTION
An improved detection circuit is provided by virtue of the present invention. In one embodiment, a detection circuit according to the present invention includes a biasing circuit for outputting a bias signal having a first state and a second state, and coupled to a comparator for comparing an input signal to the bias signal to produce a digital bi-level signal representing the detected signal. A delay circuit is coupled to the comparator output for producing a delayed version of the digital bi-level signal. A switch coupled to the biasing circuit and to the delay circuit, switches the bias signal between the first and second states responsive to the delayed version of the digital bi-level signal thereby providing a detection circuit that has the advantage that the threshold may be adjusted based on knowledge of the input signal and the circuit has a high noise margin. The circuit is useful where transitions must be detected with high accuracy.
A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.
REFERENCES:
patent: 5019722 (1991-05-01), Hess et al.
patent: 5061859 (1991-10-01), Lovelace et al.
patent: 5266884 (1993-11-01), Agiman
patent: 5341033 (1994-08-01), Koker
General Electronics Applications, Inc.
Le Dinh T.
Townsend and Townsend / and Crew LLP
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