FI86351B - Method for channel-dependent control of a voltage- controlled interphase filter in a receiver of a radio telephone - Google Patents

Description

1 86351

Method for controlling a voltage-controlled suction filter in a channel-dependent manner in a radiotelephone receiver - Förfarande för kanalberoende styrning av ett spänningsstyrt sugfilter i en mottagare av en radiotelefon 5

The invention relates to a method for controlling a channel-dependent signal-attenuating suction filter in a radiotelephone receiver 10, in which the suction filter is connected before the first mixer between the rf signal path and ground and in which the mixer local oscillator frequency is generated by a voltage-controlled oscillator PLL).

15

Typically, in radiotelephones, the received radio signal is input from the antenna to a duplexer, from where it is passed through a first amplifier and a fixed bandpass filter at the front end of the receiver section to a first mixer where it is mixed to a local oscillator frequency and filtered. For component cost reasons and for the simplicity of the structure, it would be advantageous to select a low intermediate frequency, but the resulting spurious repetition causes problems. The spurious repetition occurs in such a way that if the first intermediate frequency of the radiotelephone receiver is less than twice the reception bandwidth, a spurious repetition is generated in the mixer which cannot be filtered out by the fixed front end filters of the receiver. The spurious signal is half the frequency of the received signal. The problem arises specifically at the lower end of the reception band. In this case, when the receiver listens at the lower end of the reception band, spurious frequencies are generated at the upper end of the reception band, where there is no attenuation for the front end filters. For example, in an NMT telephone, the reception band is 25 MHz, and if the intermediate frequency is less than 2 * 25 MHz, or 50 MHz, high levels of harmful repetition signals occur at the upper end of the reception band. When the signal frequency to be received is at the upper end of the reception frequency band, the spurious frequency shifts outside the reception band, allowing the fixed filters preceding the mixer to remove them. Thus, the problems arise precisely when the rf signal of the reception band is in the lower range of the band. When switching from one receiving channel to another, the frequency of the spurious signal also changes.

10 Distractions can, of course, be reduced by using a sufficiently high intermediate frequency. Duplicate duplicates can also be eliminated by using a voltage-controlled bandpass filter or by improving the characteristics of the mixer and, in general, by dimensioning the components used with great care. If the level of signals that generate 15 distractions is very low, no special measures may be required. However, in certain radiotelephone systems, the interference suppression required from the telephone is so great that the receiver cannot be made to operate to the required specifications without a separate method for attenuating interference.

One such method is described in co-pending patent application FI-901579, 25 Nokia Mobile Phones. In the described method, a suction filter is connected between the rf signal path and the ground of the circuit before the mixer, the resonant frequency of which can be controlled by a control voltage. A suction filter is a filter that emits all frequencies except an equally narrow frequency band. 30 Its frequency response is almost constant throughout the entire frequency range except for the narrow band where the attenuation is high. The suction filter is controlled to attenuate the spurious signal when a radio frequency signal is received at the lower end of the receiving band, with the frequency of the spurious signal being at the upper end of the receiving band. When the reception frequency is at the upper end of the band, the filters at the front of the receiver are able to attenuate spurious reproduction. In this case, the suction filter must be switched off or its blocking band 3 86351 tuned away from the upper end of the receiving band so that the actual reception frequency is not attenuated.

The control voltage 5 of the suction filter of the described patent application should in some way be a function of the frequency of the receiving channel, whereby the difficulty is to obtain a suitable control voltage.

The present invention provides a method by which a control voltage which is channel-dependent can be generated on a voltage-controlled suction filter, that is, when the reception channel is changed, the control voltage changes and at the same time changes the suction frequency of the suction filter. The method is also suitable for use in controlling a suction-filter connection, such a connection being described in application FI-901579, Nokia Mobile Phones, filed simultaneously with this application.

The method according to the invention is characterized in that the control-20 voltage of the suction filter proportional to the reception frequency is taken from the control voltage of the voltage-controlled oscillator (VCO).

In modern radiotelephones, the local oscillator frequencies are formed from the frequency of the basic oscillator, the crystal oscillator, by 25 synthesizers. The synthesizer consists of a phase locked loop (PLL) in which the inputs of the phase comparator are the fundamental oscillator frequency or the frequency obtained by dividing and the output frequency of the voltage controlled oscillator (VCO) or the frequency obtained by dividing it in a controlled divider. The differential voltage of the phase comparator 30 or the control voltage filtered from the differential voltage directly controls the VCO, the output frequency of which is a frequency proportional to the differential voltage. In equilibrium, the output frequency is the desired local oscillator frequency. Since the local oscillator frequency determines the receive channel, the frequency of the receive channel is proportional to the control voltage of the loop voltage controlled oscillator (VCO), as described above. According to the invention, this control voltage can also be used to control the suction filter.

4 86351

The invention will be described in more detail with reference to the accompanying figures, in which Figure 1 shows the basic connection 5 of a suction filter in a receiver, and Figure 2 shows the basic connection of a phase-locked loop forming a local oscillator frequency.

Figure 1 shows a part of the front end of the receiver. The received radio signal 10 is amplified in the amplifier 1, filtered in the filter 2 and passed to the mixer 4. Before the mixer, a voltage-controlled suction filter 3 is connected to absorb the signal frequencies causing the repetition.

In Fig. 2, the inputs of the phase comparator 6 are the frequency of the crystal oscillator 5 and the frequency coming from the controlled frequency divider 8, and its output voltage is proportional to the frequency and phase difference of the input pulses. The filtered output voltage directly controls the voltage-controlled oscillator 7, the output frequency of which is the local oscillator frequency fL0 applied to the mixer.

From the output of the oscillator 7 there is a feedback via a controlled divider 8 to a phase comparator 6. The channel control signals of the divider 8 select the local oscillator frequency and thus the reception channel. Thus, when the receiver is operating, the lower channel 25 has the lowest control voltage of VC0 and, correspondingly, the upper channel has the highest control voltage. This control voltage can be used directly for the continuous control of a voltage-controlled suction filter if the control voltage of the oscillator 7 changes sufficiently when switching from the lower channel to the upper channel and if the control voltage values in the lower and upper channels are suitable, e.g. 2 V and 5 V respectively.

Another possibility is to use a control voltage controlled switch or comparator 35 of the VC0 to control the suction filter, the output of which is low when the control voltage of the oscillator VCO is below a certain threshold level and rises when the control voltage exceeds the threshold level. The suction filter is then adjusted in two stages. When the control voltage is below the threshold level, the suction filter blocking is above the receiving range, and when the control voltage is above the threshold level, the suction filter blocking moves above the receiving band. The comparator can be implemented in one of the known ways with a differential stage made of discrete components or with an operational amplifier. This control is well suited for use in controlling the connection shown in application FI-901579, Nokia Mobile Phones.

10 In order not to interfere with the control line of the voltage-controlled oscillator 7, the input impedance of the control line of the comparator 9 or the suction filter must be kept high in order to prevent the load on the control line of the oscillator 7.

The method according to the invention is simple and does not require any control from outside the radio parts.

Claims (3)

1. A method for controlling a voltage controlled suction filter (3) which attenuates a channel display causing signal dependency signal in a radio telephone receiver, in which the suction filter (3) is coupled before a first mixer (4) between the RF signal path and the circuit ground and in which the local oscillator frequency (fLO) of the mixer (4) is generated by a phase reading circuit (PLL) comprising a voltage controlled oscillator (7), characterized in that a raw reception frequency proportional control voltage (Vohj) is taken from the control voltage line. -controlled oscillator (7) and the control voltage (Vohj) controls the suction filter to be switched on where the error reproduction frequency is on the receiving band. 15 Method according to claim 1, characterized in that the control voltage (VohJ) is directed directly to the suction filter (3). Method according to Claim 1, characterized in that the control voltage (VohJ) is directed directly to one input of a comparator (9) and to the other input a threshold voltage is proportional to the frequency at which the suction filter (3 ) changes the suction frequency, whereby the output voltage of the comparator acts as the control voltage for the suction filter (3).