low frequency limiter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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low frequency limiter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The proposed low-frequency limiter is a modernized version of the low-frequency speech - processor, originally published by ew1mm in the journal "Radio amateur HF and VHF" No. 9 for 1995. The device is designed to improve the transmission efficiency in the SSB mode, however, it can also be successfully used in FM operation. For the manufacture of a low-frequency limiter, no special knowledge or skills are required, it is enough just to correctly install and connect the device to the transceiver.

You should not use the microphone amplifier of the transceiver, as this device completely replaces it. In essence, this is a high-quality microphone amplifier, the detailed circuit of which, using discrete elements, made it possible to bring each of the cascades to high parameters, assuming easy design repeatability and a high-quality transmission signal.

(click to enlarge)

Purpose of elements:

DA1,VD1,VD2 - input signal AGC
DA2, VT1 - microphone amplifier (MU) with frequency response boost
DA3 - active filter 300-3000 Hz
DA4, VT2, VD3 - limiter amplifier
DA5 - active filter 300-3000 Hz
VT3 - emitter follower

One of the latest design improvements is the introduction of input signal AGC. A part of the scheme published in [1,2] was taken as a basis. Having abandoned scarce imported transistors and +24 V power supply, we used the K140UD7 operational amplifier and +12 V power supply, which did not affect the quality of the circuit. The results obtained made it possible to use the AGC circuit of the input signal as part of any microphone amplifiers, as their integral part. What is input AGC for? It is known that when pronouncing in front of a microphone at a distance of 10-15 cm rolling a-a-a-a, a millivoltmeter connected to the microphone will register an audio frequency voltage of the order of 1-2 mV. However, in our speech there are many hissing sounds and deaf consonants, which instantly develop a much larger amplitude, which, depending on the type of microphone, reaches 0,5 - 1 V, and this is without any amplifying stages.

The above is true for all types of microphones, with the only difference that, unlike dynamic microphones, electret, crystalline and ceramic ones have much higher peak emissions. This is easy to check, just whistle in front of the microphone or say a phrase containing a large number of hissing sounds. When you connect a microphone to the transceiver, it turns out that the first MU stage at the peaks is pumped by the input signal, receiving a large amplitude on hissing sounds, in contrast to the average level amplitude. Even if the transceiver has a well-established ALC system, the problem is not solved, since distortion and pumping in the low-frequency path fell into the signal bandwidth and will affect the color of the transmitting signal as a whole.

Brief description of the operation of the scheme. The input signal from the microphone at a level of approximately 1 mV (average value) enters the stage assembled on DA1 - the input signal AGC, the operation of which completely eliminates the above problems. The low-frequency signal is amplified to a level of about 80 mV, then attenuated using diodes VD1 and VD2 to a level equal to the input signal. No signal clipping occurs here. The signal at the control point KT1 will have a sinusoidal shape, and the voltage is almost equal to the input, i.e. the signal developed by the microphone, but there are no instantaneous peak emissions. If the voltage in CT1 is slightly higher than the input voltage (peak emissions will also be absent), it is reduced to the input level using the trimming resistor R10.

Further, the signal is amplified using a low-noise amplifier on VT1. Here the quality of the future low-frequency limiter is set, in particular the signal-to-noise ratio. When setting up later, you should set the voltages indicated on the diagram at the terminals of the transistor. The next stage on DA2 is an amplifier with a frequency response boost in the high frequency region. All resistors in this stage, with the exception of those in the power circuit, have a plus / minus 5% nominal spread. Capacitors C14, C15 preferably film, also 5%. The output voltage is taken through potentiometer R23.

An important issue is the low-frequency filtering of the signal before it is clipped. This role is performed by an active filter assembled on DA3 with a bandwidth of 300 - 3000 Hz. Capacitors C20, C21, C23 (preferably film ones), as well as resistors in this cascade with a spread of 5%. Adjustable Amplifier-Limiter is assembled on the operational amplifier DA4. Potentiometer R30 "Restriction Level" is placed on the front panel of the device and has (preferably) a logarithmic dependence. The potentiometer is connected to the limiter board with a shielded wire, with the braid grounded on both sides.

It should be noted that all other trimming resistors in the design are located on the printed circuit board and are not displayed on the front panel. If there is no potentiometer with a logarithmic dependence, and there will be no significant changes in the level of restriction on the air, then a potentiometer with any dependence can be used. The limitation is made at the output of DA4, and one half-wave is limited by the diode VD3, the other by the base transition - the emitter of the transistor VT2.

Oddly enough, the color of the signal and its volume also depend slightly on the type of VD3 diode. We used the D311 (D311A) diode, then we used the imported 1N4148, which is not in short supply. The VD4 LED in the VT2 collector circuit is an indicator of the level of limitation. The higher the limit level, the greater the intensity of the LED. After the restriction node, an active filter with a band of 300 - 3000 Hz - DA5 follows. Capacitors C26, C27, C29 (preferably film ones), as well as resistors in this cascade, have a nominal spread of 5%.

The emitter follower is assembled on a transistor VT3. Capacitors C31 and C34 are non-polar, preferably film. A common requirement in the manufacture of such devices is the use of obviously good electrolytic capacitors, as well as the manufacture of a high-quality stabilized power supply with minimal output voltage ripple.

Setting the low-frequency limiter is reduced to the selection of voltages at the VT1 terminals in the specified sequence:

By selecting the resistor R11, +1,5 V is achieved on the collector.

Selecting the value of the resistor R13 set +0,3 V on the emitter.

Then set +0,8 V on the base by selecting R12.

After that, you should re-check the voltage at the terminals of the transistor, because. a change in voltage at one point leads to a small change at another.

Then, a low-frequency signal with a frequency of 1000 Hz with an amplitude of 1 mV is fed to the input of the low-frequency limiter circuit and we make sure with the help of an oscilloscope that the signal has a sinusoidal shape on the movable contact R23 "MU output".

Then the potentiometer R30 "Limitation level" is set all the way counterclockwise, which corresponds to the minimum limit, and the oscilloscope is transferred to the base of the transistor VT2.

We put the R23 engine in such a position that the signal in the VT2 base is not limited, but has a sinusoidal shape.

Then we put R30 in the position of 80% of the full turn of the potentiometer slider, this position will be working and corresponds to 16 dB of limitation.

We check the identity of the limitation of one half-wave and the other with an oscilloscope. With 100% clockwise rotation of the R30 engine, the limitation is 20 dB and can be used when working in Pile up (Pile up - English - "Dump" on the frequency).

In the absence of a low-frequency generator, but having an oscilloscope, it is enough to connect a microphone to the input of the device and pronounce a rolling a-a-a-a at a distance of 10-15 cm from the microphone. We set the required voltage level from the R23 engine by controlling the sinusoidal signal in the VT2 base, while R30 "Limitation level" is in the minimum restriction position.

Then we increase the level of limitation, controlling the shape of the limited signal with an oscilloscope while continuing to pronounce a rolling a-a-a-a in front of the microphone. The setup process is complete.

Those who are going to use a low-frequency limiter as part of the R143 r / station are recommended to make some replacement of the capacitances by setting the following values: C9 - 0,68 uF; C12 - 0,068 uF; C17- 0,22uF; C34- 0,1uF. For "one hundred and sixties" who have a communication receiver R160P in transceiver mode and are going to use a low-frequency limiter as part of the factory shaper of telephone types of work B4-24 from the "Lazur" Exciter, it is recommended to install the following capacitances: C9 - 0,047 uF, C12 - 0,068, C17 - 0,15, 34 uF, C0,1- 39 uF. It is desirable to use film capacitors as these capacitances. The output level to the Balanced Modulator is set using R80 "Output Level". It is advisable to use the MD380 or MD5A microphones used in official radio communications. If you don't have XNUMX% resistors and film capacitors, but want to have a low-frequency limiter, don't despair. Use the components you have. Everything will be fine. The difference between "Good" and "Excellent" is usually not noticeable by ear, but is perceptible only with measuring instruments.

Alexander (EU7AW) designed the PCB using Layout ver. 3.0. (ew1mm_lay.zip)

low frequency limiter

References:

Radio-Electronics, March 1972
Radio No. 4, 1974, p.57

Authors: Igor Podgorny, (EW1MM), Vyacheslav Sergeychuk, (EW1CA), Minsk, 2002, ew1mm@softhome.net

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