Enhancing the RF Sweep Frequency Generator’s Receiver Option
The RF sweep frequency generator, also known as the ‘wobbulator,’ featured in the October 2008 issue of Elektor, includes a receiver option enabling its use as a direct-conversion receiver. However, this receiver exhibits a noise floor of only –80 dBm, a sensitivity that falls significantly short, requiring it to be –-107 dBm to achieve a sensitivity of 1 μV. To improve the receiver’s performance, additional gain is necessary. Utilizing a wideband amplifier for this purpose, though, introduces significant additional noise, making it unsuitable for achieving substantial improvement.
Designing a Selective Receiver for Enhanced Performance
In response, the author embarked on an experiment to develop a selective receiver with an approximate bandwidth of 4 MHz. To meet the requirement of at least 35 dB gain, the preamplifier incorporates two amplifying elements. The input amplifier is centered around a dual-gate MOSFET, specifically the BF982 type. This component is chosen for its ability to provide substantial gain with relatively low noise output. The output stage employs a BFR91A to add extra gain to the amplification process.
Optimizing Drain Circuit Design for Preamplifiers
Preamplifiers that feature tuned gates and drains often encounter feedback issues due to internal capacitance. In this design, the drain circuit maintains a relatively low impedance, effectively preventing unwanted feedback. During prototype testing, the input and output were strategically positioned at right angles to each other to eliminate inductive coupling. Surprisingly, despite its high gain, the amplifier demonstrated remarkable stability even without any additional shielding. The two air-cored coils utilized in the circuit each comprise 4 turns, boasting a 6 mm internal diameter and crafted from AWG #18 (1-mm) diameter silvered copper wire, with a tap introduced after the first turn.
Versatile Amplifier for Different Frequency Bands
Primarily intended for the 144 MHz amateur band, this amplifier can also adapt for the FM broadcast band with the use of different coils. For FM detection, tuning near the edge of the IF filter proves effective. At a 15 kHz offset, signal strength only marginally decreases compared to the center of the pass-band, ensuring negligible damping. In the 2 m band, the measured sensitivity reached approximately 1 μV (6 dB), emphasizing the importance of a high-quality antenna for optimal reception. Employing a wideband outdoor antenna, such as a scanner antenna, yields excellent results.
Efficient Signal Detection and Frequency Switching
Integrating this wobbulator/receiver option enhances the setup, creating an effective monitor receiver. Users can configure the spectrum analyzer to scan frequencies between 144 and 148 MHz (or 146 MHz where applicable), rendering any signal within this range visible on the display. Upon detecting a signal, users can halt the scanning process and simply click on the signal within the display window using the right mouse button. This action causes the receiver to shift directly to the detected frequency, enabling users to listen to the signal. Resuming the scanning function allows users to explore other frequencies of interest.
Fine-Tuning for Narrowband FM Detection
For narrowband FM detection, users can select the FMN button within the receiver window. This feature provides the necessary offset for edge detection within a 25 kHz bandwidth. The offset value can be adjusted via the ‘setting’ menu, allowing users to experiment and optimize the setting for the best results.
Flexible Power Options
To power the circuit, a 9-V battery can be utilized. Alternatively, the amplifier can be powered directly from the RF sweep generator by replacing output capacitor C6 with a link. In this configuration, users need to select the ‘use probe’ option in the ‘options’ menu for seamless operation.