FI74844B - KOPPLINGSARRANGEMANG FOER REGLERING AV HOEGFREKVENS- OCH MELLANFREKVENSSTEGENS FOERSTAERKNING VID RADIO- OCH TELEVISIONSMOTTAGARE. - Google Patents

Description

74844

Switching arrangement for adjusting the gain of the high-frequency and intermediate frequencies of radio and television receivers

Radio and television receivers generally adjust 5 amplifier stages to avoid overdrive.

This avoids non-linear distortions in the output signal of the amplifier stage.

It is previously known to derive, for example, a control voltage (AGO) from the frequency amplifier, which adjusts down 10 successively different intermediate frequency stages and the input-amplifier stages of the tuning section as the antenna signals increase. In this case, it is possible to adjust the stages with lower input voltages. In the former case, there are few problems with nonlinear distortions, center modulation, etc. However, the signal-to-noise ratio remains very small. The signal-to-noise ratio is known to be equal to zero at the start of the control at a given input voltage. the possible signal-to-noise ratio is only achieved in this case at high input voltages.

The new regulations of the Federal Post Office of the Federal Republic of Germany pose new problems for the construction of reception systems, such as tele-25 vision and radio receivers, in connection with the solution of large signal characteristics. The new regulations make it necessary to start the adjustment substantially earlier in order to prevent oversteer of individual stages. However, this is at the expense of the signal-to-noise ratio, which becomes worse the lower the starting point of the control. Due to the non-linear frequency response of the amplifier stages, the gain is not constant in the received frequency band, so the amplifier system is adjusted at a certain control start point at '35 frequency. For example, at the frequency f ^ »at which the gain is lower, the start of the adjustment takes place later, 2 74844 whereby the risk of overdriving again exists, while at another frequency f ^» at which the gain is higher, the control starts substantially earlier. However, as a result, the signal-to-noise ratio only becomes very late, i.e. with larger antenna input voltages.

The invention is thus based on the task of complying with the new regulations so that the control is started very early without deteriorating the signal-to-noise ratio. This function ratio is banded by means of the measures presented as features of the claims. The advantage of the invention is that the total scattering of the system (gain scattering) is reduced. The type of control applied for works more precisely, which is very important in order to achieve the highest possible signal-to-noise ratio at an early stage. The invention is based on the finding that the switching part under overdrive hazard, here e.g. an intermediate frequency amplifier, must first be protected against overdrive before starting the adjustment in the tuning part, the amplification of which is still linear even at higher input voltages. As a result, the signal-to-noise ratio increases very rapidly, so that the maximum possible signal-to-noise ratio measured over the entire amplifier (high-frequency and intermediate-frequency amplifier) is reached at half or one-third the input voltage, depending on the noise factor of the amplifier 25. By means of the invention, the starting point of the control can be determined much more precisely, because the operating point of the input transistor of the tuning part only shifts at higher antenna voltages.

In the following, the essential features of the invention will be explained in connection with an exemplary embodiment by means of a drawing figure.

The input signals arrive via the antenna 1 at the input of the tuning section 2. They are amplified in the precursor 3 and mixed-mixed in the mixing stage H with the signal received from the oscillator 5 to an intermediate frequency ZF. According to the invention, a control member 7 is arranged between the output of the tuning section 2 and the input of the amplifier 6, which controls the input signal of the intermediate frequency stage 6. For this purpose, a circuit 9 may be connected to the output of the tuning section 2, possibly via a filter 8, to generate a signal for automatic gain control 5 (AGO. As the control element 7, an attenuator implemented in the PIN diode 10-1a or a field effect transistor stage can be used.

With an antenna voltage of 1.5 mV, the signal-to-noise ratio then improves by about 4 dB.

In the switching arrangement described above, the control criterion obtained from the output of the tuner-15 part mixer can be derived selectively or broadbandly, as appropriate. This type of adjustment makes it possible to limit the gain differences in the entire gain path from the antenna to the detector to the tuning section and thus to meet the current linearity requirements while improving the signal-to-noise ratio.