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RF - Clipper by DF4ZS
published CQ-DL 6/98 - RadCom 10/98 - CQ VFDB 2/99 - FUNK 1/2000
Joachim Münch DF4ZS - Goedeke Michel Str. 27 c - D 26434 Wangerland - Tel.: 04425 - 271002 email
There are many audio circuits to raise the average speech level. While they do compress the dynamic range of speech, they also create harmonics. As the harmonics of lower speech band, they cant´t be filtered out downstream. Greater compression means more distortion, and to much creates a reduction in readability rather then an improvement. This disadvantage can be avoid if the clipping is done at higher frequency. The harmonics and intermodulation products then appear well outside the speech bandwidth and can be easily filtered out with readily available components. The microphon signal applied to an NE612 mixer-oscillator IC. With the oscillator set to 453 kHz by means of a capacity-loaded ceramic resonator, the desired upper sideband is selected by a 455 kHz filter consistering of an LC tuned impedance transformer and a ceramic resonator filter. The resulting SSB signal is then amplified in the first of two single transistor amplifiers. Between them is the actual compression circuit. It consists of a T-attenuator in wich a pair of back-to-back germanium diodes act as a voltage-dependent shunt element, resulting in a quasi logarithmic attenuation characteristic. The downstream circuit is a reversal of the process. After another amplification, an indentical 455kHz filter removes out-of-band products generated by the non-linear attenuator. A second NE612 IC reconverts the SSB signal down to audio. Using the 453kHz local oscillator in the first IC for both conversions, up and back down, guarantees that the output frequencies are identical to the input frequencies. An LC output filter serves to suppress non-audio down-conversion products.
New developed, including ideas from operators, are in typ PEP. This typ used by myself, brings 1.5 to 2 steps more on the S-Meter with good sound.Assembled and mounted in case to be sold at original costs. Ask for Price via email. ember 2001
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