JPH0242253B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to a radio receiver in which the input level of a radio frequency signal has a large fluctuation range, and in particular, to a radio receiver of a frequency conversion type that has a low overall noise figure. This relates to the circuit configuration of the machine.

(b) Prior art and problems A conventional frequency conversion type radio receiver has a circuit configuration as shown in FIG.

The radio frequency signal input RFin is first amplified by a radio frequency amplifier (RFA) 1, and then amplified by a local oscillator (LO).
3 and the frequency converter (MIX) 2 and converted into an intermediate frequency signal (IF signal) lif. this
The IF signal lif is amplified by an intermediate frequency amplifier (IFA) 4 to a level necessary to drive a demodulator in the next stage and another frequency converter (none of which is shown). The output signal IFout of this intermediate frequency amplifier (IFA) 4 must be a signal with little noise and little distortion.

In order to lower the noise figure (NF), which represents the noise characteristics of the frequency converting radio receiver with the above configuration, it is generally necessary to use a first stage with a small NF.
This is required for the first stage circuit of RFA, MIX with low conversion loss, and IFA with low NF. Among these, the first-stage amplifier of MIX with low conversion loss and IFA with small NF has a limit due to economic efficiency, so the first-stage IFA with large amplification gain is RFA with high amplification gain as a way to equivalently improve these values. It was adopted to reduce noise. However, when the amplification degree of these RFAs and first-stage IFAs is increased, when the RF input level to the receiver becomes high, level saturation occurs in the downstream stages of the MIX and IFAs, causing amplitude distortion and phase distortion. For this reason, circuits are constructed using high-output RFAs and IFAs with high saturation levels, but this has drawbacks such as increased power consumption and expensive components. .

(c) Purpose of the Invention The purpose of the present invention is to eliminate the above-mentioned drawbacks and to provide an economical radio receiver with a low integrated noise figure and good saturation characteristics even with large fluctuations in the receiving input level. be.

(d) Structure of the Invention In the present invention, first, when the radio frequency signal input level (RF input) to the receiver is low, a radio frequency amplifier (RFA) is used to improve the noise figure (NF).
When the RF input becomes high and the output level IFout of the intermediate frequency amplifier (IFA) starts to saturate, the amplification gain of the RFA is lowered and the amplification gain of the IFA system is increased instead. Based on the idea that the overall amplification gain remains unchanged, the circuit is constructed as follows. That is, the output stage of the radio frequency amplifier (RFA) is provided with a first RF attenuator that attenuates the radio frequency level and an RF switch that controls the attachment/detachment (on/off) of the first RF attenuator. Next, there is a second IF attenuator that attenuates the IF signal level at the output stage of the pre-intermediate frequency amplifier, and an IF that controls its on/off.
Install a switch. A level detector for driving the first switch and the second switch is provided at the output stage of the pre-intermediate frequency amplifier to detect the point at which the linear amplification of the IF signal output begins to be lost and saturation.
Configure the circuit to turn on the RF attenuator and turn off the second IF attenuator.

(e) Embodiments of the Invention Below, Fig. 2 shows a circuit of an example of a frequency converting radio receiver based on the present invention, and shows the received signal input level (RFin) and intermediate frequency amplifier output level (IFout).
This will be explained with reference to FIG. 3, which shows the saturation characteristics of .

In Figure 2, 1A is the first attenuator and 1S is the first attenuator.
The switch 4A is a second attenuator, 4S is a second switch, and 5 is a level detector, which are circuits added to the conventional circuit in the present invention. Numbers other than those mentioned above indicate the same circuits as those in the conventional circuit shown in FIG.

When the level of the radio frequency signal input (RF input) of the receiver is low, the first attenuator 1A of the output circuit of the radio frequency amplifier (RFA) is turned off, and the first attenuator 1A of the output circuit of the intermediate frequency amplifier (IFA) is turned off. 2 attenuator 4A is set to the on state by level detector 5.

As the RF input level increases, the output level IFout of IFA4 increases linearly as shown in Figure 3a, but at a certain level of the RF input level (Ps in the figure)
From point ) onwards, the frequency converter (MIX) and the downstream stage of the IFA begin to saturate, resulting in a curve like that shown in Figure b, and various transmission distortions begin to occur in the IFA output signal. Here, the level detector 5 is the output stage of the IFA 4, detects this saturation start level Ps (or the level of the linear part may be selected considering an appropriate operating margin), and drives the switch 1S. to turn on the attenuator 1A, and drive the switch 4S to turn off the attenuator 4A. If the attenuation of attenuator 1A that is turned on and the attenuation of attenuator 4A that is turned off are set to the same value, they will cancel each other out and the output level of IFA4 will be as shown in Figure 3c.
There is no change just by shifting horizontally to the left by the amount of attenuation T, as shown in the figure. Therefore, saturation in MIX and IFA is avoided, and the region of linear amplification operation continues to extend as shown in FIG. 3d as the input level increases. As for the RF input level, the permissible variation range is expanded by a level equal to the value of the attenuation amount T.

On the other hand, regarding the noise characteristics, the RF signal input level is already high at the special production point Ps of switches 1S and 4S, and the signal-to-noise ratio is a large value, so attenuator 1A is inserted at the output of RFA. However, there is no adverse effect on receiver integration.

Needless to say, the above configuration can be used in common regardless of the modulation format AM, FM, PSK, etc.

(f) Effects of the Invention As detailed in the embodiments, according to the present invention, a radio receiver with low saturation and good noise characteristics even when the fluctuation range of the received input level is large can be realized for any modulation format. The effect is great because it can also be provided.

[Brief explanation of the drawing]

FIG. 1 is a conventional circuit, FIG. 2 is a circuit diagram of a frequency converting radio receiver according to the present invention, and FIG. 3 is a characteristic diagram of input level versus output level. In the figure, 1 is a radio frequency amplifier, 2 is a frequency converter, 3 is a local oscillator, 4 is an intermediate frequency amplifier,
5 is a level detector, 1A, 4A is an attenuator, 1S,
4S is a switch.

Claims (1)

[Claims] 1 In a configuration consisting of a radio frequency amplifier, a frequency converter controlled by a local oscillator, and a pre-intermediate frequency amplifier, the output stage of the radio frequency amplifier includes a first
The output stage of the attenuator and the intermediate frequency amplifier is provided with a switch for switching the second attenuator and each attenuator between an on state and an off state, and a level detector for detecting the output level of the intermediate frequency amplifier, The level detector controls a switch to turn off the first attenuator and turn on the second attenuator when the input level of the radio frequency signal is small such that the output level of the intermediate frequency amplifier is below a certain value; When the radio frequency signal input level becomes large enough to cause the output level of the intermediate frequency amplifier to exceed a certain value, the first attenuator is turned on and the second attenuator is turned off by means of a switch. Frequency conversion type radio receiver.