Motore di ricerca datesheet componenti elettronici |
|
CA3126E Scheda tecnica(PDF) 7 Page - Harris Corporation |
|
CA3126E Scheda tecnica(HTML) 7 Page - Harris Corporation |
7 / 9 page 8-39 Application Information Circuit Description (Pin numbers refer to the DIP package.) The following paragraphs briefly describe the circuit operation of the CA3126 (shown in the Block Diagram and Schematic Diagram). A detailed description of the operation of various portions of the CA3126 is given in AN6247, “Application of the CA3126 Chroma-Processing lC Using Sample-and-Hold Techniques”. The chroma input is applied to Terminal 1 through the desired band-shaping network. A 2,450 Ω resistor should be placed in series with Terminal 1 to minimize oscillator pickup in the first chroma amplifier. This amplifier supplies signals to the second chroma amplifier and to the ACC and AFPC detectors. The first chroma amplifier is gain-controlled by the ACC amplifier. A horizontal keying pulse is applied to Terminal 9. This pulse must be present to ensure proper operation of the oscillator cir- cuit. The subcarrier burst is sampled during the keying interval in the AFPC detector. The error voltage, produced at Terminal 2 and proportional to the burst phase, is compared to the quies- cent bias voltage at Terminal 3 by the sample-and-hold circuitry. This “compared” voltage controls the phase- shifting network in the phase-locked loop. The operation of the AFPC loop is inde- pendent of any external adjustments or voltages except for an initial capacitor adjustment to set the free-running frequency. The regenerated oscillator signal at Terminal 8 is applied internally to the AFPC and ACC detectors through +45 and -45-degree phase-shifter networks to establish the proper phase relationship for these detectors. The ACC detector, which also samples the burst during the keying interval, pro- duces a correction voltage proportional to the burst ampli- tude. The correction voltage is compared to the quiescent bias level using sample-and-hold circuitry similar to that used in the AFPC portion of the circuit. The “compared” volt- age is applied internally to the ACC amplifier and killer amplifier. Because the amplifier gains and killer threshold are determined by the ratios of the internal resistors, these functions are independent of external voltages or controls. The attenuated chroma signal is fed to the second chroma amplifier, where the burst is removed by keyer action. The killer amplifier, the chroma gain control, and the overload detector control the action of the second chroma amplifier, whose gain is proportional to the dc voltage at Terminal 16. The overload detector (Terminal 13) receives a sample of the chroma output (Terminal 15) and detects the peak of the signal. The detected voltage is stored in an external capaci- tor connected to Terminal 16. This stored voltage on Termi- nal 16 affects the gain of the second chroma in the same manner as the chroma gain control. General Considerations The block diagram shown is typical of the type of circuit used in the practical application of the CA3126. Several items are critical for proper operation of the circuit. 1. A series resistor of approximately 2,450 Ω (or high source impedance) must be used at the chroma input, Terminal 1. This high impedance minimizes pickup of unbalanced currents, particularly of the subcarrier oscillator signal. 2. When the overload detector is used, a large resistor (nominally 47,000 Ω) must be placed in series with Termi- nal 16 to set the required RC time constant. The same RC network series serves to set the killer time constant. 3. The setting of the free-running oscillator frequency requires the presence of the keying pulse. The free-run- ning frequency will be erroneous if Terminal 1 is DC shorted during the setting operation because of the DC offset voltage introduced to the AFPC detector. 4. Care must be taken in PC board designs to provide reason- able isolation between the oscillator portion of the circuit (Terminals 6, 7, and 8) and the chroma input (Terminal 1). Overload Detector The overload detector accomplishes two purposes: 1. It prevents oversaturation due to low burst-to-chroma ratios. 2. It prevents overload conditions due to noise. Both of these conditions are discussed in more detail in AN6247. The extent to which the overload detector is used depends upon the individual receiver design goals. If greater than 0.5VP-P output is desired, the chroma output at Termi- nal 15 can be tapped to yield any desired degree of overload detector action. Chroma Gain Control The chroma gain control operates by varying the base bias on current source transistor Q25. To ensure proper temperature tracking of the chroma gain control, it is essential that the con- trol be operated from a supply source derived from the refer- ence voltage at Terminal 12. Because the control operates from a current source, chroma gain is much more predictable and far less temperature sensitive than controls that steer current by means of a differential amplifier. The typical chroma gain char- acteristic for the CA3126 is shown in Figure 1. Subcarrier Regenerator Oscillator The oscillator filter consists of a 3.579545MHz crystal, a 680 Ω resistor, and a 10pF capacitor connected in series across Ter- minals 6 and 7. A 33pF capacitor, shunt connected from Termi- nal 7 to ground, rolls off higher order harmonics, thereby preventing oscillation at the crystal third-harmonic frequency. A VOLTAGE AT TERMINAL 16 (% OF V12) 120 0 20406080 100 60 80 100 TA = 25 oC, CHROMA INPUT = 0.5V P-P 40 20 140 FIGURE 1. CHROMA GAIN CONTROL CA3126 |
Codice articolo simile - CA3126E |
|
Descrizione simile - CA3126E |
|
|
Link URL |
Privacy Policy |
ALLDATASHEETIT.COM |
Lei ha avuto il aiuto da alldatasheet? [ DONATE ] |
Di alldatasheet | Richest di pubblicita | contatti | Privacy Policy | scambio Link | Ricerca produttore All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |