Function Generator: Resistance-Capacity Tuned Audio Oscillator with Lamp Amplitude Control (Hewlett, US 2,268,872, 1942)

The first Hewlett-Packard product: a test oscillator tuned by resistors and capacitors instead of coils. Two valve amplifiers feed their output back to their input through a series and a parallel R-C pair, which passes only one frequency in phase, so the circuit oscillates there. A second, negative feedback path through a small lamp holds the amplitude steady: as the swing grows the lamp warms, its resistance rises and the gain falls. One dial covers ten to one in frequency. Result: a sine-wave signal generator covering 20 to 20,000 cycles in three ranges, about 0.25 % distortion. Source: W. R. Hewlett, US 2,268,872 (1942). Unknown: valve types and component values. Picture: the patent drawing.

متقدم

Unknown

التعليمات

Build the two-stage amplifier

Wire two amplifying valves in cascade with resistance-capacity coupling, the first's anode to the second's grid, run as class A amplifiers. Not given: valve types, values, the supplies. Ours: pentode valves in sockets.

المواد لهذه الخطوة:

صمّام خماسي الأقطاب2 قطع

قاعدة الصمّام2 قطع

المقاومة1 طقم

طقم مكثّفات1 coupling and by-pass

علبة ألومنيوم1 case

سلك لحام1 as needed

الأدوات المطلوبة:

كاوية لحام

Fit the tuning network

From the second valve's anode, through a coupling capacitor, feed back to the first valve's grid through a resistor R1 in series with a variable capacitor C1, with a resistor R2 and variable capacitor C2 from the grid to ground. Make R1 = R2 and C1 = C2: the frequency is then 1/(2 pi R C), set by the capacitors, giving 10 to 1 on one dial. For the capacitors use a four-gang tuning capacitor with its sections paired in parallel. Switch three sets of resistors in for the ranges 20-200, 200-2000 and 2000-20,000 cycles. Ours: a rotary switch.

المواد لهذه الخطوة:

مكثّف متغيّر (عازل هوائي)1 four-gang

المقاومة1 three sets of R1, R2

مفتاح دوّار1 range switch

الأدوات المطلوبة:

كاوية لحام

طقم مفكات

Fit the lamp feedback and use it

From the same output, through the coupling capacitor, feed back to the first valve's cathode across a resistor R4 and a small incandescent lamp R3. As the oscillation grows, the lamp heats, its resistance rises, the negative feedback increases and the gain falls, so the amplitude settles below the point of grid current with no clipping. The negative feedback also steadies the gain against valve and supply changes. Take the output from the terminals; Hewlett's test unit held its output within about 10 % over 20-20,000 cycles, with about 0.25 % distortion and under 0.1 % frequency shift with supply changes.

المواد لهذه الخطوة: