DE1766682C2 - Self-oscillating mixer for the UHF range - Google Patents

Description

_R if a mixer with a tube ( "Funkschau" T 8, No. 13, p 322 and 323) it is known to evaporate the low-frequency end of the range, £ em by appropriate dimensioning of Spuk ™ of the LC Krciscs at low frequencies resistance in the The oscillator circuit is transformed into it. The use of a tube and rotating capacitor adjustment are precisely the opposite of the conditions described above.

_E Tis K-rcits a circuit with an additional inductive feedback from the output to the Im-Γ. known about the additional jerk coupling, resulting in better and more uniform oscillation. can be achieved over the entire frequency range

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The invention is based on a self-oscillating mixer stage for the UHF range with a mixer transistor operated in a basic circuit, the collector circuit of which contains the series connection of a capacitance diode and a fixed capacitor, in which the voltage generated across the fixed capacitor is also fed back to the emitter via a feedback capacitor and the IF signals are taken from an LC element connected to the collector of the mixer transistor. Mixing stages of this type are used in particular in television receivers for receiving ranges IV and V (470 to 790 MHz) (German Auslegcschrifl 251 820). The IF signals are _{picked up by the s5} grounded capacitor and generally passed through a band filter. The LC element and the band filter are dimensioned in such a way that the IF signal is allowed to pass through and the HF signal is largely blocked and any resonance frequencies of the LC element are in a range where they cannot have any effect. . Due to the greatly decreasing quality of the capacitance diode towards the end of the low-frequency range, the oscillation conditions for the oscillator are very unfavorable there, although the steepness and capacitance of the capacitance diode is greatest there, so that due to the divider ratio diode / fixed capacitor on the Autgaoc ucMciii in it, a better one and to achieve more even vibration behavior without additional feedback. This object is achieved by the invention specified in claim 1. This dimensioning of the LC element improves the oscillation conditions of the oscillator at the end of the low-frequency range so that the result is a uniform oscillation behavior over the entire range. Advantageous further developments of the invention are described in the subclaims. If the mixer stage is designed according to claim 2, the oscillator frequency can be influenced in the immediate vicinity of the lower end of the range, so that synchronization errors between the oscillator circuit on the one hand and the two UHF band filter circuits of the preliminary stage on the other hand are corrected.

An embodiment of the invention is explained in more detail with reference to the drawing.

The drawing shows the self-oscillating mixer stage of a television receiver circuit for the range from 470 to 790 MHz. The direct current operating point of the mixer transistor T is predetermined by the base voltage divider consisting of the resistors R ₁ and R ₂ and by the emitter resistor R ₃ . The base of the transistor T is connected to ground via a capacitor C ₁ of 1000 pF, so that the transistor T is operated in a base circuit for the oscillations in question. The collector of the transistor T is connected via a capacitor C ₂ of 3.9 pF to an inductance L ₁ , the other end of which is grounded.

The inductance L ₁ is formed by a piece of line which is soldered to the base of a U-shaped bent, grounded sheet metal strip and which runs parallel to its legs, so that a chamber K ₁ is created. _{A short-circuited wire bracket L 1} 'is soldered inside the chamber so that the inductance can be adjusted to the required value. The size of the inductance L ₁ therefore depends on the magnetic coupling

between L ₁ and L ₁ 'and thus from the position of L ₁ ' and can be brought to the desired value by bending the wire bracket L _1.

The series connection of a capacitance diode D ₁ of the BB 105 type and a fixed capacitor C ₃ of 15 pF is connected in parallel with the inductance L ₁ , the fixed capacitor being grounded on one side and εη on the other side via a feedback capacitor C ₄ of 1.2 pF is connected to the emitter of the mixer transistor. Therefore, vibrations are generated, the frequency of which corresponds _{approximately to the (parallel) resonance frequency of the output circuit C 3} , D ₁ , L _1. The parallel resonance frequency is in turn determined by the bias voltage of the capacitance diode D ₁ , which is fed to the capacitors C ₃ and C ₄ via the resistor R ₁ at the junction point aiii.

For alternating current, a capacitor C _{5 is} parallel to the base-emitter diode, which increases the inductive input resistance and thus also the feedback somewhat and thus improves the feedback conditions at the end of the low-frequency range. This capacitor must, however, be dimensioned relatively small so that the tuning variation is not narrowed or that there is no unstable oscillation state at the higher frequencies. A suitable value for C ₅ is 1 pF.

In addition, a coupling loop L ₂ 'is connected between the emitter and the emitter resistor R ₃ , which couples the mixer to the preceding preliminary stage, of which only the coil L ₂ and the capacitance diode D ₂ are shown in the drawing. The coil L ₈ has an ulmiicucr. Structure like that of the coil L ₁ , and the coupling loop L ₂ 'protrudes into the chamber K _{2 surrounding the coil L 2} . The connection point of the coupling loop L ₂ 'and the emitter resistor A ₃ is connected to earth via the series circuit at the inductance L _s , which is not magnetically coupled to the other coils, and a capacitor C _e of 100 pF. The fact that the connection point is not grounded directly via the capacitor, but with the interposition of the inductance L ₃ and thus the total inductance parallel to the input of the mixer is greater, ensures that the oscillations of the oscillator can be maintained in this range, especially at low frequencies .

The dimensioning of the coil L ₃ is a compromise between good vibration behavior at the lower end of the range on the one hand and the adaptation of the mixer stage to the preliminary stage at the upper end of the range on the other hand.

The inductance L ₃ would not be necessary if the inductance of the coupling loop L '' were sufficiently large. Since the coupling loop must be dimensioned so that an optimal match between the

ίο is achieved in the preliminary stage and the mixer stage, it can only be increased if the magnetic coupling with the coil L _{2 of} the preliminary stage is reduced at the same time. But that would mean that the coupling loop would be longer (= greater inductance)

and would have to have a greater distance (= smaller coupling) from the coil L _{2, so that it would no longer fit into the chamber.}

The intermediate frequency signals are taken from the mixer via a band filter (BF) which is connected to a 4.7 pF _{capacitor C 7} which is grounded on one side and connected to the collector of the mixer transistor _{on the other via a coil L 4.} This capacitor and the capacitor C ₂ are the capacitance of the primary circuit of the band filter.

The coil L ₄ is dimensioned so that on the one hand, together with the capacitors C ₂ and C _{7, it} forms an auxiliary circuit of low quality (around 20), which at a frequency just below the lowest UHF

Receiving frequency, around 450 MHz, is in parallel resonance (the inductance L ₁ can be neglected in this frequency range), but on the other hand represents a negligibly small inductive resistance for the intermediate frequency.

This puts the gain in the lower frequency range compared to the gain at the upper end of the range, so that the oscillation behavior remains approximately the same over the entire frequency range.

If the inductance of the auxiliary _{circuit (L 4} ) is made variable, one has a very effective means of influencing the oscillator frequency at the lower end of the range, which is often necessary to avoid synchronism errors between the oscillator on the one hand and the UHF band filter circuits, caused by scattering, on the other the characteristics of the capacitance diodes or other frequency-determining switching elements «; to correct.

1 sheet of drawings

Claims (4)

Patent claims: 1. Self-oscillating mixer stage for the UHF range with a mixer transistor operated in a base circuit, the collector circuit of which contains the series connection of a capacitance diode and a fixed capacitor, in which the voltage generated across the fixed capacitor is also fed back to the emitter via a feedback capacitor and the IF signals via a LC element connected to the collector of the mixer transistor can be taken, characterized in that the LC element (L ₄ , C ₇ ) together with the other elements of the output _{circuit (C 7} , L ₁ ) at a frequency slightly below the lowest receiving frequency in parallel resonance is. 2. Mixer according to claim 1, characterized in that the inductance of the LC element (coil L ₄ ) is variable. 3. Mixer according to claim 1, characterized in that a capacitor (C ₅ ) is connected in parallel to the input of the mixer transistor (T), the reactance of which is greater than the input resistance of the mixer transistor even at the highest oscillator frequency as. 4. Mixer stage according to one of the preceding claims, which contains a coupling loop (L ₂ ') effective in the emitter circuit of the mixer transistor for coupling to a preliminary stage, characterized in that one end of the coupling loop via the series connection of an inductance (L ₃ ) and a capacitor (C ₆ ) is earthed. fixed capacitor disconnects the highest possible voltage ^{1 eslKUUU} . ... ..,! "" RiirHrnnnmneskondensa- m-n unO about "the feedback condensate" em miuer is supplied. A bigger one iuckSplungskondensatoi improves the sVhWinobedineungcn at the lower end of the range, add t "ίη but difficulties due to E.nen-, enzvariation and by unstable the upper half of the frequency