WO2004023643A1 - Circuit arrangement having a frequency converter - Google Patents

Abstract

The invention relates to a circuit arrangement having a frequency converter, in which a frequency mixer (18, 19) is provided in a first direct current path and, together with a cascode transistor (13), forms a cascode circuit. An input transistor (2) is provided in another direct current path and effects a signal pre-amplification of a high-frequency input signal (RF). The first and second direct current paths are interconnected via a current interface and form a unit using circuit technology. The current interface, in conjunction with the cascode circuit, makes a high insulation possible between the high-frequency signal (RF) and a local oscillator signal (LO) of the frequency converter. The circuit is particularly suited for direct conversion in receivers and can be integrated with a low level of complexity with regard to circuit technology.

Description

description

Circuit arrangement with frequency converter

The present invention relates to a circuit arrangement with frequency converter.

Circuit arrangements with frequency converters are normally used both in radio transmitter arrangements and in radio receivers. In transmitters, the frequency converter is used to convert a baseband signal to a high-frequency position, while in a receiver, frequency conversion from a high-frequency reception position to the baseband is carried out.

A distinction is made between homodyne and heterodyne transmitter and receiver structures, depending on whether this frequency conversion between baseband and high-frequency location takes place in one step or whether it is first implemented in an intermediate frequency level.

Frequency conversion in receivers is normally implemented in terms of circuitry by means of multipliers designed as frequency converters. So-called Gilbert multiplier cells are widely used. These are supplied with a high-frequency useful signal at one input and a so-called local oscillator signal with a carrier frequency at another input.

The useful signal input of the downward frequency converter is usually designed as a voltage input, which means that the useful signal is supplied in the form of a voltage signal. A low-noise preamplifier, LNA, is normally connected to the useful signal input of the downward frequency converter, which amplifies a signal which is coupled in by an antenna and which is initially filtered, if necessary, for the further Signal processing strengthened sufficiently. LNA and mixers are therefore usually implemented as separate functional units in heterodyne receivers.

Due to the endeavor to be able to build ever smaller radio receivers with constantly decreasing power consumption and lower weight, there is, for example, a trend in GSM (Global System for Mobile communication) mobile radio devices away from heterodyne receiver structures and towards direct conversion transceivers. Receivers with a direct conversion to baseband are also known as direct conversion (DC) receivers. They ^" enable a particularly inexpensive mass producibility of mobile radio devices. In addition, DC receivers can be realized with a smaller number of components, since SAW filters can be dispensed with.

DC receivers for radio applications require high isolation between high-frequency signals and local oscillator signals of approx. 70 dB in order to meet the AM suppression values required in the GSM specifications.

The object of the present invention is to provide a circuit arrangement with frequency converter which is suitable for downward frequency conversion in direct conversion (DC) receivers.

According to the invention, the object is achieved by a circuit arrangement with a frequency converter with a first direct current path, comprising - a mixer cell which is designed as a frequency converter and comprises at least two transistors,

a cascode transistor, which is connected to the at least two transistors of the mixer cell in a cascode circuit, and a feed-in node designed to supply an input signal for the frequency converter in the form of a current signal and with a second direct current path, comprising a preamplifier for preamplifying the input signal with a current decoupling node, which is coupled to the feed-in node in the first direct current path.

According to the principle presented, preamplifiers and frequency converters are implemented in a combined circuit structure, the current decoupling node of the preamplifier, at which a preamplified signal is provided, being coupled to the feed-in node of the frequency converter in a current interface. The preamplifier serves for preamplification of the useful signal to be supplied to the frequency converter and does not work as a voltage but as a current amplifier.

The amplifier-mixer structure summarized in accordance with the present principle is distinguished in particular by the fact that it has only one AC (alternating current) signal path. This is preferably designed to carry differential signals.

In the circuit arrangement according to the proposed principle, the voltage gain at the preamplifier output, that is to say at the current decoupling node of the preamplifier, is relatively low. Thus, no large high-frequency voltage amplitude is formed on the input side of the frequency converter, which could lead to undesired coupling of the high-frequency input signal with the local oscillator signal. The AM suppression is therefore good and corresponding specifications, such as those defined in the GSM mobile radio standard, Global System for Mobile Communication, can be easily met.

An additional advantage of the circuit arrangement described results from the lower demands on the compression behavior in the mixer characteristic, since the input signal at the mixer input or at its input feed node is relatively small and therefore the transistors of the mixer cell do not easily come into compression. This results in a significantly lower outlay with regard to the frequency mixer.

A further advantage of the circuitry principle described lies in the likewise lower demands on the compression behavior of the low-noise preamplifier. Compared to two separate preamplifier and mixer blocks with a coupling by means of a voltage signal, a lower gain is achieved in the preamplifier in the case of the present object and the output, namely the current decoupling node of the preamplifier, is only later compressed in comparison with the previously customary implementation.

The compression behavior is understood to be the non-linear behavior of the transmission characteristic in its marginal areas.

According to the proposed principle, the preamplifier preferably delivers just enough voltage amplification that the noise properties in the high-frequency range, in the so-called front end of the circuit arrangement for frequency conversion, are within the applicable specification.

According to a preferred embodiment of the present invention, a coupling capacitance is provided which couples the current decoupling node of the preamplifier to the feed-in node of the frequency converter. The coupling capacitance is used advantageously for common mode decoupling of the preamplifier and frequency converter and forms the high-frequency current interface between the preamplifier and frequency converter.

The feed node in the first direct current path for supplying the preamplified useful signal is preferably formed between the cascode transistor and a current source designed against a reference potential connection. The feed at the base of the cascode stage results in particularly good isolation of the high-frequency useful signal and the local oscillator signal, which are linked together by the frequency converter with the aim of converting the high-frequency signal into a low-frequency position or into the baseband.

Preamplifiers and frequency converters are preferably designed for processing differential or symmetrical signals. Compared to the design of preamplifiers and frequency converters for so-called single-ended signals, which can be routed on just one line, the symmetrical design of the signal processing results in the advantage of significantly improved interference signal suppression.

The mixer cell is preferably designed with four transistors, each of which is connected in pairs and which are connected to form a Gilbert mixer cell.

Further details and advantageous embodiments of the present invention are the subject of the dependent claims.

The invention is explained in more detail using an exemplary embodiment with reference to the single drawing.

It shows :

The figure shows an exemplary embodiment of the present circuit arrangement with frequency converter using a circuit diagram.

The figure shows an embodiment of a circuit arrangement with frequency converter constructed in bipolar circuit technology. In the present case, NPN transistors are used as transistors. Preamplifiers and high-frequency converters form a circuitry unit in the present circuit arrangement. The preamplifier comprises two input transistors 1, 2, which are connected to one another at their emitter connections to form a differential amplifier. The common emitter node is connected via a resistor 3 to a reference potential connection 4. The base connections of the input transistors 1, 2 form the input 5 of the present circuit arrangement designed for supplying symmetrical or differential high-frequency signals RF, RFX. The collector connections of the transistors 1, 2 are each connected to a supply potential connection 8 via a current source 6, 7. The potential at the supply potential connection 8 is positive in relation to the reference potential at the reference potential connection 4.

The collector connections of the input transistors 1, 2 also form a symmetrical decoupling node for current decoupling 9. One of the high-frequency ones is at this output

Useful signal RF, RFX derived, amplified signal can be derived in the form of a current signal.

A coupling capacitance 10, 11 with one connection each is connected to each collector connection of the input transistors 1, 2. The free connections of the coupling capacitors 10, 11 are connected to a feed node 12.

The feed-in node 12 is between a cascode transistor 13, 14 and a current source 15, 16 in another

DC path between supply and reference potential connection 8, 4 formed. The current sources 15, 16 couple the symmetrical feed node 12 to the reference potential connection 4. The cascode transistors 13, 14 are connected with their emitter connections to the feed node 12 and connected to the reference potential connection 4 with their common base connection via an auxiliary voltage source 17. The cascode Transistors 13, 14 form with the transistors 20, 21, 22,

23 in a Gilbert high-frequency mixer 18, 19 each a cascode circuit. The Gilbert mixer comprises two transistor pairs 18, 19, each of which has two transistors 20, 21; 22, 23 include.

The transistors 20, 21 of the first pair of transistors 18 are connected to one another at their emitter connections and to the collector connection of the first cascode transistor 13. In analogy to this, the transistors 22, 23 of the second

Transistor pair 19 of the Gilbert mixer also connected to each other at their emitter nodes and to the collector terminal of the second cascode transistor 14. The base connections of the transistors 21, 22 are also together to form a first connection of a local oscillator input

24 connected, while the base connections of the transistors 20, 23 form the second connection of the local oscillator input 24 of the high-frequency converter.

The high-frequency signal RF, RFX, which after its amplification in the Gilbert mixer 18, 19 is combined in a multiplication with the local oscillator signal LO, LOX, is accordingly fed into the Gilbert mixer 18, 19 at the emitter connections via the cascode stage 13, 14. The output signal of this high-frequency mixture is available at the collector connections of the two transistor pairs 18, 19, which are interconnected in a cross-coupling. Specifically, the collector connections of the transistors 20, 22 are connected to one another and the collector connections of the transistors 21, 23 are connected to one another and each form a connection of the output 25 of the circuit arrangement. The output 25 is also connected via a resistor 26, 27 to the supply potential connection 8. A differential intermediate frequency or baseband signal IF, IFX can be tapped at the output 25. According to the present exemplary embodiment, this is a so-called zero IF or low TF signal. IF stands for Intermediate Frequency.

While with conventional low-noise preamplifiers in radio receivers, a voltage gain of approximately 25 dB is normally achieved, the voltage gain at the current decoupling node 9 of the present structure is only 12 dB. The amplified output voltage signal in the present circuit is not present at the preamplifier but only at the mixer output. Thus, in the circuit presented, no large RF voltage amplitude forms in the front end, which ^"could lead to undesired coupling of the high-frequency signal RF, RFX with the local oscillator signal LO, LOX. This ensures good AM suppression.

The preamplifier transistors or input transistors 1, 2 provide just enough voltage amplification that the noise characteristics of the front end are within the applicable mobile radio specification. This circuit arrangement is based on the GSM, Global mobile radio standard

System for Mobile communication based radio receivers. However, the circuit shown is also suitable for other high-frequency applications such as WLAN, wireless local area network. The present circuit arrangement is characterized in that a current interface is formed between the input transistors 1, 2, which form the low-noise preamplifier in a radio receiver, and the actual high-frequency mixer 18, 19.

Because of the high achievable with the present circuit

Isolation between high-frequency and local oscillator input signals of over 70 dB, the present circuit arrangement can be used particularly in receivers that enable a high-frequency signal to be converted directly into the baseband or into a very low intermediate frequency. An additional advantage of the present circuitry is the low demands on the compression behavior of both the down-conversion frequency mixer, and the ^'low-noise preamplifier, as for one of the mixers or de- modulator a only about 12 receives dB amplified input signal and on the other due to the reduced gain, the output of the low-noise preamplifier will later be compressed.

Of course, it is within the scope of the present principle to build the present circuit in complementary metal oxide semicon- ductor circuit technology instead of the bipolar circuit technology shown. This can be particularly advantageous if a further reduction in power consumption is to be achieved.

Furthermore, it is within the scope of the invention to design the circuit for processing so-called differential signals instead of the symmetrical embodiment shown for single-ended signals. As a result, a further reduction in the required chip area of the present circuit can be achieved in particular if a lower immunity to interference is sufficient.

In alternative embodiments, a current source can also be provided instead of the resistor 3.

Instead of the current sources 6, 7, any other, even complex, electrical loads can be provided in alternative embodiments.

In alternative embodiments, an additional amplifier stage can be connected between the low-noise preamplifiers 1, 2 and the mixers 18, 19. LIST OF REFERENCE NUMBERS

1 input transistor

2 input transistor 3 resistor

4 Reference potential connection

5 entrance

6 power source

7 power source 8 supply potential connection

9 current decoupling nodes ^" 10 ^" coupling capacity

11 coupling capacity

12 feed nodes 13 cascode transistor

14 cascode transistor

15 power source

16 power source

17 voltage source 18 transistor pair

19 transistor pair

20 transistor

21 transistor

22 transistor 23 transistor

24 local oscillator input

25 exit

26 resistance

27 IF resistance IF signal

IFX intermediate frequency signal

LO local oscillator signal

LOX local oscillator signal

RF RF signal RFX RF signal

Claims

claims 1. A circuit arrangement with frequency converter, with a first direct current path, comprising - a mixer cell (18) which is designed as a frequency converter and comprises at least two transistors (20, 21), - A cascode transistor (13) which is connected to the at least two transistors (20, 21) of the mixer cell in a cascode circuit, and - A feed node (12) designed to supply an input signal for the frequency converter in the form of a current signal and with a second DC path, comprising - A preamplifier (2, 3) for preamplifying the input signal with a current decoupling node (9) connected to the Feed node (12) is coupled in the first DC path. 2. Circuit arrangement according to claim 1, a coupling capacitance (10) is provided, which couples the current decoupling node (9) of the second direct current path with the feed-in node (12) in the first direct current path. 3. A circuit arrangement according to claim 1 or 2, so that the feed node (12) is formed in the first direct current path between the cascode transistor (13) and a current source (15) connected against a reference potential connection (4). 4. Circuit arrangement according to one of claims 1 to 3, characterized in that the preamplifier is designed as a differential amplifier, comprising two input transistors (1, 2), each with a control connection, which has a signal input (5) for supplying a differential signal (RF, RFX) form. 5. Circuit arrangement according to one of claims 1 to 4, characterized in that the mixer cell designed as a frequency converter comprises two transistor pairs (18, 19), each transistor pair (18, 19) having two transistors (20, 21; 22, 23) which are connected to each other at one connection of their controlled paths and are each coupled to an assigned cascode transistor (13, 14), the further connections of which are cross-coupled in pairs - and form the output (25) of the frequency converter and whose control connections connected in parallel form an input ( 24) to supply a local oscillator signal (LO, ^' 020. 6. Circuit arrangement according to claim 4 and 5, d a d u r c h g e k e n n z e i c h n e t that - The input transistors (1, 2) of the preamplifier and the cascode transistors (13, 14) are constructed in bipolar circuit technology and that - A coupling capacitance (10, 11) is provided, which couples one collector connection of an input transistor (1, 2) to each emitter connection of a cascode transistor (13, 14).