MHz Synthesizer Joins The Family

Figure 1. Type 1164-A7C.

The newest addition to the GR synthesizer line, the T vpk 1 164-A Coherent Decade Frequency Synthesizer (Figure 1), offers greatly expanded frequency coverage, extending well beyond the normal high-frequency, radio-communications bands and including the popular intermediate frequencies. All the features of the low-frequency models12' — modular construction, in-line readout, provision for sweeping, continuously adjustable decade (CAD), program-mability,3 ac and battery operation have been retained, and a few new ones added.

The frequency synthesizer, a relatively recent development, is nonetheless well established as an important instrument in the laboratory and in countless frequency-control applications. Also well established is GR's 1 1 60 family of coherent decade frequency synthesizers, which have until now supplied frequencies up to 12 MHz. Now a new member—the 1164 — extends coverage to 70 MHz and thus brings the considerable advantages of this type of instrument to many more users.

To review the chief characteristics of the 1160 line briefly: Each synthesizer can contain up to seven plug-in digit modules, each controlled by a front-panel rotary switch, plus a continuously adjustable decade (CAD). By the push of a button, the CAD can be electrically substituted for one or more of the step-digit modules. The output level is adjustable up to 2 volts «and is monitored by a panel meter. The synthesizer can be locked to an external frequency standard, can be swept, and, with appropriate plug-in modules, can be programmed.

Because of the modular construction, the 1 1 (50 series can be provided in a great many variations to suit customer requirements. One can, for instance, buy a synthesizer with as few as three of the maximum seven digits, adding more later as more resolution is needed. The programmable option further increases the number of available models. For the

1 A. Noyes, Jr., "Coherent Decade Frequency Syn thesizers," General Radio Experimenter, Sept-ember 196-1. 1 A. Noyes, Jr.. " 12-Mc Coherent Decade Frequency Synthesizer," General Radio Experimenter, November-December 1965.

3 G. H. Lohrer, "Remote Programming for GR Synthesisers," General Radio Experimenter, May 196o.

Communications receiver i-f with 455-kHz center frequency and 1,5-kHz mechanical filter. Horizontal scale: 500 Hz/cm. Marker frequency: 455.00 kHz. The CAD was in the 1-kHz position and was swept ± 2'/2 major divisions, or ± 216 kHz.

Response of 50-MHz crystal filter. Horizontal scale: 1 kHz/cm. Marker frequency: 50.00000 MHz. The CAD was in the 10-kHz position and was swept ±1 major division, or ± 10 kHz.

Response of GR Type 1900 Wave Analyzer tuned to 10 kHz and set for 10-Hz bandwidth. Horizontal scale: 5 Hz/cm. Marker frequency: 10,000.00 Hz. The CAD was in the 10-Hz position and was swept ± 2'/i major divisions, or i 25 Hz.

Communications receiver i-f with 455-kHz center frequency and 1,5-kHz mechanical filter. Horizontal scale: 500 Hz/cm. Marker frequency: 455.00 kHz. The CAD was in the 1-kHz position and was swept ± 2'/2 major divisions, or ± 216 kHz.

Response of 50-MHz crystal filter. Horizontal scale: 1 kHz/cm. Marker frequency: 50.00000 MHz. The CAD was in the 10-kHz position and was swept ±1 major division, or ± 10 kHz.

Response of GR Type 1900 Wave Analyzer tuned to 10 kHz and set for 10-Hz bandwidth. Horizontal scale: 5 Hz/cm. Marker frequency: 10,000.00 Hz. The CAD was in the 10-Hz position and was swept ± 2'/i major divisions, or i 25 Hz.

Figure 2.

Oscillograms showing frequency response of various filters to swept output of synthesizer CAD.

1104 alone, twenty "standard" combinations are listed.

Applications

One of the most common uses for synthesizers is frequency control of communications transmitters and receivers. Used with a transmitter, a complete 70-MITz synthesizer is roughly equivalent to a bank of 7 million crystals that can easily be switched to change frequency. (There are 700 million readable frequencies when the CAD is used.) In a receiver, the synthesizer can serve as a highly stable local oscillator.

The 70-MIIz upper frequency limit of the new synthesizer covers not only most of the commercial broadcasting spectrum but, also intermediate frequencies of many vhf and uhf systems. Frequency-translation techniques can be applied, moreover, to provide useful frequencies for devices operating in the gigahertz region.

In a telemetry receiver, for example, a synthesizer can be used as a second-or third-conversion oscillator, automatically compensated for Doppler shift.

The Doppler shift is sensed by an external phase detector, which in turn is used to control the synthesizer's CAD frequency. Or, in a nuclear magnetic resonance (NMR) study, the syn-thesizer frequency can be added to the output of a stable microwave source to produce a 70-MHz-wide microwave band under synthesizer control. Conversely, the NMR band can be beat against a stable microwave frequency and the beat frequency compared with the synthesizer output.

Of course, the synthesizer output itself can be multiplied, but, the higher the multiplier, the more noticeable any instability.

Sometimes it is desirable to divide the synthesizer frequency. In one application, for instance, an aerospace laboratory wished to measure the rotational speed of a satellite to eight significant digits, then to supply a corresponding frequency to the satellite. With the satellite rotating only 100 revolutions a minute, it was necessary to divide the ^^ synthesizer output by means of a 104 scaler; the synthesizer then served ad mirably to measure and to reproduce the satellite rotational frequency.

The sweep capability is all-important in many applications and is especially so in the measurement of the characteristics of crystal and other mechanical filters (see Figure 2). The CAD can be swept electrically over dt 5 major dial divisions, corresponding to a frequency span as wide as 1 MHz or as narrow as 10 Hz. The sweep can be centered on any point of the 12-division manual dial except on the 100-kIIz functional position, where the limits for the frequency excursion are — 100 and + 1100 kHz.

Perhaps the single most important capability of a frequency synthesizer is programmability. In the Type 1164 synthesizer, output level as well as frequency can be programmed. This means, for instance, that in a frequency-response measurement, frequency and level can be simultaneously programmed to effect 3-dB, 6-dB, 10-dB, etc, level changes at selected frequencies.

The maximum programmable bandwidth of the Type 110 4 synthesizer is at present 1 MHz. In other words, one can electrically control output frequency

HOW IT

Figure 3 is a complete block diagram of the synthesizer.

The DI-1, CAD, and AFS-1 modules have been described in an earlier article.1 The new modules are briefly described below.

DI-2, DI-3 Digit-Insertion Units

Figure 4 is a block diagram of the DI-2 unit. The frequency of the digit oscillator goes from 40 to 19 MHz in 1-MHz steps as its dial rotates from 0 to 9. The digit-oscillator frequency is rough-tuned to the proper frequency by a step switch controlled by the dial, and a voltage-control servo establishes phase lock with a multiple of the reference frequency. A gated or sampling-type phase detector Ls used here. The gate opens only during every 10th to 49th cycle (depending on the frequency selected) of i Ibid.

over any 1-MHz (or smaller) range, up to the 70-MHz frequency limit of the synthesizer. Frequency switching time is less than two milliseconds.

Constructional Features

The synthesizer consists of plug-in modules in a bench or rack frame with a panel only 5^4 inches high. Five of the seven digit modules in the Type 1164 are identical and interchangeable with one another and with corresponding modules in other GR synthesizers. The advantages of such modular construction in servicing are obvious. Less obvious, perhaps, but equally important are the manufacturing economies of this approach, which arc translated into low prices.

All etched boards in the instrument are made of fiberglass, and the improved power supply uses all silicon transistors. The rear panel is an engineer's delight: in addition to the primary output (also available at the front panel), the following are available: 100 kHz, 1 MHz, 5 MHz, 5-5.1 .MHz, 30 MHz, 42 MHz, 40-49 MHz, 50-51 MHz, 90 MHz, and +18V dc.

WORKS

the output frequency of the DI-2 unit (every 30th to 24th cycle of the DI-3) and stays open for only a small fraction of a period of the output frequency. If the phase of the output frequency is crossing zero while the gate is open, no voltage is placed on the holding capacitor, and therefore no correction voltage is applied to the digit oscillator. If the phase of the output frequency has passed zero in a negative direction when the gate opens, a negative voltage is placed on the holding capacitor; if the phase has not reached zero, a positive voltage is placed on the capacitor. With the proper phasing in the frequency-control loop of the oscillator, the phase error between the reference-fre-quency-controlled gate and the output frequency is minimized. The hitter is thus locked at an exact multiple of the reference frequency.

30 TO 39 X IOO kHz PICKET FENCE

BEAT FREQUENCY

30 TO 39 X IOO kHz PICKET FENCE

BEAT FREQUENCY

1960 Military Frequency Hardware

REF LINE

Figure 3. Block diagram of (he synthesizer.

REF LINE

TO ALL - l-7-a MODULES

SUPPLY ( PS-2)

îi TO 28 V 1-6 A

I05 TO 125 V, 195 TO 233 V, OR »210 TO 230\< 90 TO «OO W 60 * MAX

I05 TO 125 V, 195 TO 233 V, OR »210 TO 230\< 90 TO «OO W 60 * MAX

Figure 3. Block diagram of (he synthesizer.

Any change in the output frequency is immediately sensed as a phase error and corrected.

Proper phase condition for phase lock exists whenever the digit-oscillator frequency is an exact multiple of the reference frequency. Rough-tuning to the approximate frequency by the digit dial determines which multiple is chosen. A dial-light warning system indicates failure to achieve stable phase lock. As in the D 1-1*8, the presence of any appreciable ac signal in the phase-control loop is sensed, causing the dial light of the affected unit to go out.

The DI-3 unit is identical to the DI-2 except that the frequency from the ancillary frequency source is 5 MHz, and the step-tuned oscillator goes from 150 MHz to 120 MHz in 5-MHz steps as its dial setting is changed from 0 to 6.

Fixed-Frequency Multiplier (FFM-1)

The fixed-frequency multiplier is shown in Figure 5. Undesired frequencies are rejected by frequency-selective amplifiers and by the connection of pairs of diodes in push-push for doubling and push-pull for tripling. Levels are kept relatively high to minimize noise.

Mixers MM-1 and IM-1

Figures 6 and 7 are block diagrams of the multiplier mixer (MM-1) and intermediate mixer (IM-1), respectively. In each, bandpass amplifiers reject frequencies outside the desired ranges. Each uses transistor frequency multipliers and double-diode balanced mixers (one in the MM-1, two in the IM-1).

Output Multiplier Mixer OMM-3

Figure 8 shows the output multiplier mixer in block form. The 150- to 120-MHz output of the DI-3 unit, is amplified in a two-stage bandpass amplifier, doubled in frequency by a pair of diodes in a full-wave doubler, and filtered by a six-pole bandpass filter to provide the input to the final mixer, which produces the 10-MHz steps in the output. A four-diode double-balanced mixer is used to subtract this frequency from the 300- to 310-MHz output from the IM-1 unit to produce the final output frequency.

Signal levels at this mixer are kept low to minimize undesired mixing products within the output-frequency passband. A six-stage broad-

Figure 4. Block diagram of the DI-2 Digit-Insertion Unit.

STEP-TUNED DIGIT OSCILLATOR WITH VOLTAGE CONTROL

40 TO 49 MHi

Figure 4. Block diagram of the DI-2 Digit-Insertion Unit.

STEP-TUNED DIGIT OSCILLATOR WITH VOLTAGE CONTROL

l-MHi REFERENCE FREQUENCY FROM A FS

40 TO 49 MHi l-MHi REFERENCE FREQUENCY FROM A FS

FROM afs

FROM afs

Figure 5. Block diagram of the Fixed Frequency Multiplier (FFM-1),

90 MHz FROM FFM-I

5 to

90 MHz FROM FFM-I

I40 TO 141 MHz TO IM-I

30 TO SI MHz AU* OUT

Figure 6. Block diagram of the Multiplier Mixer (MM-1).

I40 TO 141 MHz TO IM-I

30 TO SI MHz AU* OUT

Figure 6. Block diagram of the Multiplier Mixer (MM-1).

SO MHz FROM FFM-I

SO MHz FROM FFM-I

200_TO 261 - MHz

m(

BAND-PASS FILTER

300 TO 310 MHz TO OMM-3

300 TO 310 MHz TO OMM-3

Figure 7. Block diagram of the Intermediate Mixer (IM-1).

ISO TO l20_MHz FROM 01-3

wOs 0.01 TO70 MHz OUTPUT

NON-LINEAR FEEDBAC*

wOs 0.01 TO70 MHz OUTPUT

NON-LINEAR FEEDBAC*

ISO TO l20_MHz FROM 01-3

Figure 8. Block diagram of the Output Multiplier Mixer (OMM-3).

■j^HHj William F. Byers was graduated with high n^^^P^^^^Hj honors from Ohio Uni-■ versity in 1943 with the _ ). degree of BSEE. He was a visiting lecturer in | electrical engineering at ^mt Ohio University prior to y coming to General Ra-dio as a development engineer late 1943. He is at present a Sec-lion Leader in the devel-^^^^^ opment group concerned with radio-frequency circuits, including standard-signal generators, oscillators, and frequency synthesizers. He is a member of the IEEE.

band amplifier covering the frequency range of 0.01 to 70 MHz follows the output filter of the mixer, increasing the level to a maximum of 2 volts behind 50 ohms. This level is maintained even with the output connector short-circuited.

Two of the earlier stages are variable in gain by diode-con trolled emitter degeneration, permitting automatic output leveling. The last two stages are operated in push-pull, using complementary transistor pairs. Emitter, shunt, and feedback compensation are all used to achieve a reasonably fiat response. The full-wave, peak-responding output rectifier senses voltage ahead of an accurate 50-ohm resistor to provide a 50-ohm source.

Following the output rectifier is a dc amplifier with a nonlinear feedback network, which makes the dc output a linear function of the ac signal applied to the rectifier. That is, it linearizes the rectifier characteristic over an ac-voltage range of 0.4 to 2 volts. This amplifier drives the level-control circuits and output meter. The linear scale on the output meter thus gives a true account of output level, and the voltage available for automatic level control is linear over this range. The automatic leveling circuit is completed by a difference amplifier, which compares the indicated output voltage with an adjustable voltage set by the output control or applied at the level program connector, and which delivers a control current to the variable-gain amplifier to minimize the difference. Enough gain variation is available so that any output level from 0.2 to 2.0 volts can be set. This level is then automatically maintained to within 0.3 dB for all load and frequency variations within the range of the instrument. An external detector can easily be connected to the automatic-level-control bus (internal control is disabled when the panel level control is turned to the off position).

Power Supply PS-2

The power supply is an improved version of the PS-1 used in earlier synthesizers. It can supply more current at 18 volts (regulated) to meet the demands of the 1164-A with a 200-mA reserve for accessories. The new, all-silicon-transistor power supply is completely short-circuit proof and current-limited.

A toroidal power transformer is enclosed in an A-metal case to minimize stray fields. A special input jack permits operation of the synthesizer from a battery, with the internal series regulator functioning to maintain normal operation with battery voltages from 20 to 28 volts.

CREDITS

The present four GR Synthesizers are the result of the combined efforts of Atherton Noyes, Jr., Group Leader, G. H. Lohrer, C. C. Evans, and the author.

In the Type 1164-A, the primary responsibility for the new modules, IM-1 and I'S-2, was that of G. H. Lohrer; for the new 1)1-2 and IJ1-3 modules, t hat of C. C. Evans. The primary responsibility for the remaining new modules, the FFM-1, MM-I, OMM-3, and the new chassis, as well as coordination of the effort, was that of the author.

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