CAF la tuner FM cu lampi

[ALVIN]

Buna seara, ma poate ajuta cineva la implementarea unui control automat al frecventei la un tuner pe lampi? Tunerul functioneaza bine dar dupa un timp "fuge" de pe post. Schema este aceasta:

[ALVIN]

http://imgfz.com/i/MdqhTXi.jpeg

Editat Decembrie 6, 2023 de ALVIN

[d2134]

Cea mai simpla varianta e cu dioda varicap:

 

Citat

Automatic Frequency Control

The voltage-variable capacitance of the Varicap and its temperature stability suit it for use as a simple, highly sensitive, afc device which performs better than some reactance tube circuits. The small size of an afc unit, containing a Varicap, four small capacitors, four resistors and an RF choke, lets you tuck it into a receiver with a minimum of disturbance to the set's circuitry. This should be welcome news to hi-fi enthusiasts whose FM receivers have no automatic frequency control.

Fig. 5 shows an afc circuit developed by Pacific Semiconductor engineers, which I adapted for bias from the 300-volt DC supply of an FM receiver. The photos show the complete unit ready for wiring into the receiver. Any type of Varicap may be used. The receiver's local oscillator is simply realigned to compensate for the shunting capacitance introduced by the biased Varicap C2 which functions as a frequency-controlled trimmer across the local oscillator tank.

The Varicap is referred to a DC bias of -8 volts obtained from the receiver's 300-volt supply through the voltage divider R3, R4. The afc DC voltage is obtained from one side of the discriminator. For other than 300-volt supplies, the values of R3 and R4 will not be the same as mine but must be worked out for an output of -8 volts from the particular supply voltage of the set that you are adding this afc circuit to.

The completed afc unit is built on a perforated phenolic board 2 3/8 inches long and 2 1/8 inches wide. The pigtails of the components are passed through the holes in the board and interconnected underneath to complete the wiring. Printed circuitry may be used. The four connections to the receiver circuit are made at solder lugs mounted along the edge of the panel. The completed unit should be mounted as close as possible to the local oscillator so that the lead from the tank to C1 will be short.

((Certain silicon diodes can also be used as variable capacitors in such applications as this. In the February-March, 1958, issue of Rectifier News, published by the International Rectifier Corp., El Segundo, Calif., a circuit is shown for using their 3DS1 silicon diode as an afc control device for an FM tuner. -Editor)

https://www.rfcafe.com/references/radio-electronics/using-varicap-may-1958-radio-electronics.htm

[ALVIN]

Multumesc pentru ajutor, cam asa ma gandisem si eu. Cred ca am citit candva ca orice dioda prezinta o capacitate(mica) variabila cu tensiunea aplicata. Mi-a mai trecut prin cap solutia cu condensatori cu coeficienti de temperatura opusi dar mi se pare mai eleganta solutia cu varicap. 

[d2134]

La 07.12.2023 la 22:08, ALVIN a spus:

Mi-a mai trecut prin cap solutia cu condensatori cu coeficienti de temperatura opusi

Asta se face in fabrica pentru a imbunatati stabilitatea oscilatoarelor.

 

Poti folosi orice dioda cu siliciu de semnal mic, de ex. 1N4148. La nevoie se pot pune 2-3 in paralel.