Theory of Operation

Let’s have a look at what makes our audio filter so special. Have a look at the schematic diagram and follow along.

First, having a look at the power supply section of the AF1 notice that diodes D3, 6, 7,and 8 form a full wave bridge rectifier. We used this configuration so that an AC or DC power source may be used for the unit (and the polarity connection for a DC supply can be either way). This “raw” voltage is smoothed by several large capacitors and routed to the voltage regulator IC to provide a crystal clear source of DC voltage for the filter IC’s. You’ll also notice a few bypass capacitors on the supply in case any of those nasty RF signals try to get in and mess up the power supply.

At the heart of our kit is a pair of the MF8 switched capacitor bandpass filter IC’s. Without these little marvels of technology this kit would not even exist! Included on each chip is a pair of bandpass filters which can be cascaded to provide sharper filter characteristics, a “Q LOGIC” binary input to set the filter bandwidth, an adjustable internal oscillator to provide the center frequency for our filter, and an extra op-amp to boot! Even the usually stingy data book devotes 16 pages to the design possibilities for this hard working IC. The overall bandwidth is controlled using switches S2 and S3 to provide a binary input at the B, C, and D inputs of the MF8 IC. Resistors R12 and R13, along with capacitor C8 provide the RC timing component necessary for the CMOS oscillator.

The rest of the circuit is pretty straight forward - with whatever features could be built in. The power switch doubles as an audio bypass, or feed through when in the “OFF” position. Input protection diodes D1 and 2 limit the audio input voltage to protect the valuable filter IC’s. Headphone jack J2 is set up to accommodate either stereo or mono headsets, and headphone use will switch the external speaker output off when the jack is inserted.

The headset / speaker output is from the LM380 audio amp IC. This chip provides about 3 watts of noise free audio with a minimum of external components.

AF1 • 5

AF1 specifications

Ramsey Electronics AF1 is a state-of-the-art audio frequency oscillator, renowned for its versatility and performance in various electronic applications. Designed for hobbyists and professionals alike, the AF1 is an essential tool for anyone working with sound generation, modulation, or detection.

One of the main features of the AF1 is its wide frequency range, spanning from 1 Hz to 100 kHz. This broad coverage enables users to generate tones suitable for a multitude of tasks, from simple audio testing to more complex signal modulation in communication systems. The AF1 provides a stable wave output, which is critical in maintaining audio fidelity and ensuring accurate readings in testing environments.

The AF1 employs advanced technologies to achieve its remarkable performance. One of these is its precision frequency control, which allows users to set the output frequency with accuracy. This is facilitated by the device’s inclusion of a calibrated variable resistor, enabling fine adjustments for precise frequency generation. Additionally, the AF1 features a built-in frequency counter that simplifies the process of monitoring the output frequency, providing users with a valuable tool for troubleshooting and calibration.

Another characteristic that sets the AF1 apart is its dual output channels. This allows for the simultaneous generation of two distinct frequencies, offering greater flexibility in applications requiring complex audio signals or dual-tone generation. The outputs are adjustable in amplitude, which helps users tailor their signals according to specific requirements, whether for testing speakers or experimenting with audio synthesizers.

The AF1 is also designed with ease of use in mind. The interface is intuitive, featuring a straightforward layout of dials and switches that enables quick adjustments and settings. This user-friendly approach is ideal for those who may not have extensive experience with electronic signal generators.

Constructed with high-quality components, the RAMSEY AF1 ensures durability and reliability, making it suitable for both fieldwork and laboratory settings. Its compact design allows for easy transport, fitting seamlessly into any toolkit.

Overall, the Ramsey Electronics AF1 is a powerful audio signal generator equipped with a rich feature set, including extensive frequency range, dual outputs, precision control, and intuitive usability. Whether you are a hobbyist, educator, or professional, the AF1 is a valuable asset for any electronic project involving sound generation.