GB2244403A - A high Q helical resonator - Google Patents

Abstract

A high Q helical resonator is described which is directly coupled and resonant for lengths of approximately lambda /2. The helical resonator consists of a helical transmission line wound between two connectors with a portion of the coil at either end connected to ground where the connection to ground could also be via a capacitor. The equivalent circuit consists of a low-loss helical transmission line and two shunt inductances or capacitors at either end connected to ground. This resonator is extremely easy to fabricate and is used as a high Q filter and as the resonator in state of the art low noise oscillators. Varactor diodes may be used to time the resonant frequency. The helix, the shunt components and the enclosure may be made of superconducting components. <IMAGE>

Description

A high Q helical resonator for oscillators and filters This invention relates to Helical filters and resonators.

Mobile radios, satellites and other small systems often have very tight specifications on adjacent channel noise performance and as channel spacings get closer together further improvements are required. Conventional LC resonators often have low Q's which the screening can degrade. They are also more difficult to make above 1 GHz. Conventional transmission lines also take up considerable space at low microwave frequencies especially when using air as the dielectric.

A helical transmission line resonator has been invented which is both compact and as the screening is part of the resonator the Q is not degraded. It differs from conventional helical resonators both in length and the form of the coupling. The new resonator is directly coupled and resonant for electrical lengths approximately equal to B/2 allowing easy manufacture and operation up to several GHz. This resonator is used both for low noise local oscillators and for front end filtering in communications systems.

Two low noise L band oscillators (900MHz and 1.6GHz) which use the new helical resonators are described.

Helical Resonator: A prototype helical resonator is shown in Figure 1 and consists of a helical transmission line (1) wound between two connectors (2,3) with a portion of the coil connected to ground (4,5) where the whole assembly is mounted in a metal enclosure (6) . The connectors can for example be BNC, SMA, OSM, OSSM or other connectors. The resonator can also be built without using connectors by passing a wire through the hole in the box as shown in Figure 2 where the helical transmission line (1) is connected between two shunt inductors (2,3) connected to ground and the input and output are connected to two wires (4,5) which pass through the box to thus forming the input and output of the filter or resonator. If necessary an insulating spacer can be placed between the wire and the box to maintain a constant position.The spacer could also be a shunt capacitor which replaces the shunt inductor but which has a similar impedance at the resonating impedance. This will still produce a filter or resonator although there will now be a through response at low frequencies.

The helical coil can be mounted on a rigid support to reduce the effect of microphony where the losses can be kept low by making the support hollow.

The Helix and the box and the shunt components can be made out of superconductors to greatly improve the Q. where the superconductors could be high Tc superconducting materials and conventional low temperature superconducting materials.

The shunt components can be outside the metal enclosure if necessary.

The equivalent circuit is shown in Figure 3 and consists of a low-loss helical transmission line (equivalent length L) and two shunt inductances connected to ground. The shunt inductances can be shunt capacitors or shunt stubs or a combination of inductors, capacitors and stubs. This is very similar to the Fabry Perot resonator in optics where the inductors are equivalent to mirrors.

If the shunt inductor has a value 1 then the normalised susceptance X= -Zo/2sfl. The value of X should be the effective susceptance of the inductor as the parasitic capacitance is often significant. If ZoT = Zo, where ZoT is the resonator line impedance and Zo is the terminating impedance, then S21 is given by the following equation, S21 = 4 #/{(1+jX)- x2(1-F2)) 1 where # = exp{-(&alpha;; + jss)L} 2 a is the attenuation coefficient of the line ss is the phase constant of the line For small aL ( < 0.05) and #f/fo, < < 1,the following properties can be derived for the first resonant peak (fro) of the resonator where #f = f - fO, fo = (Veff/2#){ 1 + (1/#) tan-1 (2/X)}, [3 where L is the effective length of the helical resonator.

S21(Sf) = S21(O)/{1 + j2QL(#f/fo)} [4] s21(O) = (1-QL/Qo)= 1/{1 + (&alpha;/2)X2} [5] QL = # s21 (0)x2/4 [6] Qo = 'r/2aL [7] From equation 5 it can be seen that the insertion loss and the loaded Q factor of the resonator are interrelated. In fact as the shunt inductors (assumed to be lossless) are reduced in value the insertion loss approaches infinity and QL increases to a limiting value of x/2aL which we have defined as QO. It is interesting to note that when S21 = 1/2, QL/Qo = 1/2.

A prototype 900 Mhz helical resonator has been built which has a total length 30 mm, where the box is 12 mm wide and deep. The helix is 21 mm long and made up of 14 turns of close wound wire with an inside diameter of 4.5 mm and wire diameter of 0.71 mm.

The total length of the helix unwound is 250 mm. The connectors in the proto-type were SMA. A typical response for the 900 MHz resonator is shown in Figure 4 showing a 3dB bandwidth of 2.66 MHz. The loaded Q is 342 for an insertion loss of 7.7 dB and therefore the unloaded Q is 582 where the insertion loss and the Q are related by the expression S21(0) = (l-QL/Qo).

The resonators can incorporate impedance transforming networks at either end in place of the shunt component which consists of a series reactance and then shunt reactance where the input and output are taken at the junction of the series and shunt component where the reactances can be capacitors or inductors or transmission line components (stubs) or any combination of capacitors inductors or stubs. If a series inductor is required then this can be part of the helix.

The resonators or filters or oscillators can be tuned by using voltage controlled variable capacitance diodes (varactors) where for example the varactors can be placed between a point on the helix and the case where the connection to the case is via a feed through capacitor and the voltage bias is connected to the lead passing through the feedthrough. In many applications two varactors placed symmetrically on either side of the centre of the helix to ground via feedthroughs can be used where the bias is applied on the side of the feedthrough outside the box. The amount of coupling of the varactors into the resonator is highest close to the centre and lowest near the shunt inductors where the resonator is usually high impedance in the middle for the fundamental resonance. These resonators can also be used at their higher resonances.

The enclosing box can be rectangular or cylindrical or any any shape which will enclose the helix to form a transmission line.

These resonators can be formed into high order filters as shown in Figure 5 by using coupling networks (1,2,3,4,5,6) to interconnect the helical transmission lines (7,8,9) where any number of sections can be used where the coupling networks consist of for example a T or It network of reactances where the reactances can be capacitors or inductors or transmission line components or any combination of capacitors or inductors or transmission line networks.

The transmission lines can be made on a substate by using a meander line an also by using a spiral coil suspeded above an earth plane as in microstrip or with earth planes on both sides as in tri-plate.

Oscillators: Two low noise feedback oscillators have been built using these helical resonators as shown in Figure 6. The oscillators consist of an amplifier (1), power splitter(2), helical resonator(3) and phase shift network(4). The oscillators have been designed based on the theories described in references 2,3,4 in which it is shown that that there is an optimum coupling coefficient, QL/Qo and hence insertion loss (3-10 dB) for optimum noise performance.

Both helical resonators had an insertion loss between 7 and 8 dB.

The SSB noise performance as shown in Figure 7 of a 900 MHz oscillator was measured, using the phase detector method, to be -127 dBc/Hz at 25 KHz offset for an oscillator with 0 dBm output power, 6 dB amplifier noise figure (Hybrid OM345 amplifier) and the unloaded Q of 582. The 1.6 GHz oscillator had a noise performance of -120 dBc/Hz at 25 KHz offset for Qo of 382, output power of O dBm and amplifier NF of 3 dB. The noise performance of both oscillators is within 2 dB of the theoretical minimum noise performance available from an oscillator with the specified Qo, Qs, output power and noise figure. In both oscillators the noise performance is 6 dB lower at 50 KHz offset demonstrating the correct 1/8f2 characteristic and that Flicker noise is unimportant at these offsets.

References 1 A.I. Zverev "Handbook of Filter Synthesis John Wiley and Sons, chapter 9 on Helical Resonators pp. 499-521.

2. J.K.A. Everard "Low Noise Power Efficient Oscillators: Theory and Design" IEE Proceedings Part G, Vol. 133, No.4, August 1986, pp. 172-180.

3. J.K.A. Everard, "Low noise transmission line oscillators".

Patent Application number 8816489, 12th July 1988.

4. J.K.A. Everard and K.K.M. Cheng. "Novel Low noise 'Fabry Perot' Transmission Line Oscillator". Electronics Letters, 17th August 1989, Issue 17, Vol. 25, pp.1106-1108.

Claims (10)

1. A High p Helical Resonator for oscillators and Filters

   Claims 1. A helical resonator comprising a conducting helix grounded at both ends, means for directly coupling into and out of the helix, means for enclosure of the helix.
2. 2. A helical resonator as described in claim 1 in which the coupling point is adjusted to vary the insertion loss and ratio of loaded to unloaded quality factor (QL/QO).
3. 3. A helical resonator as described in claim 1 in which the enclosure is a metal or superconducting container.
4. 4. A helical resonator as described in Claim 1 in which the conductor is a superconductor.
5. 5. A helical resonator as described in claim 1 in which the grond contact occurs outside the box.
6. 6. A helical resonator as claimed in claim 1 connected to other helical resonators using coupling networks to form high order Filters.
7. 7. A helical resonator as claimed in claim 1 in which varactor diodes are used to tune the resonant frequency.
8. 8. A helical resonator as claimed in claim 1 connected to an amplifier and power splitter and phase shift network to form an oscillator.
9. 9. A helical resonator as claimed in claim 1 in which supports are included to reduce microphony.
10. 10. A helical resonator substantially as described with reference to the accompanying drawings.