HK1027918A - Receiver if system with active filters - Google Patents

Description

Receiver IF system with active filter

The invention relates to an integrated intermediate frequency circuit for a radio receiver with an active band-pass filter.

In a conventional superheterodyne receiver as shown in fig. 1, a received signal is passed through a bandpass filter 10 before being amplified by an RF amplifier 12. The band pass filter 10 filters out-of-band signals that may saturate the RF amplifier 12. That is, the band pass filter 10 ensures that only those desired signal components are amplified. After signal amplification, the output signal produced by amplifier 12 is passed through a second bandpass filter 14. The band pass filter 14 filters out any out of band signals that are not completely suppressed in the band pass filter 10. In addition, the band pass filter 14 reduces noise and interference at other frequencies that may cause unwanted responses to the mixer. The output signal of the second filter 14 is received by a mixer 16. Mixer 16 converts the received frequency to an intermediate frequency suitable for further processing by a conventional receiver, such as demodulation represented by demodulator 20, by mixing the signal from local oscillator 18 with the filter output signal.

Additional filtering may sometimes be required and a double down-conversion superheterodyne receiver may be used. As shown in fig. 2, a dual down-conversion superheterodyne receiver 30 receives a signal via an antenna 31. The received signal is then filtered in an RF filter 32 and amplified in a low noise amplifier/mixer 34 and mixed with a signal from a first local oscillator 36 in the receiver described above and shown in figure 1.

The mixed signal is then filtered in a first Intermediate Frequency (IF) filter 38. In cellular wireless applications, the first IF filter is typically a crystal filter. The signal output by the first filter 38 is then amplified in an IF filter/mixer 42. The amplified signal is then mixed with the signal from the second local oscillator 44. The mixed signal is then filtered in a second filter, typically a ceramic filter, and then output to amplifier 48. In the past, radio designers have attempted to reduce the cost of the receiver by replacing the passive crystal or ceramic IF bandpass filter with an active filter circuit.

One way to remove the IF filter is to use a quadrature mixer at the first or second down conversion stage and then mix the signal down to the in-phase (I) or quadrature (Q) components of its baseband, which are then low pass filtered. This method for receiver design is known as Homodyne (Homodyne) or Zero-IF (Zero-IF). In principle, an IQ radio receiver can be constructed according to fig. 3, in which the radio signal 56 from the antenna 51 is supplied directly to two balanced, quadrature mixers 52a, 52b, in which the signals are multiplied by a sine and a cosine wave, respectively, of the carrier frequency of the signal generated by the local oscillator 53. In this way, an I-channel or in-phase signal, and a Q-channel or quadrature signal are generated. The output produced by the multiplication means comprises both the sum frequency components around 2F and the difference frequency components around zero frequency. The low pass filters 54a, 54b filter out the former and accept the latter. The zero frequency component may then be amplified to any suitable level by the low frequency amplification stages 55a, 55b rather than by the high frequency amplifiers.

In practice, in single operation, the zero-IF receiver eliminates transition Conversion (Interim Conversion) to an intermediate frequency by converting the received signal directly to baseband. The advantage of the low-pass filter is that it is easier to construct than the band-pass filters that they replace. In practice, however, the zero-IF approach suffers from various practical problems, one of which is that the balanced mixer is less than perfect compared to a perfect mathematical amplifier. The most troublesome aspect of this deficiency is that the dc offset or constant voltage produced can be many orders of magnitude higher than the expected signal. The low frequency amplifier receiving the mixer output is forced to saturate with a large dc offset long before the desired signal is sufficiently amplified.

Theoretically, another way to remove ceramic IF filters is to replace them with active band-pass filter circuits. The active filter circuit may be combined with other IF circuits such as amplifiers, mixers, Voltage Controlled Oscillators (VCOs), and detectors to form an IF system. The main difficulty with active bandpass IF circuits is that sufficient dynamic range has not previously been achieved. The dynamic range of the active filter is limited to a high signal level due to compression in the circuit and to a low signal level due to noise. Theoretical maximum dynamic range of an active filter and quality factor (Q) defined by the following equation_f) In relation to maximum dynamic rangeNot more than C_T(V_RMS)²/(2πkTφQ_f) (equation 1) wherein C_TTotal filter capacitance

V_RMSMaximum RMS input signal voltage

Kt ═ Boltzman constant x temperature

Phi is a constant determined by the circuit arrangement and layout

Q_fFilter quality factor (center frequency)/(3 dB bandwidth)

Detailed study of equation 1 will reveal that since the wireless power supply voltage is fixed, for a given operating temperature and device technology, 2 π kT and φ are constants, the total capacitance C_TDetermined by the die area available on the integrated circuit, then V_RMSIs that only Q is predetermined in consideration of these factors_fThe factor is a variable that can be adjusted to increase the dynamic range of the filter.

In a given application, the signal bandwidth is predetermined, which means that the center frequency of the filter must be reduced to reduce the quality factor Q_fThereby increasing the dynamic range of the filter. However, there is a problem that the filter at some point in the receiver stage described above will not be able to remove (reject) the picture as the intermediate frequency is reduced since it is closer to the frequency of the picture.

It is an object of the present invention to overcome the above-mentioned drawbacks by providing a radio receiver and an integrated IF circuit with an active band-pass filter, in which the quality factor Q of the filter is reduced by lowering the operating intermediate frequency_fDown to a level that is achievable. Lowering the intermediate frequency reduces the image Rejection (Rejection) provided by the first IF filter. This drawback is overcome by using an image compression mixer before the filter.

Despite the reduced quality factor Q_fCan be liftedHigh dynamic range, active filters cannot achieve the dynamic range required for high performance wireless communications. The dynamic range can be further improved by dynamically adjusting the gain of the amplifier before the active filter. That is, if the signal becomes too large, the gain decreases.

Most 2-way wireless systems require the mobile station to return the received signal strength to the base station. This is achieved by a circuit whose output voltage is proportional to the logarithm of its input voltage. This voltage is commonly referred to as the Received Signal Strength Indication (RSSI). Typically, RSSI is K log (a Vin), where K is a constant and a is the total gain before the RSSI circuit. The RSSI is unreliable because the gain is changed to accommodate the limited dynamic range of the active filter.

The present invention includes an apparatus that reconstructs the RSSI from varying gains. This has the advantage that an intermediate frequency is selected which is an even multiple of the channel bandwidth of the signal.

These and other features and advantages of the present invention will become apparent to those skilled in the art from the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a conventional superheterodyne receiver;

FIG. 2 illustrates a double downconversion superheterodyne receiver;

fig. 3 shows a schematic block diagram of a receiver using zero-IF technology;

fig. 4 shows an IF integrated circuit with an active filter according to an embodiment of the invention;

fig. 5 shows an IF integrated circuit with an active filter according to another embodiment of the invention;

fig. 6 shows an IF integrated circuit with an active filter according to another embodiment of the invention;

fig. 7 shows an IF integrated circuit with an active filter according to another embodiment of the invention; and is

FIG. 8 illustrates a dual band IF integrated circuit with an active filter in accordance with another embodiment of the present invention;

figure 4 is a schematic block diagram of a system embodying the present invention. It will be appreciated that while one of the particular applications of the invention is in a cellular mobile radiotelephone receiver, the invention may be used with any signal receiving device.

The IF integrated circuit 60 shown in fig. 4 receives an input signal from the first IF filter 38 shown in fig. 2. The input signal to the IF integrated circuit 60 is amplified in a Variable Gain Amplifier (VGA)64 and then provided to an image compression mixer 66. Alternatively, a variable gain amplifier 66 may be connected to the output of the image compression mixer shown in FIG. 5. In the present invention, the intermediate frequency is reduced to reduce the quality factor Q of the band-pass filter_fThereby increasing the dynamic range of the filter. The image signal is compressed by an image compression mixer 66 so that the combination of the first intermediate frequency filter 62 and the image compression mixer provides sufficient image Rejection (Rejection) for the wireless system. The image compression mixer 66 outputs a signal to an IF amplifier 68, and the IF amplifier 68 amplifies the signal. The variable gain amplifier is controlled by an automatic gain control loop 70 which detects the signal level at the signal input of the active filter 72 by, for example, an envelope detection or logarithmic detection circuit and adjusts the gain of the variable amplifier accordingly. Also, the variable gain amplifier 64 may be digitally controlled using a microcontroller. When the detected signal level is high, the dynamic range of the receiver can be increased by reducing the gain of the variable gain amplifier. The amplified signal is then provided to an active bandpass filter 72, which filters out unwanted components from the second IF signal.

The active filter of the IF integrated circuit may be followed by a limiting amplifier 74 and a logarithmic detector 76, which are typically found in FM radio receivers. The limiting amplifier 74 severely (Hard) limits the second IF signal before it is sent to either the demodulator detector or the discriminator, which may be integrated into the present IF system. The log detector 76 outputs a Received Signal Strength Indication (RSSI). The logarithmic detector includes an input from the AGC circuit such that the logarithm of the gain is subtracted from the RSSI. That is, RSSI (log (a) Vin) -log (a)) K log (Vin) where K is a constant. Furthermore, as shown in fig. 6, the active filter may be followed by a down conversion stage 80 so that the IF circuit can output a baseband signal in phase or ninety degrees out of phase. Alternatively, as shown in fig. 7, the active filter may be followed by an up-conversion stage 82, such that the IF circuit outputs another intermediate frequency at a frequency that is more suitable for the particular type of demodulator detector or discriminator circuit.

According to one embodiment of the invention, the second intermediate frequency is preferably placed at an even multiple (e.g., 2x, 4x …) of the channel bandwidth of the signal. For example, in a cellular radio system in the united states, the channel bandwidth is 30 kHz. The second intermediate frequency is then selected to be 4x30kHz, equal to 120 kHz. The reason for this is that the preferred embodiment of the active filter is a balanced or differential circuit which provides good rejection of even order distortion components of the signal. Thus, only the lower levels of even harmonics of adjacent channels at 30kHz and 60kHz can fall on the desired channel at 120 kHz.

Another aspect of the preferred embodiment is to place a second local oscillator at a frequency lower than the first intermediate frequency, referred to as low-side injection, which means that the second intermediate frequency will be in the high-side band and the image compression mixer 66 will reject the lower-side image. This is an important option for operating this IF system because the image compression mixer cannot provide a sufficient rejection of the image frequency by itself and some additional rejection from the first intermediate frequency crystal filter is required. By their design and construction, crystal filters have very good rejection at the low end of the pass band and less effective rejection at the high end. Thus, the combination of this crystal filter, the image compression mixer and the low second intermediate frequency requires low-side local oscillator injection to achieve sufficient image REJECTION (REJECTION).

According to another embodiment of the invention, the possibility of supporting dual bandwidth operation in a receiver incorporating the IF system of the invention is disclosed. As shown in fig. 4-8, the IF system includes an integrated Voltage Controlled Oscillator (VCO). The second intermediate frequency may be programmed if VCO78 is phase locked by a programmable synthesizer. Alternatively, as shown in fig. 8, a second active bandpass filter 84 may be added to the IF system, so that for one signal bandwidth one of the filters may be used, and for another signal bandwidth, which may be centered at another second intermediate frequency, the other filter is switched into the signal path. One application of this dual bandwidth operation is to use both the 800MHz cellular and 1900MHz pcs bands in a radio receiver, where the channel bandwidth in the cellular band is 30kHz and the channel bandwidth in the 1900MHz band may be wide to support a GSM type system operation or code division multiple access. For example, setting the intermediate frequency to the cellular band at 120kHz makes the quality factor of the active filter 4. If the radio receiver is also to support GSM with a channel bandwidth of 200kHz, the intermediate frequency may be reset to 800kHz and the figure of merit of the GSM filter is still 4.

The present invention has been described in terms of specific embodiments for ease of understanding. The above embodiments are, however, illustrative and not restrictive. It will be apparent to those of ordinary skill in the art that changes can be made to the specific embodiments described above without departing from the spirit and scope of the invention. Accordingly, the present invention is not limited to the above-described examples, but is to be defined by the scope of the following claims.

Claims (17)

1. A high dynamic range receiver IF system using a limited dynamic range IF filter, comprising:

a variable gain amplifier for amplifying the first IF signal;

an image compression mixer for compressing the image component of the amplified first IF signal to provide sufficient image compression for the wireless receiver system, said image compression mixer outputting an image compressed second intermediate frequency signal;

active band-pass filter means for filtering undesired components from the second IF signal;

limiting amplifier means for limiting the filtered signal as much as possible and outputting a limited IF signal; and

a logarithmic detector means for providing an indicator of the strength of the received signal.

2. A receiver IF system according to claim 1, wherein the variable gain amplifier is controlled by an automatic gain control loop.

3. A receiver IF system according to claim 1, wherein the variable gain amplifier is digitally controlled by a microcontroller.

4. A receiver IF system according to claim 1, wherein the logarithmic detector means includes means to eliminate the effect of gain variations such that the strength indicator of the received signal is not corrupted by the gain variations of the variable gain amplifier.

5. The receiver IF system of claim 1, further comprising:

a down conversion stage for converting the second IF signal to a baseband in-phase signal and a quadrature signal.

6. The receiver IF system of claim 1, further comprising:

an up-conversion stage for converting the second IF signal to another frequency.

7. The receiver IF system of claim 1, further comprising:

a voltage controlled oscillator for providing an adjustable frequency signal to the image compression mixer, the mixer mixing the adjustable frequency signal with the first IF signal to change the frequency of the first IF signal.

8. A receiver IF system according to claim 1, further comprising:

a second active band-pass filter means centered at a second intermediate frequency; and

switching means for selectively connecting said first and second active band-pass filter means to said amplifier and said logarithmic detector means.

9. A high dynamic range receiver IF system using a limited dynamic range IF filter, comprising:

an image compression mixer for compressing the image component of the first IF signal to provide sufficient image compression for the wireless receiver system, said image compression mixer outputting an image compressed second intermediate frequency signal;

a variable gain amplifier for amplifying said second IF signal;

first active band-pass filter means for filtering undesired components from the second IF signal;

limiting amplifier means for limiting the second IF signal as much as possible and outputting a limited IF signal; and

a logarithmic detector means for providing a received signal strength indicator from said filtered signal.

10. A receiver IF system according to claim 9, wherein the variable gain amplifier is controlled by an automatic gain control loop.

11. A receiver IF system according to claim 9, wherein the variable gain amplifier is digitally controlled by a microcontroller.

12. A receiver IF system according to claim 9, wherein the logarithmic detector means includes means to eliminate the effect of gain variations such that the strength indicator of the received signal is not corrupted by the gain variations of the variable gain amplifier.

13. A receiver IF system according to claim 9, further comprising:

a down conversion stage for converting the second IF signal to a baseband in-phase signal and a quadrature signal.

14. A receiver IF system according to claim 9, further comprising:

an up-conversion stage for converting the second IF signal to another frequency.

15. A receiver IF system according to claim 9, further comprising:

a voltage controlled oscillator for providing an adjustable frequency signal to the image compression mixer, the mixer mixing the adjustable frequency signal with the first IF signal to change the frequency of the first IF signal.

16. A receiver IF system according to claim 9, wherein said first active band-pass filter means is centered about the first intermediate frequency.

17. A receiver IF system according to claim 16, further comprising:

a second active band-pass filter means centered at a second intermediate frequency; and

switching means for selectively connecting said first and second active band-pass filter means to said variable gain amplifier and said logarithmic detector means.