DE2545725C3 - Circuit arrangement for reducing the inter-channel modulation - Google Patents

Description

4 y

The most important properties of a mixer for transposing high-frequency signals to an intermediate The basic frequencies of a radio receiver are:

1. low mixed attenuation,

2. low noise figure,

3. the best possible linearity,

4. The best possible decoupling of the high frequency and the oscillator frequency input from the intermediate frequency output.

The requirement for a low noise figure and the best possible linearity also applies to the bo entire part of the receiver up to the crystal filter.

If the mixer - or one of the linear stages possible up to the crystal filter - strong distortion generate third order, this results in poor resistance to interchannel modulation. In t >> F i g. 2 shows the conditions under which reception is disturbed by interchannel modulation. Of the The receiver is set to the receiving frequency / k.

At a distance of Af or 2Af for the frequencies

U ₁ = Ie + ■ «/.
U ₁ = Ie + 2 if

two powerful jammers may be in operation. Third order distortions are then obtained a mixed product

/ = 2 / s ,, - Is ₁₁ = Ie >

the reception on the frequency fe depending on the amplitude more or less disturbs

In a receiver for frequency-modulated signals known from DT-AS 12 63 113, an IF amplifier connected downstream of the mixer is designed as a low-pass amplifier with an RC filter that is not specified in more detail. It can therefore be assumed that this RC filter, due to its task-specific nature, is not suitable for improving the interchannel modulation resistance of a receiver, especially since the problem of interchannel modulation in the DT-AS 12 63 113 is not addressed.

From DT-OS 21 40 680 it is known at a receiving station for VHF audio broadcast signals for Purpose of interference-free reception of frequency-adjacent transmitters with large level differences to arrange a limiter amplifier between two mixer stages, one on the input and one on the output side IF filter of high phase linearity is switched on. Also this reference are no measures for Removable increase in interchannel modulation resistance.

The German magazine "cq-DL", Volume 8, 1973, pages 450 to 458 can already be seen that Push-pull or ring mixers with high-current field effect transistors have a particularly high intermodulation distance to have.

The invention specified in claim 1 is based on the object of creating an arrangement by which the interchannel modulation resistance of radio receivers is increased considerably.

Advantageous further developments of the invention are given in the subclaims.

With the help of the matching network according to the invention, the strength of radio receivers can be increased increase by more than 1OdB against interchannel modulation, which is a considerable advantage in practice.

An advantageous embodiment of the invention is shown in FIGS. 1 and 2 and is shown below described in more detail.

In the exemplary embodiment, the input stage of the intermediate frequency amplifier has a low-resistance input of the intermediate frequency amplifier a bipolar high-current transistor in a basic circuit.

F i g. 1 shows a block diagram of part of a radio receiver, consisting of a receiving antenna 1, a transforming high-pass filter 2, two amplifiers 3 and 4 filters each, a mixer 5 with Mixing oscillator 6 for transposing the high-frequency received signals to an intermediate frequency, a IF amplifier 8 and a crystal filter 9, in which between the mixer 5 and the IF amplifier 8 a Matching network 7 is inserted, such that the output of the mixer on the one hand via a series circuit from a first capacity 71 and one

first ohmic resistor 72 is connected to ground and on the other hand via a series connection of a Inductance 73 and a second ohmic resistor 75 to the input of the IF amplifier is connected, the connection between the inductance and the second ohmic resistor is connected to ground via a further capacitance 74.

The following signals appear at the mixer output: hj

1. the intermediate frequency signal, the frequency of which is equal to the difference between the frequency of the high-frequency received signal and the oscillator frequency of the mixer oscillator 6,

2. a signal whose frequency is equal to the sum of the received high frequency and the oscillator frequency is,

due to asymmetries in the transformers and diodes of the mixer (ring modulator) also appear still:

3. the signal of the mixer oscillator 6 with its harmonics,

4. one or more strong high-frequency signals of the antenna in the passband of the high-frequency quenzselektionsglieder ² ^ 2 are up. 4

Investigations have shown that there is an adaptation for all of these signals at the mixer output must be provided so that they are not in the mixer jo be reflected. If this is not observed, the resistance to interchannel modulators deteriorates. noticeably.

When using a ring modulator as a mixer with an output resistance of around 50 Ω, one η Intermediate frequency of 10.7 MHz and an intermediate amplifier with an input resistance of 5 to 10 Ω becomes, for example, a value of 180 pF for the first capacitance 71 and for the first ohmic resistance 72 a value of 33 Ω, for the inductance 73 a value of 1 μΗ, for the second ohmic resistance Value of 47 Ω and for the further capacitance 74 a value of 180 pF was chosen.

The IF signal reaches the input of the via the inductance 73 and the second ohmic resistor 75 Intermediate frequency amplifier. The terminating resistance of the mixer for the intermediate frequency signal is mainly determined by the second ohmic resistor 75, so that adaptation is given. This Ohmic resistance causes a level reduction and a pronounced current control at the same time Intermediate frequency amplifier, which largely eliminates non-linear distortion at this stage be avoided. Interestingly, the sensitivity of the radio receiver is determined by the second Ohmic resistance is not affected, since the gain loss due to this resistance, which is the noise figure of the radio receiver itself would have to deteriorate, obviously due to the improved noise adaptation is almost balanced.

The part of the oscillator signal and the antenna signals at the output of the mixer, which are listed above under 3 and 4, reach the first capacity 71 (short circuit for these signals!) To the terminating resistor in the form of the first ohmic resistor 72, see above that their energy is not reflected in the mixer. The same applies to all high-frequency mixed products. The low-pass characteristic of the matching network also prevents the high-frequency signals from being transmitted generate new, undesired mixing products in the intermediate frequency amplifier.

When using an IF amplifier with a higher input resistance (e.g. field effect transistor in gate circuit) or bipolar transistor in emitter circuit) can be the value of the second ohmic resistor 75 can be chosen equal to zero.

1 sheet of drawings

Claims (4)

Patent claims: 1. Circuit arrangement for reducing the inter-channel modulation in radio receivers, characterized in that between the mixer (5) of the radio receiver, which is provided for transposing the high-frequency received signals to an intermediate frequency, and the intermediate frequency amplifier (8), a low-pass matching network ( 7) is arranged in such a way that the output of the mixer is connected to ground on the one hand via a series circuit consisting of a first capacitance (71) and a first ohmic resistor (72) and on the other hand via a series circuit made up of an inductance ( 73) and a second ohmic resistor (75) connected to the input of the intermediate frequency amplifier ^; st and that the connection between the inductance and the second ohmic resistor is connected to ground via a further capacitance (74) (FIG. 1). 2. Circuit arrangement according to claim 1, characterized in that the mixer (5) is a ring modulator is (Fig. 1). 3. Circuit arrangement according to claim 1 or 2, characterized in that when using one Mixer (5) with an output resistance of around 50 Ω, an intermediate frequency of 10.7 MHz and an intermediate frequency amplifier (8) with a low-impedance input for the first capacitance (71) a value of 18OpF, for the first ohmic resistor (72) a value of 33 Ω, for the Inductance (73) a value of 1 μΗ, for the second ohmic resistor (75) a value of 47 Ω and r> for the further capacitance (74) a value of 180 ρ F is selected (Fig. 1). 4. Circuit arrangement according to one of claims 1 to 3, characterized in that the value of the second ohmic resistor (75) when using an intermediate amplifier with a higher Input resistance equal to zero can be selected.