Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-08-19
2001-02-27
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06192760
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
Transmit/Receive (T/R) switches are used extensively in radio communications electronics. In these applications, the T/R switch is used to couple an antenna to the transmitter and receiver electronics in a manner such that when transmitting, the majority of the transmitter power goes to the antenna and when receiving, the majority of the signal received by the antenna goes to the receiver. The T/R switch also protects the receiver circuitry from being damaged by large transmitter signals through limiting the power that gets to the receiver input section. Several different kinds of electronic circuits have been employed to perform the T/R switch function.
U.S. Pat. No. 4,637,065 to Ruppel discloses one type of T/R switch used in radio communications circuits and contains a good description of the prior art. These types of T/R switch circuits utilize PIN diodes, giving rise to a long switch over time of several milliseconds, which is adequate for most radio communications applications. However, a T/R switch suitable for use with Electromagnetic Acoustic Transducers (EMATs) must be very fast acting; that is, capable of switching from transmit mode to receive mode in a few microseconds or less. Thus, a drawback to using these type of T/R switches for EMAT operation is that the time it takes for the circuit to recover from the transmit mode and then switch to the receive mode is too long.
Another method of coupling the transmitter and receiver to the same transducer is to attach the output of the transmitter directly to the transducer and attach the input to the receiver to the transducer via resistors. As illustrated in
FIG. 1
, a back-to-back diode (CR
11
and CR
12
) arrangement is placed at the input to the receiver to prevent damage to the receiver circuit from the large transmitter voltages with the current being limited by the resistors. This allows rapid switching from transmit mode to receive mode. However, the use of resistors to couple the transducer to the receiver input results in transmit power loss in the resistors and signal to noise reduction from the receiver because of the increased resistance at the receiver input, so that this arrangement is not well-suited for use with EMATs.
FIG. 2
illustrates another T/R switch circuit known to the inventor prior to the current invention. In this T/R switch, the transmitter output is coupled directly to the transducer. The transducer is then coupled to the receiver input via a power limiting circuit formed by diodes CR
1
and CR
2
, inductor L
1
, resistor R
1
, voltage source V
1
, and transformer T
1
. Direct current (D.C.), supplied from V
1
and limited by resistor R
1
, flows through the diodes CR
1
and CR
2
. This D.C. is set by R
1
such that the diodes are biased “on” for small signals; that is, a small received signal passes from the transducer to the receiver input unimpeded because the diodes are put into a conductive state (low resistance) by the D.C. bias currents. Inductor L
1
provides a high impedance for the radio frequency (RF) signals preventing them from flowing through voltage source V
1
. High voltage RF signals applied to the transducer cause the diodes to become reverse biased, switching them into a nonconductive state. This switching time depends on what diodes are employed in the circuit.
In the circuit illustrated in
FIG. 2
, the diodes employed had switching times on the order of {fraction (1/10)} of a microsecond. However, the {fraction (1/10)} of a microsecond switching time limited the operation to frequencies below approximately 2 MHz because the diodes must switch on and off with each cycle of the transmitter RF toneburst. Consequently, one disadvantage to using this circuit for EMAT operation is that it is single ended (one side of the transducer is connected to ground), and those skilled in the art have found that, in order to prevent noise pickup, the EMAT coil is best kept isolated from ground and operated into a differential input receiver which provide high common mode noise rejection. Likewise, the D.C. bias current flowing through the primary windings of T
1
can cause the transformer core to saturate if it is not of adequate size resulting in larger transformer size than would otherwise be necessary. Therefore, the circuit illustrated in
FIG. 2
is not well-suited for use with EMATs.
In sum, it is apparent that an improved T/R switch suitable for use with an EMAT is needed to overcome the deficiencies discussed above. Moreover, such an improved T/R switch for use with an EMAT would be welcome by the industry.
SUMMARY OF THE INVENTION
The present invention seeks to improve the T/R switching capabilities during EMAT operation. Specifically, the T/R switch disclosed in the present invention provides several advantages for EMAT operation. This circuit blocks the transmitter pulses from being sent to the input to the receiver, while allowing the small received signals to be applied to the input to the receiver with very little attenuation or added source resistance. Additionally, since this circuit is balanced with respect to ground, good common mode noise rejection is obtained.
Assuming that the input impedance is infinite, the use of resistors to couple the received signal from the EMAT to the input to the receiver reduces the signal to noise ratio by a factor of two, as compared to an ideal T/R switch. That is, in prior art circuits, the input impedance of the receiver may actually be low enough, especially at high frequencies, to produce significant attenuation of the received signals if resistors are placed between the transducer and the receiver input. In lab testing of the present invention, an improvement in signal to noise of a factor of three was observed when this T/R switch was substituted for resistor coupling between the transducer and the EMAT receiver during operation at 5 MHz. In many applications, these improvements in signal to noise ratio can mean the difference between being able to perform the test and not being able to perform the test.
Also, receiver input transformer core saturation is prevented in the present invention by using a center tapped primary with the bias current flowing in opposite directions in each half of the primary windings. This use of a center tapped primary allows very small receiver input transformers to be constructed.
One aspect of the present invention is drawn to an improved electrical circuit which utilizes an EMAT as both a transmitter and a receiver by allowing the fast switching of the EMAT from transmit to receive mode, and vice versa. In addition to an EMAT coil, this circuit contains a tuning capacitor which allows the EMAT to be operated at a desired frequency when transmitting. The EMAT is also coupled to a pair of capacitors which are, in turn, connected to two separate diode strings. The diode strings preferably contain at least one fast switching silicon diode, although the use of similar diodes is possible. The diode strings are also resistively connected to each other at their respective junctions with the capacitors. The other end of each diode string is coupled to a single transformer, which is center tapped at its primary winding. Finally, the transformer is coupled to an input receiver.
In operation, the frequency of the EMAT is set by appropriately energizing and tuning the tuning capacitor. A voltage is then applied in the center tapped transformer and in the resistive link between the diode strings. The current automatically switches the conductivity of the diodes and permits the EMAT to switch modes as desired.
Another aspect of the present invention is drawn to an improved method for selectively switching an EMAT between transmitting mode and receiving mode. Essentially, this method utilizes a circuit, as described above, and involves several steps. A sinusodial toneburst voltage is applied across the terminals of the EMAT coil to induce transmission by the EMAT. The large applied voltages reverse bias the diode strings, effectively switching off the input to the receiver s
MacLauchlan Daniel T.
Overby Charles B.
Baraona R. C.
Edwards R. J.
Marich Eric
McDermott Technology Inc.
Miller Rose
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