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Free programmable LOW/HIGH/BANDPASS FIR filter in the audio range 0-4kHz

CW keyers, Logikey, and antenna rotator controll like rotor-ez, and active audio low pass filters.

Presents a QRP AM/CW transmitter project specifically designed for the 10-meter band, utilizing a crystal oscillator and a collector-modulated AM oscillator. The design employs a 2N2219(A) transistor in a Colpitts configuration, generating 100 to 350 mW of RF output power depending on the 9-18 Volt supply voltage and modulation depth. Frequency stability is maintained by a 28 MHz crystal, with fine-tuning possible via a Ct1 trimmer capacitor for approximately 1 kHz adjustment. The resource details the RF oscillator stage, implemented with a 2N2219 NPN transistor, emphasizing frequency stability and low power dissipation. It also covers the amplitude modulation stage, managed by a 2N2905 PNP transistor, which impresses audio information onto the carrier. Selective components (C3, C4, C7, C5) enhance voice frequencies within a +/- 5 kHz bandwidth, and modulation depth is controlled by R2 and R3. The project includes a 3-element L-type narrow bandpass filter (Ct3, L3, C10) to suppress harmonics and ensure a clean output signal. The project provides a complete schematic diagram, a comprehensive parts list including specific capacitor, resistor, and inductor values, and construction notes for the coils (L1, L2, L3). It also offers practical advice on enclosure requirements, suggesting an all-metal case or a PVC box with graphite paint for RF shielding. Operational parameters such as current draw (27mA@9V to 45mA@16V) and input impedance (50 Ohms) are specified, alongside guidance on antenna matching and the importance of a valid amateur radio license for 10-meter band operation.

Details the construction of an **HF converter** designed by M1GEO, George Smart, specifically to extend the frequency range of the FunCube Dongle Pro (FCD) for amateur radio reception. The FCD natively covers 64 to 1,700 MHz, but this project enables reception from 0 Hz to 64 MHz by up-converting signals to the FCD's operational range. It employs a **double-balanced mixer** with a 100 MHz local oscillator (LO) to translate incoming HF signals; for instance, a 1 MHz signal appears at 101 MHz within the FCD's passband. The design incorporates a 7th-order Chebyshev low-pass filter with a 62 MHz cutoff frequency at the input to mitigate image frequencies, ensuring cleaner spectral presentation. George provides the schematic, PCB masks, and Gerber files for replication, noting that Far Circuits also offers PCBs. The resource includes test results for the low-pass filter and measurements of LO leakage, identifying -36.8 dBm at 100 MHz as a potential sensitivity concern. M1GEO discusses potential improvements, such as adjusting the mixer's LO drive, adding a balance pot, or incorporating a post-mixer high-pass filter to reduce LO breakthrough. Audio recordings from 40m and 17m demonstrate the converter's performance with WRplus SDR software.

A CW sidetone bypass for the NESCAF audio filter, a small and low cost circuit that works great.

Adding 2 RCA inputs, 1 high impedance filtered audio output RCA 3, 3 HIGH PASS filters, 3 LOW PAS filters to the JRC NVA88 Speaker

A dual insert microphone design for the Icom IC-7300 transceiver utilizes a **Besson BZ2400 M4 Rocking Armature** insert for frequencies from 500 Hz to 3 kHz, exhibiting a rising response of approximately 11 dB. A generic Electret Condenser insert, powered by the transceiver's microphone line, covers the low-frequency range from 100 Hz to 500 Hz. A Low Pass Filter is incorporated after the Electret insert to prevent frequency overlap, and a pre-set potentiometer (VR1) adjusts the low-frequency response, balancing the output of both inserts. The design emphasizes a "Close Talking" arrangement and addresses audio "colorization" by housing the Besson insert in a thick rubber holder with a foam boot, separate from the circuitry, with the Electret insert also wrapped in a foam boot. Critical importance is placed on using the correct BZ2400 M4 insert with 12 holes in its face plate. The frequency response table for the BZ2400 M4 insert shows 0 dB at 500 Hz, rising to +11 dB at 3000 Hz, while the Electret insert with the Low Pass Filter provides 0 dB at 100 Hz, rolling off to -9 dB at 500 Hz and -50 dB at 3000 Hz. This combination ensures a broad, balanced audio spectrum for SSB operation. The project includes a circuit diagram, a comprehensive parts list detailing components like a 1 Henry iron-cored inductor (L1) and various capacitors, and a board layout within the metal tube. The completed unit provides a tailored audio profile for the IC-7300, enhancing transmit audio quality.