evaluation of a simplified doppler frequency shifter

International Journal o f Infrared and Millimeter Waves, Vol. 11, No, 9, 1990

E V A L U A T I O N OF A S IMPLIFIED D O P P L E R F R E Q U E N C Y SHIFTE R

R. A. Marr and U. H. W. Lammers

Rome Air Development Center Hanscom Air Force Base, Massachusetts, 01731

Received July 27, 1990

Abs t r ac t

Mechanical Doppler frequency shifting of a millimeter or submilllmeter wave signal provides a means to implement a sensitive and highly coherent heterodyne receiver in a laboratory system. The rotary shifter, which we have previously described, is a precision-machined device suitable for use down to submillimeter wavelengths. We found at 140GHz, however, that the coherence of such a system is not affected by mechanical imprecision of the frequency shifter, and that the system dynamic range is only moderately affected by mechanical imprecision and by certain design simplifications. We have therefore built several versions of simpler and less precise Doppler frequency shifters and report here on their performance at 140GHz.

Key words: Doppler frequency shifter, mechanical frequency translation, single sideband modulator, heterodyne receiver, selfmixing system.

1099

O19J-9271/90/OgOO-lO9950&O0/O �9 1990 Plenum Publishing Corporation

1100 Marr and Lammers

Introduction

In two earlier papers [I], [2] we presented the principle, a design, and several applications of a rotating Doppler frequency shifter for millimeter and submillimeter wavelengths. Three characteristics make this device useful for sensitive, narrowband measurement tasks: I) The phase coherent sequencing of repetitive, Doppler shifted wave- trains leads to a largely monochromatic output signal. 2) The frequency conversion is of high efficiency, since it is based on reflection from stationary and moving metallic surfaces. 3) Frequency and phase instabilities of the source signal are retained by the Doppler shifted signal, and are eliminated in a first heterodyning process. Here the Doppler shifted component of the source signal, after transmission through the measurement system, is down con- verted to a first intermediate frequency (IF) by mixing it with an unshifted component of the source signal. Since the Doppler shift and hence the IF depends on the shifter rotation rate, a second heterodyning process to baseband (zero IF) follows, where the second local oscillator signal is derived from the same oscillator that determines the shifter motor drive frequency. These operations make the measurement largely independent of frequency and phase instabilities of the two oscillators involved. Avoiding the necessity to independently frequency and phase stabi- lize sources for heterodyne applications is a major advantage, and even more so as the source frequency increases.

Although millimeter and submillimeter system components have to be built to great precision almost by defini- tion, we felt that the relative insensitivity of system performance to some frequency shifter characteristics war- ranted an investigation of relaxed design parameters. Experimentally, this work was carried out only at 140GHz. It is possible that results might not be as favorable at higher frequencies.

A major task in building the frequency shifter [i] had been the machining of the stationary involute-contoured reflector segments. For best Gaussian beam refocussing the reflecting surface was two-dimensionally curved, which required three-dimensional, numerically controlled milling. Making the reflecting surface one-dlmensionally curved (involute cylinder) was the first modification tried. This opened up much simpler manufacturing techniques. The involute-shaped stationary reflectors are a likely source of shifter performance degradation.

The rotating dual-plane-mirror assembly was originally

Doppler Frequency Shifter 1101

built with great accuracy. This was done to minimize external centrifugal forces at high rotational speed and also to facilitate optical alignment of the completed shitter with a HeNe laser. Subsequently, we investigated a much simpler mirror assembly based on the notion that some mirror pointing inaccuracies would be tolerable as long as they were applied evenly through the whole 360deg range of rotation.

We also considered other methods of launching a tan- gentially rotating beam toward the stationary involute segments from a source radiating coaxially with the motor axis. Internal and external centrifugal forces on the original dual-mirror assembly limit the rotation rate, the Doppler shift, and in turn the receiver sensitivity [2]. A lighter and stronger design of the rotating parts would benefit the frequency shifter operation substantially.

Apart from the objective of a more easily manufactur- able device, the underlying philosophy in this effort has been that narrowband coherent operation afforded by the frequency shifter is such an advantage over conventional systems that it makes acceptable some loss in frequency shifter conversion efficiency.

Phase Adjustment Technique

Originally [1], our procedure had been to adjust the frequency shifter for optimum performance by observing the difference frequency between the shifted and unshifted source signal components after mixing. The segments were aligned for best phase continuity at segment transitions and for spectral shape, that is, for the highest power ratio between the desired and undesired spectral lines. Subsequently, the double-conversion technique and its in- herent phase stability as described in [2] opened up a way to precisely observe the phase progression along each segment. This is a much more sensitive criterion by which to align the frequency shifter. It is a means for recognizing where the phase progression deviates from linear along the segment and for taking corrective action. Figure 1 shows the process of heterodyning and signal comparison as used for phase adjustment. The source signal f is divided in a power divider, one component frequency shifted to fl, and then mixed with f to obtain the IF, i'f2. Here i is an integer and f2 is the repetition rate of the frequency shifting process. The synthesizer that generates f2 also synthesizes a reference signal i,f2, where both are derived


J DRIVE [ [ REQUENC'~L GENERATOR -- --'~,]~'-- F ! ,SHIFTER l-

f2

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l i.f2

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L

~i*f2

ANALYZER |

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f

Figure I. Arrangement to measure amplitude and phase progression on segments

from the same o s c i l l a t o r . In a d d i t i o n to observ ing the ou tpu t l i n e spectrum on a spectrum a n a l y z e r , we d i s p l a y on an o s c i l l o s c o p e with de layed sweep the IF time waveform, the r e f e r e n c e waveform, and t h e i r phasor sum. By a d j u s t i n g the r e f e r e n c e s i g n a l phase and ampli tude a p p r o p r i a t e l y i t can be used, fo r example, to cancel the IF waveform. Since the de layed sweep al lows one to in spec t each per iod of the f requency s h i f t e d 140GHz waveform in d e t a i l (28 per iods per segment in our ea se ) , l o c a l imper fec t ion of the i n v o l u t e contour or d isp lacement of the i n v o l u t e contour from i t s proper p o s i t i o n can thus be made v i s i b l e . We found t h a t the p r e c i s i o n - m i l l e d i n v o l u t e contours were q u i t e good, but the segments were not o p t i m a l l y a l igned by our o r i g i n a l method. Actually, for ease of alignment a redesigned frequency shifter would include not only rotational but also translatory adjustment means for the segments.

Figure 2 is the currently best spectral result in frequency shifter performance achieved through inspecting and correcting for proper phase progression. The shifter rotation rate is f2 = 100kHz/2048 = 48.83Hz for an IF fre-


Figure 2. Spectral output of all-aluminum frequency shifter at 140GHz

quency of i'f2 = 4"28"f2 = 5468.75Hz. The horizontal and vertical scales in the figure are 96Hz and 10dB per division, respectively. All sidelines are at least 33.2dB down from the desired line. Earlier we had reported 20dB. Note that at R = 19mm and f = 140GHz, phase continuity has to be achieved by slightly swivelled segments [i].

Involute Segment

The involute of a circle is generated by the end point of a string, held taut while unwinding it from the circum- ference of the circle. We found that blocks of granular- structured styrofoam packing material could be accurately sliced with a hot wire. Figure 3 shows a simple jig used to cut an involute cylinder contour out of a block of foam, using the basic geometric procedure above. The involute surface was afterwards made reflective by gluing on a thin sheet of copper.

On an aluminum base plate in Figure 3 is mounted a phenolic cylinder of radius R, where R is the generating radius for the involute. In accordance with the original frequency shifter [I], R = 19mm was chosen. A thin, flexi- ble, but inelastic metal band is partially wound around the phenolic cylinder and bolted to it on one end. The other, free end is soldered to a bushing. The bushing pivots around a shaft mounted in the center of a phenolic disk. The latter is free to slide on the aluminum base, except that its center is constrained in outward direction by the metal band to move along an involute boundary. A support structure attached to the phenolic disk carries an exten-


Figure 3. Cutting jig for foam-backed involute segments

sion shaft positioned coaxially above the center shaft. Narrow concentric holes are drilled into the center shaft and its upper extension to hold the cutting wire. The center shaft extension glides in its support and is spring loaded to hold the cutting wire taut after thermal expan- sion. The wire is 0.01in diameter Constantan of 2 I/2in length heated with a current of 2A. This resulted in a fine cut at reasonable cutting speed. The cutting assembly slides partially under a raised table, which is parallel with the base plate and carries the foam block to be cut. The foam block is of rectangular cross section, precut to size, and bolted to the table with two 8-32in screws. These fit through the segment mounting holes. The raised table was contoured such as to let the cutting wire pass by unimpeded. Care was taken to adjust the cutting wire to a normal direction relative to the raised table, to lock the phenolic cylinder in an orientation, which covers the desired portion of the involute during the cut, and to adjust the holding screws to the proper distance from the cutting wire as it passes by. Apart from simple lathing operations the jig was handmade.

We also used another technique based on precise templates for manufacturing foam segments. The handheld cutting tool in this case consists of a Y-shaped carrier which holds the hot wire taut between the elastic branches of the


Y. The stem serves as a handle. A raw block of foam is first mounted between a set of 1-in wide guides to cut a l- in wide slab of foam with two parallel sides. In this operation the hot wire is held against the guides under slight pressure while moving it along. This causes the hot wire to make a straight cut between the two guiding edges. The 1-in slab is then mounted between two templates which have the exact contour of the involute segment. Again, the hot wire is drawn under slight pressure along both template edges to finish the part. The guides were made from 1-in aluminum U-channel stock. The templates, shown in Figure 4, were made from 1/16in fiberglass material, copperclad on both sides (G-10 type circuit board). The bolts holding the template/foam assembly are guided by threaded bushings, soldered to the templates. This helps to align the templates for the circumferential cut (including the involute surface) orthogonal to the two parallel surfaces which were cut first. Before making the templates by hand, the involute line was inscribed on the copperclad material by the same basic geometric procedure described above. Figure 4 shows a finished foam segment between the fiberglass templates.

In both cases, the foam backing is thick enough to prevent deformation by the reflective copper layer, which was glued on. Of several materials available to us, we found a slightly springy vinyl-backed copper foil of 5mil thickness most suitable. Visual inspection was made of the reflection of a distant fluorescent light tube while its image travelled along the involute surface. Image deformation was about equal between the two foam backed segment types, and noticeably more than on the original segment, which was machined to a nominal accuracy of l-mil.

The ability to adjust the involute contour locally to an optimum fit led us to try another design. The same springy copper/vinyl foil was supported instead of by foam, by 6 pairs of 4-40in screws mounted on a carrier, where the screws were adjusted such that their tips aligned on two

/

Figure 4. Dual-template assembly for segment cutting


parallel involute contours, much like the two templates in Figure 4. Final alignment could then be effected by observing the 140GHz phase progression while changing the screws' insertion depth. A push/pull force on the involute surface would have been better than the push only force provided by the screws. Overall, this appeared to be a promising design, but we did not explore it further.

Rotating Launcher

Continuing the simplification process, we built a rotating dual-mirror assembly mostly from 1/16in copperclad fiberglass material. Its construction is apparent from Figure 5. The brass hub for the motor shaft was soldered to the bottom copperclad disk. The top disk with a large center hole is held in position by four equispaced brass bushings and screws. Both mirrors are of the same copperclad material. The center one is braced against the bottom disk with a triangular back plate. It is spot soldered to the brace and to the bottom disk. The outer mirror and its balancing counterpart are spot soldered between the two disks. The top hole and the cylindrical outer surface between the disks are covered with l-mil sheet mylar to reduce drag.

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Figure 5. Simplified dual-mirror launcher


Relinquishing Gaussian beam propagation as was sug- gested in [2], one might consider a waveguide feed according to Figure 6. Here the coupling antenna is coaxial with the motor shaft and is held by, but is insulated from the stationary upper feed waveguide. The lower waveguide ro- tates on the motor shaft. Its outer end curves around sufficiently to launch a tangential beam at distance R. The outer end is suitably flared. Figure 6 shows only the principle of operation. The rotating portion needs to be properly balanced and streamlined.

More radically yet, if all wave guidance components (coupler between stationary and rotating parts, transmission line, and endfire antenna) could be etched onto a single thin disk, this would further help raise the limit on rotational speed. The latter two launcher designs will operate at lower frequency.

Experimental Results

For concept evaluation , the remaining components of a complete frequency shifter were also assembled from 1/16in copperclad material. Figure 7 shows how some degree of structural rigidity was achieved with reinforcing ribs, spot soldered to the back and base plates of the device. In detailing our results we will refer to all components of

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Figure 6. Conceptual waveguide launcher

1108 Mart and Lammers

Figure 7. Frequency shifter made from G-10 material and styrofoam segments

this frequency shifter by adding the prefix copper. The original device was made from aluminum and will be referred to by that prefix. To investigate how critical individual shifter components are to overall electrical performance, we conducted measurements on: i) the all-aluminum shifter, 2) the aluminum shifter with copper segments, 3) the copper shifter with aluminum segments, and 4) the all-copper shifter. Copper segments of 1-in height used in case 2 were manufactured according to Figure 4. Owing to the different launcher design, the all-copper shifter accommo- dates segments i i/2in in height. These were manufactured according to Figure 3.

In Figure 8a the upper trace represents the IF waveform of the aluminum shifter, which corresponds to the spectrum in Figure 2. One segment response is displayed

Figure 8. Time domain waveforms of a) all-aluminum frequency shifter and b) aluminum shifter with copper segments


per 2 horizontal divisions. Only alternating segment transitions are discernible. The lower trace represents the phasor sum of the IF and reference signals, adjusted for optimum cancellation. When more highly resolved, the traces show that the transition discontinuities are in amplitude, not phase. They cannot be eliminated by reposi- tioning the segments. We had pointed out previously [I] that the dual-mirror reflection causes the electric field vector of the beam inside the frequency shifter to rotate 360deg once per 360deg of launcher rotation. The beam reflecting off the segment edge is polarized parallel with the edge on one pair of opposing segments and polarized orthogonally to the edge on the other pair. This apparently leads to a higher diffraction loss in the parallel-orientation case. We did not appreciate the effect until all experimental data had been taken. Obviously, the asymmetry can be reduced by having the transitions between segments occur with the E-field at a +/-45deg angle relative to the transition edge. We verified this experimentally by rotating the antenna, which launches the Gaussian beam, with a 45deg waveguide twist. The sidelobe suppression of the all-aluminum shifter improved by another dB. As expected, the time domain waveform showed lesser but identical dips at all four segment transitions. Polarization rotation does not occur in the waveguide launcher (Figure 6). Figure 8b was obtained with the same configuration as Fig- ure 8a, except for copper segments replacing the aluminum segments. The upper trace of this figure is the IF waveform again. It is expanded after the first horizontal division to show individual IF periods. The middle trace in Figure 8b is a replica of the upper one. The lower trace depicts the reference signal. Subtraction of the lower two traces would yield a display similar to that at the bottom of Figure 8a. A comparison of 9 periods out of 28 shows a reasonably good match. The IF signal lags the reference a little more at the beginning than at the end. There is amplitude variation evident in the expanded and condensed portions of the trace. The displayed values I0 and 20uS should read 100 and 200uS instead (readout mal- function).

Representative spectral displays of all four experimental component combinations prior to the 45deg beam rotation can be seen in Figure 9. In Figure 9a we duplicate Figure 2 for reference (the all-aluminum shifter, sidelines at -33.2dB or less). In the other three figures nothing was changed from the Figure 9a measurement, except for frequency shifter components and attenuator settings. The vertical scales are lOdB per division and the horizontal


Figure 9. Output spectra of frequency shifter: a) all- aluminum, b) aluminum with copper segments, c) copper with aluminum segments, and d) all-copper

scales are 96Hz per division. Because of attenuator ad- justments, power levels in Figure 9a through 9d cannot be compared directly. Relative output levels and sideline suppression for all component combinations are summarized in Table i.

In Figure 9b an output level reduction of 3dB resulted when replacing the aluminum segments with copper segments. At a first glance this appears to be the penalty for cylindrical involute shape, which refocusses the beam in only one plane and perhaps for imprecision in the contour due to manufacture. The minimum sideline level separation was measured as 17.9dB. Disregarding the fourth sideline higher in frequency than the main line, one finds all sidelines to be below -25dB. Individual sideline levels were quite dependent on segment adjustment. Compared to Figure 9d or to the bottom line in Table I, this single high sideline level is surprising. It did not yield to readjustment. Since different copper segments were used in Figures 9b and 9d, we attribute the higher sideline to imperfections in


Table I:

All-aluminum shifter

Frequency Shifter Performance

Output relative to Minimum sideline all-aluminum shifter suppression

(dB) (dB)

0 33.2

-3 17.9 Aluminum shifter with copper segments

Copper shifter with aluminum segments

2 28.6

All-copper shifter 2 24.8

one or more segments. In Figure 9c we have confirmation that the copper dual-mirror launcher operates quite satis- factorily in combination with the aluminum segments. In fact, output from the frequency shifter is higher by about 2dB than in Figure 9a. This is a direct consequence of the wider beam passages than those in the aluminum design. At 140GHz the aluminum launcher is constraining the Gaussian beam. It cannot be said, whether or not limited precision in the copper launcher offsets the benefits of the wider aperture to some extent. At any rate, the sideline suppression of 28.6dB is quite good. Finally, Figure 9d displays the all-copper shifter spectral performance. Output is still 2dB up from the all-aluminum shifter. This is better than expected, judging from the 3dB loss in Figure 9b, which is only partially compensated for by the good performance of the copper launcher in Figure 9c. The sideline suppression is good at 24.8dB. A comparison of cases 3 and 4 suggests that refocussing in both planes is not as important as thought when comparing cases I and 2. Rather, with a sufficiently wide launcher aperture the cylindrical involute segment performs equally well. The dielectric lens is underilluminated by the beam returning from the aluminum launcher. With straight segments and the wider copper launcher aperture the lens apparently is still able to capture the wider returning beam. Segments produced by the template method are assumed to yield a similar output level in case 4. A single set of copper segments in these measurements would have been better for comparison. The dual set resulted from the two manufacturing methods and from the differences in frequency shifter design. Of the


four combinations studied only the all-aluminum and all- copper shifters appear to be practical. The others helped in assessing component performance.

Conclusions

We reasoned in an earlier paper [2] that the minimum bandwidth achievable in a frequency-shifter based heterodyne system would be largely independent of the frequency and phase stability of the two signal sources involved and be independent of the mechanical precision of the frequency shifter. There is only a requirement for the mechanical rotation of the frequency shifter to be phase coherent with the signal source driving it. This is achieved with a synchronous motor. We have demonstrated here that a simplified device can be constructed successfully, using only a lathe and hand tools. The foam-backed segments produce acceptable results. We did not attach them individually to base plates for solid mounting nor did we provide them with provisions for fine adjustment. This made the measurement difficult. Such features would be a necessity in a practical design. The performance of the simplified frequency shifter in terms of undesired-sideline suppression is quite satisfactory. The frequency conversion efficiency is high by any millimeter or submillimeter standards. Disk aperture size was found to be a critical parameter relating to efficiency. It remains to be seen, whether or not the increased sideline levels present a problem relative to the performance obtained from the best rotary frequency shifter built so far. If the received signal bandwidth is near zero, appropriate signal processing can further suppress the effects of existing spectral sidelines.

References

[I] U.H.W. Lammers, R. A. Marr, and J. B. Morris, 1990, Int. J. of Infrared and Millimeter Waves, Vol. II, No. 3, pp. 367-382.

[2] U.H.g. Lammers, R. A. Marr, and J. B. Morris, 1990, Int. J. of Infrared and Millimeter Waves, Vol. ii, No. 6, pp. 701-716.

evaluation of a simplified doppler frequency shifter

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