Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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In the universal low-frequency path of the Tselina transceiver (see the author's article "Tselina Transceiver" in the "KB Journal" No. 1 and 2 for 1996), the K140UD1B op-amp is used. As you know, the voltage gain (Ku) of this op-amp has a large spread (from 103 to 12.103) When using an instance with a maximum Ku in the path and applying a signal with a high level to the input, distortion occurs, especially in the high frequency region.
This drawback can be eliminated by turning on the capacitor C1 with a capacity of about 1000 pF in parallel with the VD18 diode (see figure) in the negative OOS circuit covering the op-amp DA1.
As a result, the frequency response of the path will gain a rise in the frequency region of about 1,2 kHz, which will favorably affect the CW reception mode, and a decline in the high frequency region. To obtain a uniform frequency response in the frequency band 100 ... 3500 Hz (for operation in SSB), it is necessary to additionally introduce another diode D220 (VD1') and a resistor with a resistance of 39 kOhm (R1) into the device. You can correct the frequency response of the tract in the high-frequency region by selecting the capacitor C1'. If necessary, change it in the low-frequency region in parallel with the VD1 'diode, you need to turn on the capacitor, the capacitance of which is selected during adjustment.
Transistors VT11 and VT12 for the upgraded path should be selected with a minimum current transfer coefficient h21E (about 30). In addition, it is necessary to replace R60 and R65 with resistors with nominal values of 2,7 and 3 kΩ, respectively (by selecting the latter, the gain of the path as a whole is set).
Author: Vladimir Rubtsov (UN7BV)
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