JP2818188B2 - Direct conversion receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a receiver used in a wireless communication system, and more particularly to a direct conversion receiver employing a direct conversion system.

(Conventional technology) In the direct conversion method, the received RF signal is
This is a reception method in which the output is mixed with a local oscillator output having the same frequency, directly converted to a baseband frequency, and detected. FIG. 2 shows a configuration example of a direct conversion receiver. The RF signal received from the antenna 201 passes through the RF filter 202, is amplified by the RF amplifier 203, is divided into two channels, and is mixed with the carrier having the same frequency as the RF signal from the local oscillator 206 in the mixers 204 and 207. You. This local oscillator is connected to a first mixer 204 and to a second mixer 207 via a 90 ° phase shifter 205. The RF signal is converted into a baseband signal having a 90 ° phase relationship by the first and second mixers, passes through the low-pass filters 210 and 211, is amplified by the baseband amplifiers 212 and 213, and detected by, for example, a flip-flop 214 or the like. . Note that A after the mixer
The C couplings 208 and 209 are inserted for the purpose of removing a DC component appearing at the mixer output in order to prevent the amplifiers 212 and 213 from being saturated. The direct conversion method does not have an intermediate frequency, and since the RF signal is directly converted to baseband in frequency, there is no image response in principle.
Due to the fact that a steep filter for image removal, which is usually used in the stage, is not required, and that most of the gain can be obtained in the baseband stage, the recent advances in LSI technology have led to the miniaturization of receivers. , As a receiving method that can realize low power consumption.

The state of frequency conversion will be described with reference to FIG.
In (a), when the FSK signal 302 is directly frequency-converted to the baseband, the RF carrier frequencies fc and the same frequency fb as fc are used.
By mixing the outputs of the local oscillators, a baseband signal as shown in FIG. Here, fd in (a) is the maximum frequency shift of FSK. The output signal from the mixer is folded around zero frequency, and the DC component generated by mixing appears in the middle (zero frequency) of the signal spectrum. This DC component is mainly caused by leakage of the local oscillator output to the RF section and mixing with the local oscillator output again, and causes saturation of the amplifier at the subsequent stage.
Removed by couplings 208,209. This is shown in FIG. 3 (b), where 303 indicates the characteristics of the AC coupling.

The operation of the direct conversion method in FIG. 2 has been described above. In an actual receiver, the R
F-LO isolation is not perfect, or R
The carrier wave from the local oscillator 206 leaks to the RF unit due to leakage from the common bias power supply line for the F component, and is radiated to the air from the antenna 201 through the RF amplifier 203 and the RF filter 202. The same applies to the case of the superheterodyne method. However, in the superheterodyne method, the carrier frequency of the RF signal and the oscillation frequency of the local oscillator inside the receiver are different (if zero,
455 kHz), so that it is usually attenuated by an RF filter having a pass band of an RF signal, so that the power radiated into the air is suppressed to some extent. In the direct conversion method, on the other hand, the carrier frequency of the RF signal is the same as the oscillation frequency of the local oscillator of the receiver. The only part that attenuates the radiated local oscillator leakage is the RF amplifier. In this regard, the direct conversion method is disadvantageous compared to the superheterodyne method.

The output level of the local oscillator of the receiver is usually -30 to-
Since it is 20 dBm, which is larger than the normally received RF signal, the above-mentioned leak output to the air of the local oscillator cannot be ignored.

The output from the local oscillator to the air is not a hindrance to the adjacent receiver having the same reception frequency because the reception frequency and the local oscillation frequency are different in the superheterodyne method, but in the case of the direct conversion method, Since it is the same frequency as the RF signal to be received,
An adjacent receiver using the same frequency becomes an interference wave, which causes a decrease in sensitivity. In a wide area communication system, if a direct conversion receiver is used in a relatively small place such as a premises, radio interference between the receivers becomes a serious problem.

(Problems to be Solved by the Invention) As described above, in the direct conversion receiver, the radiation of the RF carrier into the air due to the leakage of the output of the local oscillator when the local oscillator or the active state is in operation state is different. It was an interference wave to the receiver. As a system that uses a direct conversion receiver,
Taking the paging method as an example, a method is adopted in which a plurality of slave stations simultaneously communicate with subscribers belonging to one group in the same time slot. Since the local oscillators of all the receivers belonging to are turned on, radio interference interference occurs between multiple slave stations that use the same RF frequency in close proximity, leading to a decrease in reception sensitivity, etc. Was causing the problem.

[Configuration of the Invention] (Means for Solving the Problems) As described in detail above, in the present invention, a circuit for extracting a part of the local oscillator output and a circuit for weighting the extracted local oscillator output are provided. A weighting circuit, a subtractor for subtracting an output from the weighting circuit from a leak component of a local oscillator output to an RF port, and a circuit for extracting a part of the subtractor output and a part of the local oscillator output. A correlation detector for taking a correlation with the output of the local oscillator, and a low-pass filter that passes only low-frequency components of the output of the correlation detector,
A local oscillator unnecessary output canceling circuit that adjusts the weight of the weighting circuit in accordance with the output of the low-pass filter and operates to cancel a leakage component of the local oscillator output to an RF port. .

(Operation) In the direct conversion receiver according to the present invention, since the receiver is provided with a circuit for reducing the unnecessary radiation power of the local oscillator so as to be at least lower than the reception sensitivity of another receiver, the same reception frequency is used. Even if the direct conversion receivers are used at adjacent positions, radio interference does not occur between the receivers, and the problem that has occurred in a system using a conventional direct conversion receiver is solved.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

The present invention is obtained from the result of actually measuring the unnecessary radiation power of the output of the local oscillator using a small receiver (superheterodyne system, base injection mixer) having the block diagram of FIG. The numerical values in the figure are approximate values indicating the distribution of the attenuation of the local oscillator output in the RF stage. Considering from this approximate value, it can be seen that most of the unnecessary radiation from the output of the local oscillator (405) comes from circuit components, and the filter (403) → the amplifier (40)
2) → It can be seen that radiation is in the air at the route of the antenna. Therefore, when a receiver having the same performance and specification exists in the vicinity, it is necessary to prevent the outputs of the local oscillators from interfering with each other. It is necessary to keep the local oscillator unnecessary radiation power level from the receiver at a lower level than the reception sensitivity. To give a specific numerical example for this, taking into account the receiving sensitivity of a general receiver, assuming that the receivers are at a distance of 1 m, in a direct conversion receiver with the above specifications , Isolation between the local oscillator (206) and the RF port (21
5,216) must be at least 26 dB or more. Further, it is necessary that the reverse attenuation of the RF amplifier (203) be 15 dB or more. By designing the RF stage of the direct conversion receiver by using these, it is possible to solve the problem of radio interference caused by unnecessary radiation from the local oscillator output between direct conversion receivers, which has been a problem in the past.

FIG. 1 is a diagram for explaining another means for preventing unnecessary radiation of the local oscillator output of the direct conversion receiver. A portion 103 shown in FIG. 1A is a circuit for canceling unnecessary radiation from the output of the local oscillator (113). 104 is a correlation detection unit, and 110 is a weighting circuit. As shown in FIG. 1 (a), the operation principle will be described on the assumption that the complex signals in each section are υ, υ ′, w. Of the output of the local oscillator (113), the component extracted from the local oscillator by the directional coupler (112) to cancel ν leaking to the RF port due to imperfect isolation of the mixer (111) ν ′ is multiplied by a weight w in a weighting circuit (110) (110), and subtracted from the leakage component ν (1
07). From the residual component after the subtraction, y = {− w} ′ is extracted by the directional coupler (106), multiplied by υ ′ by the mixer (109) by the correlation detection unit (104), and the correlation with υ ′ is detected. Then, a loop for controlling the weighting function w is formed after passing through the low-pass filter (108) which operates as a loop filter. If this operation is explained using an equation, the basic equation of a loop filter (first order) In steady state, Holds. A circuit for extracting a part of the subtracter (107) (10
6) does not necessarily need to be a directional coupler, but may be an ordinary distributor. In this case, a loop filter (10
For example, when the received signal is FSK, the band of 8) needs to be sufficiently smaller than the maximum frequency shift.

FIG. 1 (b) is one of more specific configuration examples for realizing the circuit of FIG. 1 (a).

This is an example in which both the correlation detection unit and the weighting circuit are configured by a mixer π / 2 phase shifter.

By using the above method, unnecessary radiation of the output of the local oscillator can be prevented, and there is no fear of causing radio wave interference between adjacent receivers.

[Effects of the Invention] As described in detail above, in the direct conversion receiver of the present invention, even if the output of the built-in local oscillator is radiated from the antenna to the air, the sensitivity is lower than the sensitivity of the receiver having the same reception frequency in the surroundings. The use of a direct-conversion receiver eliminates the problem of radio interference between adjacent receivers, which has been a problem in wireless communication systems that consist of direct-conversion receivers. The effect is that the system can be operated reliably.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a direct conversion receiver according to the present invention. 2 and 3 are diagrams for explaining a direct conversion receiver and a receiving system. FIG. 4 is a diagram for explaining a local oscillator unnecessary radiation power. 201 …… antenna, 202 …… RF filter, 203 …… RF amplifier,
204, 207 mixer, 205 π / 2 phase shifter, 206 local oscillator, 208, 209 AC coupling, 210, 211 baseband filter, 212, 213 baseband up, 214
…… Detector.

Claims (1)

(57) [Claims] A circuit for extracting a part of the local oscillator output; a weighting circuit for weighting the extracted local oscillator output; and an output from the weighting circuit based on a leakage component of the local oscillator output to an RF port. A subtractor for subtracting, a correlation detector for calculating a correlation between a part of the output of the subtracter and a local oscillator output extracted by a circuit for extracting a part of the local oscillator output, and an output of the correlation detector. A low-pass filter that passes only the low-pass component of the local oscillator that adjusts the weight of the weighting circuit in accordance with the output of the low-pass filter, and operates to cancel the leak component of the local oscillator output to the RF port. A direct conversion receiving device comprising an unnecessary output canceling circuit.