Principle and production of FM signal generator

The FM signal generator introduced has high precision and reliable operation and can be used as the high frequency, intermediate frequency and discriminator signal source of FM receiver.

The basic working principle of FM signal generator

The block diagram of the FM signal generator is shown below. It consists of a field effect transistor high-frequency oscillator, a dual varactor diode, a low-frequency oscillator, a low-impedance converter, a 50-ohm attenuator, etc. The complete circuit is shown in Figure 2.

The high-frequency oscillator has a simple structure and only one band, with a frequency range of 8.6~11.5MHz. Its 10th harmonic can be fully covered by a dual variable capacitance diode (BB294B or KV1820) and coil L1. The required capacitance change is:

The capacitance change of the above-mentioned varactor diode can meet this requirement. The coil uses 30~36# enameled wire, 30 turns around the 6mm magnetic core, and tapped at the 5th turn, or 39 turns of 5mm coil bag and tapped at the 5th turn. The two coils are connected in series, and the inductance is adjustable between 5.5-10uH.

VRD1 variable capacitance diode is connected to R9 and R10. The DC current is allowed to vary between 0.8V and 9.6V. The minimum voltage is adjusted by RV3. At this time, the dial scale is at 180. The maximum voltage is adjusted by RV2. Its value is approximately:

Field effect tube Q1 is a high frequency oscillator with stable performance and almost constant amplitude in the whole frequency range. The drain output of Q1 is about 2V. R12 and R13 form a voltage divider to reduce the output intensity. Q2 and source follower