Of course, the most commonly used circuit of the LM1881 is very simple. The two RC devices set the field synchronization time to be OK, and the effect is also very good.
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However, the LM1881 has another feature, which is expensive. When I first used it, 8 yuan, now it seems to be 4 to 5 yuan, the replacement seems to be almost the same price, robbery! The circuit designed for others, absolutely use it, its own circuit, can not dare to use.
The separation circuit should be designed. Isn't the TV the whole transistor at first? The first one designed by myself was realized by a separate circuit, and it was N years. The device is also full of pins. I remember that I had found more than a dozen textbooks, monographs, and circuit diagrams. I also went to the library twice. I couldn’t find a circuit that I could use. Later I found a circuit in a book published in the late 1970s. The negative power supply, according to the principle of changing to a positive power supply, is like this:
Troublesome, the two power supplies, first reverse phase, and then according to the principle of the length of discharge time of the capacitor, the voltage maintained by C2 makes the transistor Q2 turn on only when the synchronization arrives, and the TTL level is only amplified after the first step, but the work Very good. This circuit was originally drawn with OrCAD SDT 3.34, and the files are now converted, and can only be redrawn once.
Once again, when designing the synchronous separation circuit, it has entered the era of comprehensive SMD, and then went to find a lot of books. This time, I still have a lot of papers in the journal online, but I still have not found the circuit that can be used directly, but also a bunch of Knowledge waste. Later, I thought of the old Toshiba two-chip TV. In the introduction of the principle of TA7680/TA7698, there is a similar circuit. I took it and changed it. I changed the N tube to the P tube and made a new synchronous separation circuit:
This circuit is much simpler and works well. However, it was very dissatisfied with it. First, the electrolytic capacitor C2 and the diode D1 were too large, and the second was not isolated. It was not safe to simply move to other circuits. Once again designing this circuit, touch it on the basis of this circuit and change it to a relatively simple circuit:
The principle is the same. The DC voltage accumulated at both ends of the relatively large capacitor is used to separate the sync head with a large change from the DC level point. This circuit boldly replaced the electrolytic capacitor with 104 capacitors, and it also worked very well, and the volume was greatly reduced. However, it was found that the load variation was not stable enough, and the CSYN output was cluttered. The most important adjustment here is the bias resistor R25. In the final application, either a third-level transistor is added to the rear, and the signal is shaped to eliminate clutter that has not reached the TTL level; or the first-stage shot is added to isolate the influence of the load change.
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