DE960728C - RC generator with frequency-calibrated fine tuning - Google Patents

Description

ISSUED MARCH 28, 1957

W ii386VlIIa / 2ia ^

has been named as the inventor

RC generators or beat transmitters are used to generate low and medium frequencies. The beat transmitter enables a very wide, constant variation of the frequency and is used for orienting and automatically running measurements. The bridge-stabilized i? C generator According to Fig. ι gives the greater frequency security, especially at low frequencies, and becomes more accurate Investigations used.

There is often a need to make very small, defined changes to the transmission frequency. This is made possible with the beat transmitter by making the coarse adjustment of the frequency with one auxiliary oscillator and the fine tuning with the other. The fine tuning scale can be calibrated in frequencies. A frequency-calibrated fine tuning is also possible with the .RC generator. If the two capacitances C of the bridge (Fig. I) are represented by a series connection of two capacitances C _a and C _b , then the frequency change that is achieved by changing the capacities C _{b is} independent of the size of the capacitance C. _a . The capacitor C ₁₁ can therefore be used for the coarse adjustment of the frequency and C _b for the frequency-calibrated fine detuning. to serve. Even if the two resistors R of the bridge are represented by two parallel-connected resistors R _a and R _b , a frequency change can be achieved with R _b , the size of which is independent of R _a . The resistor R _a can be used for a coarse frequency setting and the resistor R _b for fine tuning.

The disadvantage of these two arrangements is expressed in the dimensioning. if namely the range of variation of the frequency through

the fine detuning is not to be significantly narrowed, the maximum values of C _b and Rj must be very large compared to the maximum values of C ₀ and R ₀ . A two-speed variable capacitor with a very large final capacitance or a two-speed variable resistor with a very high final resistance value is required for the fine detuning. These switching elements can hardly be manufactured with the required precision, especially since the capacitance C _b or the resistance R _{b have to change} their values reciprocally with the angle of rotation if the frequency scale is to be linear. Another disadvantage is that in the case of C fine tuning, only a C coarse setting of the frequency is possible, so that only one scale is necessary for the fine tuning. Correspondingly, only a. ^ - coarse adjustment is possible with the Z? Fine tuning.

The invention is shown in two circuits (Figs. 2 and 3) which enable a frequency-calibrated fine detuning in the bridge-stabilized i? C generator and thereby largely avoid the disadvantages of the known circuits described. Detuning is carried out with a single variable capacitance Cp or with a single variable inductance Lp . The frequency detuning range is independent of the set basic frequency of the transmitter, and both the resistors R and the capacitances C for changing the basic frequency can be changed here. A fine detuning of a maximum of about -10% of the set basic frequency is possible.

The mode of action of the fine detuning is easy to see and to calculate if one assumes that the gain of the oscillator amplifier is infinitely large; then the bridge output voltage U _g (input voltage of the amplifier) must be zero if the bridge input voltage U ₀ (output voltage of the amplifier) is to remain finite. This is true when

H ₁ = H ₂ (i)

is. One can calculate that

^ - = 3 + jcoCR +

(2)

jtoCR is. In the circuit according to Fig. 2 also results

R ₂ j ω Cf R ₂ and in the circuit according to Fig. 3

H ₂

R ₂

JO) Lj?

(3)

(4)

Using equation (1), by comparing the real parts of equation (2) with equation (3) or equation (4)

R ₁ = 2 R ₂ .

(5)

If the resistor R _{1 is formed} by a thermistor or the resistor R _{2 is formed} by a PTC thermistor, the transmission amplitude is regulated in such a way that this condition is met. By comparing the imaginary parts of (2) and (3) one obtains

cr

CR ZpR ₂

CR

(6)

For the circuit according to Fig. 3, by comparing the imaginary parts of (2) and (4) one obtains

ω =

R ₁ CR

CR

(7)

For Cp = 00 or Lp = 00 , the transmission frequency of the transmitter is obtained without fine tuning

CU _n = -

Rc

With equation (8) in (6) and (7) one obtains

CO «a CD _n - ·

2 Cτ R ₂

(I) Ri (B ₀ - ■

ι R ₁

CO ₀

R,

(9)

(10)

(8th)

The results obtained are approximate solutions and only apply to small detunings. A more precise evaluation of the root expressions of (6) and (7) shows that a detuning of about - 10% is possible without any errors worth mentioning. If one takes into account the real amplification factor of the amplifier, which was assumed to be infinitely large in the calculation, it follows that at V ^ 30 the results are not significantly influenced.

From equations (9) and (10) it can be seen that the capacitance Cp or the inductance Lp must change reciprocally to the angle of rotation if the frequency scale of the fine detuning is to be linear. It is therefore expedient to form Cp by a capacitor in which the plate spacing is changed linearly, or to represent Lp by a coil with a ferromagnetic core (ferrite), in the path of which a linearly variable air gap is inserted. This automatically results in a largely reciprocal change in Cp or Lp, i.e. a linear frequency scale.

When the circuit according to Fig. 2 and when the circuit according to Fig. 3 is to be preferred, must be decided on a case-by-case basis. The circuit according to Fig. 2 will often have to be ruled out because the capacitance Cp assumes such large values that it is difficult to implement.

Claims (3)

PATENT CLAIMS: ι. Circuit arrangement for frequency-calibrated Fine tuning of i? C generators with the Vienna-Robinson bridge in one Bridge arm a series connection of a first resistor with a first Capacitance and in series with it a parallel connection of a second resistor a second capacitance and in the other branch of the bridge a third and fourth resistor are connected in series, of which the third is expediently a thermistor or the fourth a PTC thermistor, characterized in that either in series with the third resistor (R ₁ ) an adjusting capacitance (C _P ) or a setting inductance (Lp) is connected in parallel to the fourth resistor (R ₂ ). 2. Circuit arrangement according to claim i, characterized in that a plate capacitor with variable plate spacing is provided as the setting capacitance (Cp) , such that its capacitance changes approximately reciprocally with the distance and thus the frequency changes approximately linearly with the distance. 3. Circuit arrangement according to claim 1, there characterized in that the setting inductance (Lp) is a magnetic coil which has a ^{magnetic circuit partially made of ferromagnetic material 1} , the air gap of which can be changed in such a way that with a linear change in the air gap, the ^T nduHativeness of the magnetic coil is approximately reciprocal and thus the frequency is approximately changes linearly with the air gap. Considered publications: Funkechnik, Issue 20, 1952, p. 560; Funkschau, Issue 8, 1953, p. 146; "Wireless World", Sept. 1949, p. 346; ^ ₀ A. Hund, "High Frequency Measurements," Mc. Graw Hill Book Inc., New York, 1951, p. 252 / 253- 1 sheet of drawings © 509627/71 1.56 (6Oi 845 3.57)