Generator / vobulator DSG-108

This device was designed mainly to radio transmitters and receivers service and in constructing all kinds of amateur transceivers. DSG-108 generator has a frequency stabilized with a quartz generator and the accuracy of setting is 1Hz. The frequency range is from 50KHz to 108MHz, so it would also be excellent for tuning radio-receivers. It is equipped with AM (500Hz signal) and FM (signal 500Hz, deviation 5KHz, 10KHz, 25KHz, 50KHz, 75KHz) modulator and adjustable level of output signal in the range from -20dBm to 10dBm. The vobulator also works in the range of 50KHz to 108MHz. The returning range is from 0 to 108MHz and is set with an accuracy of 1Hz. The signal level is the same as in the generator. In conjunction with the oscilloscope, it is possible to display the frequency response of the whole receiving circuit as well as individual blocks such as an input filter, high frequency amplifier, mixer, intermediate frequency filters or intermediate frequency amplifier. Characteristics on the oscilloscope screen is displayed with a frequency of 25Hz, so turning the system is very pleasant and changing the displayed characteristics occurs smoothly.
Below a sample of several characteristics of the receivers tuning.
Characteristics of the input receiver filter in the radio Alan 18:



Characteristics of the input circuit and the intermediate frequency track I of the receiver in the radio Alan 18:



Characteristics of the input circuit, intermediate frequency track I and II of the receiver:



Characteristics of the input circuit and intermediate frequency track of the receiver in the radio Alan 18:



View of the FM signal after the demodulator in the radio receiver :



View of an amplitude modulated with a signal of 400Hz carrier



Construction
The device is made on the basis of the popular system of direct synthesis DDS AD9851 from Analog Devices, controlled by a microprocessor Atmega16 from Atmel. The system AD9851 is clocked at 216MHz with quartz generator. For the safety, the system has a heat sink which also serves as the screen for DDS synthesis. This solution works even when the generator runs 24 hours a day. Microcontroller's software was written in C and assembler, which resulted in a full program FM modulation. AM modulation signal is created by a microcontroller, but it is connected to the synthesis through an external circuit. Of course, a synchronization signal at vobulator work is also created by the microcontroller. To communicate with the user, the device uses alphanumeric display 2x24 characters and 7-button keyboard.




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