METHOD AND ARRANGEMENT FOR FREQUENCY CONVERSION

Abstract: The present invention relates to a method and an arrangement for frequency conversion for a first information-carrying signal (Uin2) with a frequency ( omega b) to a second information-carrying signal (UM) with a desired frequency ( omega ut). A frequency converter (300) comprising an adder (301) is used for adding the signal (Uin2) to an oscillator signal (Uin1) with a predetermined frequency ( omega a), therewith to obtain a sum signal (Uadd) with said frequencies ( omega a, omega b). An output (304) on the adder (301) is connected to an input (305) on an amplifier (306). The amplifier (306) is designed for non-linear amplification of the sum (Uadd), whereby an output signal (UF) with intermodulation components depending on said frequencies ( omega a, omega b) is obtained. An output (307) on the amplifier (306) is connected to an input (308) on a filter (309), which is used for removing all undesired frequencies, so only the signal (UM) with the desired frequency ( omega ut) is obtained on an output (310) on the frequency converter (300).

Description: METHOD AND ARRANGEMENT FOR FREQUENCY CONVERSION FIELD OF INVENTION The present invention relates to a frequency converter which with the aid of two signals of predetermined frequencies generates a signal whose frequency is dependent on the two aforesaid frequencies . The frequency converter may be included in a transmitter and a receiver intended for radio communication, for instance. BACKGROUND ART Practically all devices that are intended for the transmission of radio frequency signals include a frequency converter. The frequency converter is constructed to convert a base band signal to a radio frequency signal or vice versa, by mixing. Mixing of the signals may be carried out in several steps. Stepwise conversion may be effected by first mixing the radio frequency signal to obtain an intermediate frequency and thereafter converting this intermediate frequency signal to a base band signal. Frequency conversion can, in principle, be carried out from any frequency whatsoever to any other frequency whatsoever. Since many radio communications systems are battery powered, such as a mobile telephone for instance, it has become important to construct these devices in a manner which will enable them to be operated at the lowest possible bias currents. A current-lean frequency converter is thus a step in this development. Mobile telephones are tending to become smaller and smaller. Consequently, there is a reason for implementing mobile telephone circuitry that requires the smallest possible space . One earlier known frequency converter is a mixer that is implemented as a multiplier. This multiplier multiplies together two signals each having a specific frequency component. The multiplication process results in a product signal that has new frequency components. For instance, if the signals U inl =a sin ω a t and U in2 =b sin ω b t are connected to the multiplier as input signals, there is obtained the output signal U ut = — {cos ( (ω a -ω b ) t) -cos ( (ω a +ω b ) t) } , which is the product of the signals U inl and U in2 . The signal U ut includes the frequency components ω a -ω b and ω a+ ω b . The signal U ut is then filtered through a filter of specific band width and a signal that has one of the aforesaid frequency components is obtained. Typical and earlier known circuitry for multipliers is the so-called Gilbert multiplier. The Gilbert multiplier includes a number of transistors mutually connected in a specific circuit configuration. The Gilbert multiplier requires a relatively high supply voltage and high bias current in order to operate in the manner desired. This results in the drawback of high power consumption. Another drawback with the Gilbert multiplier is that it is constructed of many transistors, which makes implementation of the multiplier difficult to achieve in order for the multiplier to operate properly. Another drawback with the Gilbert multiplier is that comparatively powerful input signals are required as a result of the relatively high bias current. Diode mixers are typical in microwave contexts. In this case, mixing is implemented by adding a frequency signal to a local oscillator signal so as to obtain a sum signal. This sum signal is applied to a diode which is connected in series with a suitable load. Because the non-linear properties of the diode are used, there are obtained intermodulation signals with frequencies that are dependent on the signal frequency and the local oscillator frequency. A desired frequency is selected from among the intermodulation frequencies with the aid of an appropriate filter. This type of diode mixer requires the local oscillator signal to be much stronger than the frequency signal. It also gives very low mixing amplification, i.e. strong attenuation, which, in turn, results in poor signal/noise ratio on the signal obtained. Another type of diode mixer is a so-called ring diode mixer. This type of mixer has better amplification than the diode mixer described above. However, it is necessary to use high local oscillator power, often in the region of hundreds of mW, in order to obtain good amplification and linearity. The ring diode mixer has also low impedance, resulting in high signal currents from the local oscillator. This is a serious drawback in, for instance, integrated circuits, where it is difficult to achieve loss-free impedance matching. Poor impedance matching means that the loss effect will be several times greater, since the current is then taken from the supply voltage . DISCLOSURE OF THE INVENTION The present invention addresses a problem of how to effect frequency conversion with a frequency converter that requires a relatively low supply voltage and a relatively low bias current. Another problem addressed by the invention is one of reducing the number of components in a frequency converter. Another object of the invention is thus to construct a frequency converter that requires a lower supply voltage and a lower bias current and therewith obtain a frequency converter that consumes relatively little power. Still another object of the present invention is to construct a frequency converter that is comprised of only a few components and therewith provide a frequency converter of simpler construction. In accordance with the present invention, these problems are solved with a frequency conversion function that is implemented with solely one adder function, one amplifier function and one filter function, where the amplifier function has a non-linear characteristic. The frequency converter includes an adder, an amplifier and a filter. The adder and the amplifier are implemented with solely a few components. For instance, the adder can be implemented with the aid of two differential amplifiers, and the amplifier can be implemented with the aid of one differential amplifier. Described in a little more detail, the adder of the inventive frequency converter includes two inputs and one output and is adapted to add together the signals applied to each of said inputs. Each of the signals has a predetermined frequency component. For instance, one signal may be an oscillator signal and the other signal an information signal of given frequency. The sum of the signals is obtained on the adder output as a sum signal that includes two frequency components that are the same as the frequency components of the two signals applied to the inputs. The amplifier functions to amplify the sum signal delivered by the adder in a manner to obtain intermodulation components of the frequency components of the sum signal. This can be achieved, for instance, by biasing the amplifier towards a non-linear working range. The amplifier output is connected to at least one filter that filters-off undesirable intermodulation components, therewith obtaining at least one signal having one frequency component that constitutes the desired product of the multiplied frequency components. One advantage afforded by the inventive frequency converter is that it requires a lower supply voltage and a lower bias current than present-day frequency converters. Another advantage afforded by the invention is that simple circuitry solutions can be readily found for the inventive frequency converter. Another advantage is that the circuitry of the inventive frequency converter includes fewer components than the circuitry of present-day frequency converters. This makes the inventive frequency converter more cost-effective, since fewer components are required in its construction. Another advantage afforded by the present invention is that the frequency of the oscillator signal need be only half as great as, e.g., the frequency of the oscillator signal in respect of the diode mixer. This means that the frequency content of any oscillator signal leakage out towards the antenna will lie far from the current frequency band and can therefore be filtered out. An oscillator for lower frequencies can also be given a low noise level more easily. The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings . BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a typical symbol for a frequency converter that is implemented as a multiplier. Figure 2 illustrates implementation of a known multiplier intended for the radio frequency range . Figure 3 illustrates an embodiment of an inventive frequency converter . Figure 4 illustrates an embodiment of the adder and amplifier of the inventive frequency converter. Figure 5 is a flowchart that illustrates frequency conversion in accordance with the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 illustrates the manner in which a block element for a mixer 100 is normally shown in a block schematic. The mixer 100 is constructed to multiply two signals in a manner earlier known. A signal U inl = a sin ω a t is received on a first input 101 and another signal U irι2 = b sin ω b t is received on a second input 102, therewith obtaining a signal nh U ut = — {cos ( (ω a -ω b ) t) - cos ( (ω a +ω b ) t) } on an output 103. The output signal U ut is a product of the input signals and include the frequencies ω utl = ω a - ω b and ω utl = ω a + ω b . One of the frequency components is then filtered out, so as to obtain a desired signal that has one of the two obtained frequency components . Figure 2 is a simplified circuit diagram showing the implementation of a multiplier in the form of a so-called Gilbert multiplier 200. The Gilbert multiplier 200 includes a first differential stage having two transistors Zl and Z2 , a second differential stage having two transistors Z3 and Z4 , and a third differential stage having two transistors Z5 and Z6. These three differential stages are connected to one another in a known manner. The Gilbert multiplier 200 also includes a first input 201 intended for an input voltage U inl , and a second input 202 intended for an input voltage U in2 . The third differential stage that includes the transistors Z5 and Z6 is a voltage-current converter. The input voltage used having U in2 is converted to an input voltage I in . An output signal is obtained as the voltage difference between a voltage U^ on an output 203 and a voltage U 2 on an output 204, i.e. said signal ~ U inl xU in2 = ϋ^-U^. In this type of Gilbert multiplier, two transistors are connected in series between a supply voltage V and ground (GND) . For instance, the supply voltage V is shown connected to the collector Cl of the transistor Zl in Figure 2. The emitter El of the transistor Zl is connected to the collector C5 of the transistor Z5, and the emitter E5 of the transistor Z5 is connected to ground (GND) via a current generator. A series coupling of this kind requires a relatively high supply voltage . Figure 3 is a block schematic illustrating an inventive frequency converter 300 that has a first input 302, a second input 303 and an output 310. The frequency converter also includes an adder 301, that has the input 302, the input 303 and an output 304. The converter also includes an amplifier 306 that has an input 305 and an output 307, a filter 309, for instance a bandpass filter or a lowpass filter, that has an input 308 and the output 310. The adder 301 is connected to add an input signal U inl having a specific frequency ω a on the first input 302 to another input signal U in2 of specific frequency ω b on the second input 303, therewith obtaining a sum signal U add on the output 304. For instance, when U inl =a sin(ω a t) and U in2 =b sin(ω b t) are each applied to a respective input 302, 303, the signal U add =a sin(ω a t) + b sin(ω b t) on the output 304. The adder input 304 is connected to the input 305 of the amplifier 306. The amplifier 306 is adapted to amplify in its non-linear region, in other words the output signal from the amplifier 306 is not linearly dependent on its input signal. A well-known phenomenon resulting from the non-linearity of the amplifier is that when its input signal U add includes at least two frequency components, there is obtained subsequent to amplification a signal U F which includes intermodulation components of the input signal frequency components. When the input signal of a non-linear amplifier includes at least two frequency components, there are typically obtained intermodulation components whose magnitudes are dependent on the frequency components of the input signal. When the amplifier input signal consists of two frequency components, there are obtained, among other things, third order intermodulation components that consist of four different frequencies. Following four frequency components are obtained in this case: two times the frequency component of the first input signal ± the frequency component of the second input signal, and two times the frequency component of the second input signal ± the frequency component of the first input signal . For instance, when the aforesaid sum signal U add =a sin ω a t + b sin ω b t is amplified non-linearly by the amplifier 306, there is obtained, among other things, the third order intermodulation signal S3 = a 2 b(sin(2ωa + ωb) t + sin(2ωa - ωb)t) + ab 2 (sin(2ωb + ωa)t + sin(2ωb - ωa)t). This signal includes four different frequencies: 2ωa + ωb, 2ω-ωb, 2ωb + ωa and 2ωb - ωa. The amplifier output 307 is connected to the input 308 of the bandpass filter 309. The bandpass filter 309 is adapted to allow the frequency components obtained with the non-linear amplification to pass through. For instance, if the desired frequency is ω ut = 2ω a - ω b , there is used a bandpass filter 309 that allows frequencies having the centre frequency 2ω a - ω b to pass through, wherewith the signal U M = a 2 b sin(2ω a - ω b ) is obtained on the output 310 of the bandpass filter. When several filters are connected to the output 308, several frequency conversion can be implemented due to the intermodulation signal obtained. Bandpass filters having different centre frequencies are connected to the amplifier output 307, depending on which frequency is desired. An embodiment of the adder 301 and the amplifier 306 of the inventive frequency converter will now be described with reference to Figure 4. The adder 301 is implemented with two differential amplifiers 400, 401. The differential amplifier 400 includes transistors Tl and T2 , and the differential amplifier 401 includes transistors T3 and T4. The input 302, intended for a differential signal U inl , is comprised of a connection to the base Bl of transistor Tl and a connection to the base B2 of transistor T2. Similarly, the input 303, intended for a differential signal U in2 , is connected to the transistors T3 and T4. The collector Kl of the transistor Tl is connected to the connector K3 of the transistor T3. The transistor T2 is connected to the transistor T4 in a similar manner. The collectors Kl, K2 of respective transistors Tl and T3 are connected to the supply voltage Vcc via a common collector resistor R. The transistors T2 and T4 are connected to the supply voltage via another collector resistor R, in a similar manner. Each of the differential amplifier 400, 401 also includes a current generator, for instance a current generator 402, 403. The current generator 402 is connected to the emitters El, E2 of respective transistors Tl and T2. The current generator 403 is connected to the transistors T3 and T4 in a similar manner. The amplifier 306 shown in Figure 4 is comprised of a differential amplifier 404 that includes transistors T5, T6, a current generator 405, and two collector resistors Rl . These are connected to each other in a known manner such as to obtain the differential amplifier 404. The output 304 of the adder is comprised of a connection 408 to a point 410 located between the collector resistor R and the collector K3 , and a connection 409 to a point 411 located between the collector resistor R and the collector K4. The connection 408 is coupled to the base B5 of the transistor T5 and the connection 409 is coupled to the base B6 of the transistor T6. These connections correspond to the connection between the adder output 304 and the amplifier input 305. The signal U add , which is a differential signal by virtue of being obtained as an output magnitude from a differential amplifier coupling constituting the adder 301, is applied to each gate of the transistors T5 and T6. The amplitude of U add is sufficiently great to over-excite the differential amplifier. The value of U add can be made sufficiently great by, e.g., selecting sufficiently large input signals U inl , U in2 , or by allowing the adder 301 to amplify the input signals U inl , U in2 . An over-excited differential amplifier typically generates an output signal whose frequency components are comprised of intermodulation frequency components of the frequency components of the input signal. The generated signal above mentioned includes, inter alia, third order intermodulation components . The signal U F , which represents the output signal from the differential amplifier 404, is obtained as a voltage difference between outputs 406, 407. The signal U F is filtered with a bandpass filter 309 that has a predetermined centre frequency. The filter construction is known generally and will not therefore be described in any detail. The frequency conversion method will be described with reference to the flowchart according to Figure 5. Frequency conversion is commenced in a step 500 by determining the frequency components of those signals that constitute input signals to the frequency converter 300. If the frequency components of the input signals are chosen in a predetermined, beneficial way, there is obtained on the amplifier output 307 a signal that contains several frequencies. For instance, if an oscillator frequency that is half the frequency of the centre frequency of the information-carrying signal for instance, i.e. ω a = ω L0 and ω b = W INF = 2 x ω L07 the frequency ω ut is obtained in the base band when the frequency 2 x ω a - ω b is filtered-out from U F . There is chosen with this selection an oscillator frequency that lies far from information-carrying frequencies. The oscillator frequency can thus be easily filtered-out by suitable choice of filter, so as not to disturb the information-carrying signals. The two input signals are added in step 501, to obtain a sum signal. This sum signal is amplified non-linearly in step 502, therewith obtaining a signal having intermodulation frequency components that are dependent on the frequency components of the input signals. That frequency component or those frequency components to be obtained is/are determined in the following step 503. The non-linearly amplified sum signal is therefore filtered in a bandpass filter whose characteristic corresponds to the signal frequency to be recovered in step 504. Those frequency components which the frequency converter is intended to generate are obtained subsequent to this filtration. The invention is not restricted to the addition of solely two signals of predetermined frequencies, but includes the addition of more than two signals and therewith obtain a result which is analogous with the result explained above. It will be understood that the invention is not restricted to the aforedescribed and illustrated embodiments thereof and that modification can be made within the scope of the following Claims.

Claims: CLAIMS 1. A frequency converter (300) which is adapted to generate with the aid of at least two signals having respectively a first and a second determined frequency (ω a , ω b ) , a signal (U M ) of desired frequency (ω ut ) in dependence on said first and said second frequencies (ω a , ω b ) , said frequency converter including at least one adder (301) which is adapted to add the signal that has said first frequency (ω a ) to the signal that has said second frequency (ω b ) , so as to obtain a sum signal (U add ) , characterized in that the frequency converter further includes at least one non-linear amplifier (306) which functions to amplify said sum signal (U add ) , wherewith a signal (U F ) that includes intermodulation frequencies of said first frequency (ω a ) and said second frequency (ω b ) is obtained; and in that said converter includes further at least one filter (309) which is adapted to filter the signal (U F ) , thereby obtaining the signal (U M ) that has the desired frequency (ω ut ) . 2. A frequency converter (300) according to Claim 1, characterized in that the non-linear amplifier (306) is a differential amplifier (404) which is biased in its nonlinear region. 3. A frequency converter (300) according to Claim 2, characterized in that the desired frequency (ω ut ) corresponds to the value of 2 x ω a - ω b . 4. A frequency converter (300) according to Claim 3, characterized in that the signal having the first frequency (ω a ) is an oscillator signal and the signal having the second frequency (ω b ) is an information-carrying signal. 5. A method of frequency conversion comprising generating a signal having a desired frequency (ω a , ω b ) with the aid of at least two signals having respectively a first and a second determined frequency (ω ut ) in dependence on said first and said second frequencies (ω a , ω b ) , said method further comprising adding the signal that has the first frequency (ω a ) to the signal that has the second frequency (ω b ) such as to obtain a sum signal (U add ) , characterized in that the method includes the further steps of non-linearly amplifying the sum signal (U add ) such as to obtain a signal (U F ) that includes intermodulation frequencies of the frequency (ω a ) and of the frequency (ω b ) ; and filtering the signal (U F ) such as to obtain at least the signal that has the frequency (ω ut ) . 6. A method according to Claim 5, characterized in that the obtained frequency (ω ut ) corresponds to the value 2 x ωa - ωb. 7. A method according to Claim 6, characterized in that the signal that has the first frequency (ω a ) is an oscillator signal and the signal that has the second frequency (ω b ) is an information signal.

Inventors: CARLSSON MATS, RYDIN ARNE, CARLSSON, MATS, RYDIN, ARNE
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1997
Main classification: H03D7
IPC codes: H03D7