HF SPEECH - processor. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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HF SPEECH - processor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment

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To improve the transmission efficiency in the SSB mode, high or low frequency limiting is used. The most effective devices are high frequency (HF) limiters - HF Speech processors, which allow increasing the average power of the SSB signal by 4 - 6 times (6 - 8 dB). RF clipping can be done in two ways. In the first case, the SSB signal is limited directly in the transmitter according to the scheme: SSB signal shaper, (1st EMF) - limiting amplifier - 2nd EMF. In the second, the RF limitation is performed outside the transceiver, i.e. without interfering with its circuit, between the microphone and the transceiver's SSB driver. This article proposes a device made according to the second method.

Fig.1 (click to enlarge)

The block diagram of the device is shown in Fig. 1, and the schematic diagram is shown in Fig. 2. 1. The VT1, DA300 preamplifier-equalizer is used as a microphone amplifier. The band of the low-frequency signal is in the range of 3000 - XNUMX Hz.

Fig.2 (click to enlarge)

The proposed preamplifier-equalizer scheme eliminates the decay of the high-frequency part of the speech spectrum and significantly improves intelligibility. In the proposed scheme, the amplitude rise of 12 dB occurs at a frequency of 2700 Hz. Equalizer resistors R5, R10, R6, R11, R8, R9 and capacitors C7, C8 must have a value spread of no more than ± 5%. Through the emitter follower on VT2, the signal enters the actual RF limiter circuit (VT3 - VT5, Z1, Z2.DA2), developed by UV9DZ. The first mixer was made on VT3 KP303I. A signal from a 500 kHz crystal oscillator with an amplitude of 0,6 - 0,7 V is fed to the VT3 gate. The VT3 drain receives an LF signal with an amplitude of 150 - 160 mV. At the output Z1, an SSB signal is selected, limited by diodes VD 1, VD2 and filtered by Z2. Further, the signal is demodulated using VT5 and amplified by DA1.

If it is necessary to adjust the output signal level, instead of R9-51 K, use the chain shown in the diagram with a dotted line. The microphone amplifier of the transmitter is not used, because. That is unnecessary. In a 500 kHz crystal oscillator, using R30, the output voltage level is set to 0,6 - 0,65V. The degree of limitation can be defined as the ratio of the RF voltage at the VT4 drain with the diodes VD1, VD2 off to the voltage at the same point after the diodes are connected (-0,6 V). This value is 7 - 8 dB. To increase the clipping level to 15 dB between Z1 and VT4, you can turn on an additional amplification stage on VT7 (Fig. 3).

HF SPEECH - processor
Ris.3

VT4 gain control is performed using a variable resistor R19 or preamplifier output control - equalizer R14. It is convenient to choose the value of the resistor R19 in such a way that, with a minimum amplification of the cascade on VT4, it is not possible to reduce the optimal level of the SSB signal with a minimum of limitation. This is important when comparing a radio's transmit signal at the minimum and maximum clipping levels.

After a long operation of the device, bypass relays were installed (for input and output). Such switching is convenient when making local QSOs, because the signal level at the receiving point is high and clipping can reduce the intelligibility of the signal.

When making a DX QSO, or under QRM conditions, the QRN limited signal is of high quality and intelligibility.

Author: I. Podgorny, Minsk; Publication: N. Bolshakov, rf.atnn.ru

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