Widerange Rc Oscillator With Inline Digital Frequency Readout

Central to the popularity of the general-purpose RC oscillator is its continuously adjustable frequency dial. In fact, the RC oscillator did not become popular imtil variable air capacitors were used to provide continuous adjustment over 10:1 frequency bands. This method of frequency selection, with dial and vernier, provides a convenient compromise between ease of reading, settability, resolution, and ability to sweep. It will no doubt continue to be most popular for general laboratory use.

As requirements for greater accuracy and resolution and for digital program-mability have had their effect on signal sources, noncontinuous or discrete fre quency controls have replaced the continuously variable air capacitor. This trend is most apparent in sources using synthesizer techniques, where seven or eight digits of resolution are common, and also in RC oscillators where pushbutton and rotary switches are used to control three or four digits. Since the use of discrete steps allows closer tracking of the tuning elements, this type of tuning most often appears on high-performance, high-cost instruments, where the objective is to improve performance characteristics of the source, such as stability of output, distortion and noise. In almost all cases the actual method of frequency selection was resorted to as an undesirable necessity, z

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Figure 1. The Type 13T2 Decade Oscillator.

Figure 2. In-line readout of frequency with positioned decimal point and units. Least significant digit is continuously variable with a detent at zero.

and little attempt was made to utilize the discrete steps to provide a more readable or more easily set control. The use of XI, X10, etc range multipliers and confusing decimal-point locations with strange units is still common.

And yet, this type of frequency selection can be quite desirable in its own right. Its greatest advantage is that the same frequency can be reset very accurately, over and over again, limited only by the stability of the oscillator. And it can eliminate perhaps the worst disadvantage of the continuously adjustable dial: difficulty in reading the frequency quickly, accurately, and unambiguously.

The new General Radio Type 1312 Decade Oscillator provides for the first time a general-purpose oscillator with a discrete repeatable type of frequency selection, at a price comparable to that of other general-purpose solid-state oscillators. The 1312 embodies many of the characteristics of the popular Type 1310-A1, with additional features of an 80-dB step attenuator on the output and a rack-width package with front and rear output terminals.

The frequency range is 10 ITz to 1 MHz with an accuracy of ±1% of reading. The frequency is determined by four rotary controls, which provide an in-line three- or four-digit readout

' R. E. Owen, "A Modern. Wide-Ranee RC Oscillator." General ftailio Experimenter, August 1905.

with positioned decimal point and frequency units. The decimal-point location and units have been selected so that the frequency appears as it is normally written. Twenty kilohertz appears as 20.0 kHz, not as 20000 IIz, 2.00 X 10 kHz, or 200 Hz X 100. A single exception is that 1 megahertz appears as 1000. kHz, a frequency indicated by some oscillators as 999 kHz.

The first two most-significant digits and the units control are multi-position rotary switches with a light but positive detent mechanism for easy setting. The third digit is a continuously adjustable potentiometer with a detented zero position. This allows the selection of 100 discrete, highly repeatable frequencies within each decade and continuous coverage in between. Rotary controls were selected over push-buttons because of operator preference in high-rate testing. This in-line readout of the rotary switch is generally preferred over the columnar readout of the pushbutton, just as it is on electronic counters.

The high repeatability of this type of frequency selection is illustrated by the two examples shown in Figure 3. These show the frequency of a typical 1312 after warmup in a normal production environment. Three frequencies, one decade apart, were selected. The same three frequencies 10 minutes later are all within 0.002% of their original

Figure 3. Frequency re-settability of the 1312. Difference in frequency at 100 Hz, 1 kHz, and 10 kHz after (a) 10 minutes and (b) 40 hours.

Figure 3. Frequency re-settability of the 1312. Difference in frequency at 100 Hz, 1 kHz, and 10 kHz after (a) 10 minutes and (b) 40 hours.

value. Even after 40 hours the change is less than 0.01%. The effects of line-voltage changes on the output frequency are also seen to be small (Figure 4),

The oscillator output is 20 volts open-circuit behind 600 ohms. The voltage is quite constant with changes in frequency (Figure 5). Frequency-

response measurements can be made quickly, since controls do not have to be adjusted to keep the output constant. The distortion in the output is low, particularly in the middle of the frequency range, and remains low regardless of the size of the load or attenuator setting. Even a short circuit at full output will not cause clipping of

Figure 4. Stability of 10-kHz output frequency with dblO-voIt line voltage changes.

Figure 5. The output-voltage-vs-fre-quency characteristics of a typical 1312 with 600-ohm load.

10 Hi

I kHz FREQUENCY

I MHz

10 Hi

I kHz FREQUENCY

I MHz

IO Hz

I kHz

FREQUENCY

Figure 6. Typical output distortion of the 1312 with 600-ohm load.

IO Hz

I kHz

I MHz

FREQUENCY

Figure 6. Typical output distortion of the 1312 with 600-ohm load.

the waveform. Noise at frequencies far from a 1-kIIz fundamental, measured in a bandwidth of 5 Hz to 500 kHz, is typically less than 0.02%. Noise close to the fundamental is also low, as a close-in spectrum analysis of the 1-kHz fundamental shows (Figure 7).

The output level of the oscillator can be reduced to 200 ¿¿V open-circuit by means of a stepped 80-dB attenuator (20 dB/step) and a continuously adjustable attenuator with a range of more than 20 dB. The continuous con trol is not calibrated but is marked with the approximate open-circuit output voltage. The closely spaced numbered graduations permit convenient return to a given attenuator setting.

Between the 20-dB step positions of the attenuator are so-called "zero-volt-output." positions. This feature, d/lso available on other GR oscillators, provides a convenient transient-free means of reducing the output to zero without disturbing the continuous control setting or shorting or disconnecting a

Figure 7. Noise spectrum close to 1-kHi output. Note — 100-dB resolution.

Figure 7. Noise spectrum close to 1-kHi output. Note — 100-dB resolution.

FREQUENCY

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OUTPUT

Figure 8. Oulpul attenuator has intermediate positions where voltage is reduced to zero with 600-ohm output impedance maintained.

carefully shielded system. Regardless of where the attenuator is set, a zero-output position is always adjacent. The output is not shorted, but instead the normal 600-ohm output impedance is maintained. This aids in locating sources of extraneous signals and ground loops that would be masked by the presence of the normal output or by shorting. A rear-panel female bnc output is in parallel with the front-panel binding posts.

The 1312 has the external-synchronization feature 2 originally introduced on the General Radio line of RC oscillators. This permits locking the oscillator frequency to an external signal's frequency, and it also provides a constant 0.8-volt output regardless of the attenuator setting. Connection is made by way of a rear-panel female bnc connector.

The 1312 has obvious applications in production or quality-control test ing. Repetitive measurements at a variety of widely separated frequencies can be made very accurately. The high repeatability assures uniform testing and allows pre tuned filters and distortion meters to be used to speed the operation.

The 1312 is also very valuable in applications in which, contrary to the above, frequency is changed very seldom. In capacitance-measuring sys tems, for example, almost all measurements are made at GO ITz, 120 Hz, I kITz, 100 kHz, or 1 MHz. Once the frequency is set it is rarely changed, but it must often be verified that the frequency is in fact the desired one. The unambiguous readout makes this easy even with low-skill operators. A misad-justment is more apparent than on conventional continuous dials, and the ±1% accuracy of the 1312 is more compatible with system requirements than is the 2% or 3% of dial oscillators. The compact 1312 takes up a minimum of valuable eye-level space in a test rack, and the rear-panel connections leave the front of the rack free of clutter.

How It Works

The 1312 Decade Oscillator uses the modified Wien circuit shown in Figure 10. This circuit oscillates (i.e., its trans it C

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