A double-tuned circuit is realized from 1/2.lambda..sub.1 transmission lines for connecting a pair of inductors in series at a first frequency and in parallel for a second frequency where said first and second frequencies are in the ratio of a power of two.

FIELD OF THE INVENTION 
The invention is in the area of NMR probe circuits particularly such probe 
circuits exhibiting double-tuned response. 
BACKGROUND OF THE INVENTION 
In the area of nuclear magnetic resonance, the need for apparatus 
concurrently sensitive to non-adjacent spectral regions is encountered in 
several contexts. One common example occurs where a sample is irradiated 
at some (high) frequency for one purpose while the same sample is 
concurrently irradiated at another (low) frequency for some other purpose. 
This is typical of a decoupling experiment wherein, for example, carbon 
13-hydrogen chemical bonds are decoupled while separately exciting the 
carbon 13 resonance concurrently. 
One variation of such a double-tuned arrangement is the need for concurrent 
excitation and observation of chemically distinct samples where one such 
sample is a control employed for instrumental purposes such as the 
establishment of a field frequency lock, while the second sample is under 
study. One example of this arrangement is to be found in U.S. Pat. No. 
3,434,043, commonly assigned. Another similar circumstance is the desire 
to concurrently excite selected different nuclei for acquisition of 
corresponding spectral response. 
A double-tuned circuit ordinarily utilizes a single inductor common to two 
resonant circuits. Each sub-circuit in such an arrangement is separately 
tuned and impedance matched to its respective rf source (or sink). In one 
class of double-tuned arrangement it is necessary for an isolation element 
to be inserted between high frequency and low frequency sources. This 
permits concurrent excitation from respective rf sources. Double-tuned 
circuits are known which employ a transmission line of length .lambda./4 
(at the high frequency) to provide such isolation. For an example of such 
an arrangement see Stoll, Vega, and Vaughan, Rev. Sci. Inst., v. 48, pp. 
800-803 (1977). Balanced circuits exhibiting electrical symmetry are also 
known for the purpose of supporting double-tuned apparatus. Such circuits, 
exhibit among other properties, the virtue that a symmetry plane (or other 
surface) is defined which has a property of electrical neutrality, which 
is to say, a virtual ground. 
Inductive elements in rf probe circuits are known to include "split 
inductors" such as taught in the work of Alderman and Grant, J. Mag. Res., 
v. 36, pp. 447-451 (1979). 
An example of a balanced double-tuned circuit with split inductors and 
capacitances for NMR observe coils is to be found in U.S. Pat. No. 
4,833,412. A double-tuned balanced circuit using lumped elements for a 
birdcage geometry is described in U.S. Pat. No. 4,916,418. 
A saddle coil for multiple tuned response is realized by providing a 
segmented sequence of reactances wherein each such reactance includes 
alternate rf current paths exhibiting widely distinct impedance 
characteristic of the respective resonant frequencies. This work is 
described in U.S. Ser. No. 07/603,966, commonly assigned. 
The present invention is motivated by a desire to observe C.sup.13 NMR 
signals at about 50 MHz while concurrently irradiating the sample with 
decoupling radiation at about 200 MHz. 
The resonant frequencies in hydrogen and C.sup.13 are almost exactly in the 
ratio of 4 to 1. The ratio of the LC (inductance capacitance) product for 
a simple tuned circuit resonant at the high frequency carbon resonance to 
the LC product for a similar circuit resonant at the low frequency (e.g. 
same magnetic field) is about 16. In one common approach to double-tuned 
circuits, the inductance L is common to both resonant sub-circuits forming 
the double-tuned resonant circuit. Thus, the capacitance ratio in such 
respective sub-circuits will also be 16. 
In the present invention the double-tuned character of the circuit is 
realized with two inductors (which could comprise saddle coils or surface 
coils) which are in parallel for the high frequency sub-circuit, but which 
form a series combination for the low frequency sub-circuit. The effective 
inductance ratio in this case is thus 4:1 (200 MHz:50 MHz), reducing the 
required capacitance ratio from 16:1 to 4:1.

DETAILED DESCRIPTION OF THE INVENTION 
Portions of a typical NMR data acquisition instrument are schematically 
illustrated on FIG. 1. An acquisition/control processor 110 communicates 
with an rf transmitter 112, modulator 114 and receiver 116, including 
analog-to-digital converter 118 and a further processor 120. The modulated 
rf power irradiates an object (not shown) in a magnetic field 121 through 
a probe assembly 122 and the response of the object is intercepted by 
probe 122 communicating with receiver 116. The response typically takes 
the form of a transient oscillatory signal, or free induction decay. This 
transient waveform is sampled at regular intervals and samples are 
digitized in ADC 118. The digitized time domain waveform is then subject 
to further processing in processor 120. The nature of such processing may 
include averaging a time domain waveform with a number of nominally 
identical such waveforms, and transformation of the average time domain 
waveform to the frequency domain yields a spectral distribution function 
directed to output device 124. The latter may take on any of a number of 
identities for the display of further analysis and data. 
The magnetic field 121 which polarizes a sample is established by an 
appropriate means indicated in FIG. 1 as maintained in cryostat 123 for 
the establishment and maintenance of a superconducting phase in a 
solenoid, not shown. The cryostat comprises a bore 123A in which the probe 
and sample are housed at room temperature. 
Turning now to FIG. 2 there is shown an example of a double-tuned 
arrangement following the present invention, which is concurrently 
resonant at 50 and 200 MHz. Rf excitation at 200 MHz is applied at 12 to 
inductors 14 and 16 in parallel through identical transmission lines 18 
and 20, each of length one wavelength (at 200 MHz; .lambda./4 at 50 MHz). 
The other ends of coils 14 and 16 are connected through transmission line 
22 also of unit wavelength at 200 MHz which is effectively a short to 
ground. 
The 50 MHz excitation is applied at 26. Transmission line 22 appears to the 
50 MHz source as a shorted .lambda./4 transmission line and thereby 
presents a high impedance. The transmission lines 18 and 20 in series with 
the 50 MHz source comprise a .lambda./2 transmission line (at 50 MHz). 
Consequently the 50 MHz excitation is directed in series through coils 14 
and 16. 
Capacitive combinations 27 and 28 provide impedance matching and tuning for 
the respective 50 MHz and 200 MHz sub-circuits. FIGS. 3a and 3b show the 
computed response of a representative system. The circuit of FIG. 2 was 
studied for the case where the inductances 14 and 16 are each 200 NH with 
a Q of 100 at 200 MHz. The three transmission lines are each one 
wavelength at 200 MHz exhibiting an attenuation of 20 db/10.sup.2 meter at 
200 MHz. The tuning and matching networks are characterized by C.sub.1 
=10.9 pf; C.sub.2 =13.62 pf; C.sub.3 =3.73 pf; and C.sub.7 =2.64 pf. 
In FIG. 3b there is shown the reflection coefficient corresponding to rf 
power at 200 MHz and 50 MHz applied at the respective inputs 12 and 26. In 
FIG. 3a there is shown the relative isolation in db for the same circuit. 
Resonant behavior at other frequencies corresponds to the non-idealities 
of any realization of the circuit including couplings and effects of 
secondary current paths. 
It is important to recognize that the series combination at the lower 
frequency occurs through an additional 1/2 .lambda. (lower frequency) rf 
path. Consequently there is a 180.degree. phase increment in the series 
combination of the two inductors. The significance of this observation is 
best explained in reference to FIG. 4. Inductors 14 and 16 (of FIG. 2) are 
of some selected helicity and relative geometry. Assume that the 
instantaneous magnetic field B.sub.1 is as shown for the high frequency 
case of FIG. 4a. The low frequency energy is driven from point 26 and for 
this situation the low frequency rf path includes a 1/2 .lambda. line 
(18+20). Accordingly, the resulting instantaneous magnetic field vectors 
B.sub.1 for the respective inductors 14 and 16 remain in the same relative 
relationships for both high and low frequency resonances. 
Efficiency of the above described arrangement is the ratio of rf power 
dissipated in the inductors 14 and 16 to the rf power applied at the ports 
12 and 26 for each respective frequency. For the above described example, 
the efficiency is largely affected by losses in the transmission lines. 
Considering such losses for two different types of transmission lines, one 
finds: 
______________________________________ 
Efficiency @ 
50 MHz 200 MHz 
______________________________________ 
20 db/10.sup.2 m @ 200 MHz 
52.6% 27.6% 
6 db/10.sup.2 m @ 200 MHz 
72.4% 46.4% 
______________________________________ 
No attempt has been made to maximize the efficiency or sensitivity of the 
example here described, which is based on commercially available coaxial 
cable. The efficiency of the circuit can be improved, particularly at the 
lower frequency, by using special transmission lines such as strip lines 
or rigid coaxial cables. It should be noted that traditional methods of 
deriving the signal-to-noise ratio of N.M.R. detection circuits based on 
proportionality to the square root of the Q or quality factor can be 
misleading when applied to these transmission line circuits which can show 
a higher Q than non-transmission line circuits but show a lower 
signal-to-noise ratio. A discussion of the losses and efficiencies of this 
class of circuits related to the present invention, which incorporate 
transmission line elements, may be found in co-pending U.S. Ser. No. 
287,789, commonly assigned, and incorporated by reference herein. 
The above discussion was primarily motivated by the desire to facilitate 
NMR experiments where resonances of C.sup.13 and protons might be 
concurrently studied. The respective gyromagnetic ratios are in the ratio 
of 3.977. Frequencies in the exact relationship of a power of 2 are often 
encountered, by accident or design, as harmonics of the identical 
fundamental frequency. The present invention is capable of responding to 
harmonics of order N and N+2.sup.k. Consequently, apparatus for the 
generation or monitoring of such frequencies can advantageously employ the 
principles herein. 
For example, the above described arrangement may be adapted for operation 
with a pair of frequencies in the ratio of 2:1 if (high frequency) tuned 
transmission lines 20 and 18 are cut for .lambda..sub.hi /4 and the 
transmission line 22 is cut for .lambda..sub.hi /2. The generalization of 
the circuit to frequency pairs in the ratio 2.sup.k, k.gtoreq.1 is 
evident, e.g. for (f.sub.hi /f.sub.low)=2.sup.k, the required cable 
lengths for lines 22, 20 and 18 is (.lambda..sub.Hi /4)(2.sup.k). 
The multiple resonant behavior discussed herein in the context of 
excitation of rf resonances from distinct radio frequencies applied to the 
inputs should be understood to include similar multiple resonant response 
for inductive coupling to the coils of rf energy radiated from a nuclear 
resonant sample following excitation. Such multiple resonant behavior may 
be employed in either excitation of resonant condition (s), the 
observation of such resonance(s) or both of the foregoing. 
The above invention has been described as referenced to a particular 
embodiment and example, however, other modifications and variations will 
occur to those skilled in the art in view of the above teaching. It is to 
be understood that this invention may be practiced otherwise than as 
specifically described and is limited only by the scope of the dependent 
claims.