New Series of Microwave Sweep Oscillators with Flexible Modulation and Leveling

In microwave work one of the valuable and inherently appealing test instruments is the microwave sweep generator. Such generators can greatly simplify difficult microwave investigations, as discussed in several past articles' on sweep-frequency techniques.

A new series of microwave sweep oscillators has now been designed which incorporates a number of new conveniences. These oscillators, especially when used with some of the advanced components recently developed in the -hp— laboratories1, make microwave sweep methods simpler, more accurate and more meaningful than ever.

The new sweepers collectively cover the fre quency range from 1 to 40 gigacycles (kilomega-cycles), A major factor in their convenience derives from their use of a p-'t-n diode type modulator in the output circuits of those generators covering the 1 to 12.4 Gc region. This type of modulator permits the generators to be modulated or leveled with virtually no reaction on the generated carrier frequency, since the voltages and currents in the oscillator tube remain constant. As a result, more accurate frequency specifications are possible in all operating functions.

I P. D. Lacy and D. E WliMler. "J* New 3 - 12 kMc Voltage Tuneil Sweep flsc1 later (or Fni'er Microwave Eiralmtioni", Hewlett-Packard Jaurnal Vol S N';. S. Feb., 1»?.

P. D. Lacy attd D E. Wheeler. "Ptrmanirlt Retort) and Oscilloscope Techniques «ith the Microwave Sweeu OscillJlor", Hiwielt-Pai urd Journal, Vol. 9, No. 1-2. Sept.-Oct. 1957.

'ft, B Riley. "A New Coaxal Crystal Detector w.th Ejtremely Flat Frequency Response". Hewlett-Packard Journal, Vd. 15. No. 3. No*. 1963. N J, KuM, "A New Microwave Modulator". HewlettPacMrd Journal, Vol. 14, No 7 S. Mar. ■ Aor., 1463.

High resolution microwave spectrum examination, p. 8.

Fig. 2, Comparison of leveled power incident upon u typical-VSWR load when external leveling is used (upper curve) and when internal leveling is used (lower catve). Equipment arrangements for making curves are shown i/t Fig. 5 and discussed in text.

Fig. 1. New -hp- Microwave Sweep Oscillators collectively cover frequency range from 1 to 40 Gc, hare many conveniences ¡or sweep-frequency type measurements. Either internal or external "leveling" am he used, hut external leveling is to he preferred, as discussed in text.

Fig. 2, Comparison of leveled power incident upon u typical-VSWR load when external leveling is used (upper curve) and when internal leveling is used (lower catve). Equipment arrangements for making curves are shown i/t Fig. 5 and discussed in text.

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Fig, 3(a). New -hp- Model 692B Sweep Oscillator operates from 2 to 4 Ge. "B" suffix on model /lumber indicates instrument has p i n diode type modulator/ attenuator in its output circuit for maximum flexibility in modulation and leveling.

Fig. 3(b). New -hp- Model C,S3A Sweep Oscillator operates from 4 to S (le. Both ".4" und "B" series of Oscillators hare exclusive panel tamp to indicate proper leveling operation.

For applications where this improved performance ts not required, ;i second series of rlie generators ("A" series) is provided with a grid-cathode type modulation circuit. The accompanying table shows the frequency ranges and minimum rated power output of the generators in each of the series.

Other conveniences achieved in the new generators include ar< u-rately-calibrated wide- and narrowhand sweeps, independeni start and stop frequency controls, a manual sweep control for setting end-point and marker frequencies with wave-meter accuracy, and other features described later. SWEEP ARRANGEMENTS

Fig. 3 shows ihe front panel of the new sweepers. As indicated by the four Ft notion push-button switches at the lower left hand center of the panel, there are four types of sweeps with a choice of four sweep modes (sweep slu.ctok switch) foi each sweep. In ihe star! stop function (l-ig. 4a), the frequenty at which the sweep starts is set by the start cw control, and the frequency at which the sweep stops by the stop aF con trol. Both controls are independently adjustable to any frequency in the entile band and tan he st-t so that the sweep will occur toward a higher or a lower frequency with respect to time.

In the AF function, the instrument sweeps across a narrow band centered upon a frequency which is set by the start/cw control, as shown in Fig. 4(b). Tile width of the band being swept is set In the stop .iF control which is calibrated (lower scale of slide-rule dial) from a maximum sweep width of 10";. ol the total frequency hand to a minimum of approximated 0.1',' of the total band. In automatic sweep, then, the instrument sweeps from the <:w —ViiF frequency U> the cw + WA* frequency.

Two independent markers set on separate digital dials (marker t and marker n) and direct reading in Gc can be positioned anywhere on the start-stop or ¡IF sweeps. The mark ers amp)itude-modulate the RFoUi put power and are triangular in shape. They are about 2 to I db of power in amplitude and approximately 0.1% of the frequency hand

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SWEEP GENERATOR RANGES AND DESIGNATIONS

Minimum

Frequency Range

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17 4 Gc to 18.0 Gc 1B.0 Gc to 265 Gc 26 5 Gc to 40.0 Gc

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60 mw 40 mw

10 mw 10 mw in width. Since their width is a function of RF frequency and not of time, their resolution is constant regardless of the sweep speed, i.e.. of the sweep time setting.

A third independent sweep arrangement is extremely useful when narrow-band and wide-band measurements are required alternately on the same device. I his sweep, called viarkf.r sweep, uses as end [joints the frequencies set by the \i \hkkr i and m \kki:k it ( ontrols. H\ use of this sweep arrangement, it is easy to change from the main sweep to a different sweep range without t hanging the end [mints of the main sweep. For example, the arrangement enables a wide-band sweep lo be set up on the start stop sweep, and the portion of this sweep bra< k eted by markers to be expanded in marker sweep for higher resolution. Also, one sweep can be set up on start stop or _iF, while a second sweep covering a different part of the generator band can l>e set tip in marker sweep. In either case one i an quic kly i hange from one sweet> to another with the push-button switches without resetting end frequencies. marker sweep has the same art nracy specifications as the main start stop sweep.

A fourth sweep arrangement (Ext km) provides for external sweeping or for control of the Rl- frequency by external voltages. The center RF frequency is established in the start cw control, and the full range of the generator can be swept. "T he

FREQUENCY {GO (ui Typical Start /Stop sweep cohering a largn portion of Oscillator's 1-2 Cc frequency range. Internal murker* are here set at 1.42 and, 173 Cc, find a wavemeter pip is set at 1.56 Gc.

1 56

FREQUENCY (GC)

(b > Typical use of a/-' sweep in which RF output is being swept ±5 Mc from l.oti Gc utauemeter pip in (a).

1 56

FREQUENCY (GC)

(c) Typical use of Marker Sweep in which RF output is being swept between tiro markers at 1.42 and 1.72 Gc in (a).

Fit;. 4. Oscillogram« demonstrating three of the sweep arrangements in the new -hp- 690 Series Sweep Oscillators.

external FM input, located immediately under the ext fm push button on the panel, is direct-coupled for programming and automatic frequency- control application*» and has a bandwidth greater than kt. SWEEP MODES

The four sweep modes provided by the sweep sei ector switch at the left of the panel can l>e used with any of the three intei rial sweep functions, i.e., with any of the h notion push buttons except ext fm. Some of ihese sweep modes are self explanatory, but there are several associated considerations that have special interest. One is that the generators have been provided with an exclusive manual sweep control Which can he used to manually sweep the RF frequeue y between the precise end point settings. Phis control thus greath simplifies establishing setups involving X-Y retord ers or osc illoscopcs. It also fa< ilitates selling the end point markers with a wavemeter when higher acc uracy is desired than is provided !>\ llie marker calibrations.

The TRitiCK» mode permits the sweep to be initiated either by the panel iricckk pushbutton or by an external signal. The triu;ek pushbutton will also return the RF iie-quency to its start position before the end of the normal sweep when the pushbutton is depressed. Thus, on slow sweeps, this arrangement is particularly useful when it is desired to terminate the sweep prematurely.

In auto sweep the sweep is continuously adjustable from 11» milliseconds to 100 seconds, as determined by the swff.p time controls. \n end jjositton on the sw n p ttvit vernier synchronizes the sweep to the power line frequency.

The CW position is normal!}

used when single-frequency output is desired.

When the instrument is sweeping, a sweep on light glows at the top of the panel as a positive indication of sweeping. This is particularly designed for slow sweep applications and is useful lo indie ate that the sweep has ended when no indicating equipment suc h as an osc iHose ope or recorder is being employed.

SWEEP-RELATED VOLTAGES

In all internal sweep functions two sweep-related voltages are provided for use with external equipment. One is a sawtooth for driving the horizontal system of an X-Y recorder or oscilloscope. The second is a voltage directly related to output frequency regardless of the function or mode used. This voltage is thus useful for triggering voltage -coincident circuits or to give an external voltage indication <>r the RF frequent y.

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Fig. 5(a). "'External leveling" equipment setup used to make record shown in Fig. 2(a). I 'se of long cable between generator and had still results in good leveling.

Fig. 5(b). "Internal leveling" equipment setup used to make record shown in Fig. 2(b). If ALC circuit is internally contained, leveling at SO-okm load is poorer in typical case where filter, cable, etc . (simulated by VSWR = 1.4 block) is required ahead of load.

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Fig. 6. Block diagram t>f circuit arrangement o\ "Ii type of new - h /> 690 Serie> Sweep Oscillators which uses p i n modulator in output circuit.

POWER LEVELING

To permit leveling ihe RF output power, a wide-band high-gain amplifier has been included in the in-sirument. \ directional coupler and c rystal detector, required to complete the AFC (automatic level control) loop, can be lota ted either inside or outside of the instrument, at ihe option of the user. There is, however, a decided advantage in externall) leveling the RF power at the load. If the power is interna 11\ leveled, poor directivity* in the sampling coupler, together with mismatch and transmission losses between the sampling coupler and the load, will cause a considerable variation a<ross the frequent y band of power incident upon ihe load. An example of this is shown in Fig. 2 (b) , an \-V recorder plot of power incident upon a medi um-VSWR load as a function of RF frequency, lo make this plot, a high-directivity toupier and detector were located at the front panel of a f>91B Sweep Ost illator to simulate internal leveling, The block diagram of the equipment arrangement is shown in Fig. 5(b). A feed-through device with a measured maximum VSWK of 1.4 was placed between the sampling coupler and the load. The load.

' The equivalent source match tor the 1 sue led source « i function of ttiu coupler's directivity consisting of a 50-ohm termination (VSWR = 1.05) anil a -I78A Thermistor Mount connected to an —hp— 431B Power Meter {VSWR—1.2) Connected in parallel, provide a load load VSWR of approximately 2.0. The resultant power variation across the frequency band, measured With ilie power meter and thermistor mount, was approximately 1.0 db. In contrast to this. Fig, 2(a) shows tlie power incident on the same load when the same sampling coupler is located in an external leveling arrangement, as shown in ihe block diagram of Fig. 5(a). In (his case ihe power variation across the frequency band was only about .25 db.

In addition to producing flatter power output across the frequency band, external leveling has the convenience of producing good leveling when the sweep oscillator is placed a considerable distance away from the load under lest. Any length of cable, or any component, such as a low-pass filter, placed between the sampling coupler and the load will result in a sign ifit anil v greater powet variation fate as a result of mismatch losses and low sampling couplet directivity, hi addition, external leveling is particularly advantageous when the

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