Background of the Invention There are several methods for calibrating an Intermediate Frequency (IF) crystal filter of a radio receiver. The most common method is to calibrate the IF crystal filter to give a maximum Receive Signal Strength (RSS) level of a desired signal.

As a first step in the method, a wideband Radio Frequency (RF) signal is applied from a signal generator to an antenna of a radio receiver.

Next, the IF crystal filter is tuned to provide maximum RSS by changing a varactor diode's tune voltage. This changes the capacitance at the input, and the output of the IF crystal filter, and tunes the filter because the frequency response is dependant on the capacitance. The method described above provides a radio receiver with good linearity but not optimal linearity.

This invention seeks to provide a method of calibrating a radio receiver which provides to a radio receiver with better linearity.

Summary of the Invention According to a broadest aspect of the invention there is provided a method for calibrating a radio receiver. The method includes the steps of calibrating the crystal filter in order to maximise a Received Signal Strength (RSS) level and calibrating the crystal filter to minimise a third order intermodulation product received by the radio receiver.

Intermodulation products are a results of a combination of two or more signals. The third order intermodulation product is typically with the highest level of all other Intermodulation products, therefor representing the undesired signal of the radio receiver.

With the use of the present invention, the linearity of the radio receiver will improve in one exemplary embodiment the linearity improved by about 5 dB.

In the preferred embodiment of the invention, the step of calibrating of the crystal filter further includes the step of injecting two radio frequency signals into the input of the radio in order to provide a third order intermodulation product In this manner, the step of calibrating the crystal filter further includes the step of calibrating the radio receiver crystal filter to minimise the RSS level of the third order intermodulation product.

Preferably, the crystal filter is Intermediate Frequency (IF) crystal filter.

In a second aspect of the present invention a radio receiver including a crystal filter and a calibrator for calibrating a crystal filter to promote linearity of the radio receiver. The calibrator includes means to calibrate the radio receiver to maximise the RSS level of a desired signal, means to produce a third order intermodulation product in the radio receiver and means for calibrating the radio receiver to minimise a RSS level of the third order intermodulation product.

The radio receiver can automatically self calibrating its crystal filter at a predetermined interval or every time the receiver is activated.

In the preferred embodiment of the invention, the crystal filter is an IF crystal filter.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawing.

Brief Description of the Drawings FIG. 1 is a block diagram of a radio receiver according to a preferred embodiment of the invention ; FIG. 2 is a flow chart showing a method of calibrating a radio crystal IF filter according to a preferred embodiment of the invention ; and FIG. 3 is a graph showing a RSS level and third Intercept Point (IP3) level as a function of varactor tuning voltage in accordance to a preferred embodiment of the invention.

Detailed Description of the Drawings

With reference to Figure 1, a radio receiver 19 operating in accordance with the invention, comprises an antenna 1 operably coupled to an antenna switch 2, an RF filter 3 a mixer 4 and associated local oscillator 5, a crystal filter 6, a receiver back-end 7, a varactor 8, a further varactor 9, a received signal strength (RSS) decoder 10, a microprocessor controller 11, a digital to analog converter 12, and a signal source 13.

The antenna switch 2 has an output which is connected to an input of the RF filter 3. The RF filter has an output which is connected to an input of the mixer 4. The mixer 4 has an output which is connected to an input of the crystal filter 6. The crystal filter 6 has an output which is connected to an input of the receiver back-end 7. The signal source 13 has an output which is connected to the antenna switch 2. The varactors 8 and 9 are connected to the input of the crystal filter 6 and also the output of the crystal filter 6 respectively. The other end of the varactors are connected to ground. The RSS indicator 10 is a part of the receiver back-end 7. An output of the RSS indicator 10 is connected to the controller 11. The controller 11 is connected via the digital to analog converter 12 to each of the varactors 8, 9. The controller 11 is also connected by control lines to the antenna switch 2 and also to the signal source 13 and furthermore, is connected to the receiver back-end 7.

As will be readily understood by a man skilled in the art, the RF filter 3 filters received signals to reduce the amplitude of signals lying outside the desired range of operation of the radio receiver 19. The mixer 4 and local oscillator 5 in effect, convert the received radio frequency signals to intermediate radio frequency signals. These are signals of a lower frequency that can be processed by the other components of the radio receiver 19.

The crystal filter 6 is a narrow pass band filter which filters the intermediate frequency signal to reduce the amplitude of undesired signals.

The receiver back-end 7 includes a signal demodulator which demodulates the intermediate frequency signal to retrieve the information, for example speech, and an amplifier and speaker, the amplifier amplifying the demodulated signal prior to inputting it to the speaker.

The RSS indicator 10 is of conventional type and produces an output indicative of the received signal strength that is then passed to the controller 11. In a manner to be later described, the controller 11 is responsive to the output provided by the RSS indicator 10 to control the capacitance of the varactors 8, 9. This involves the use of a digital to analogue converter 12 connected to control inputs of the varactors 8, 9.

The signal source 13 comprises two signal generators 14 and 15, a switch 16 and a combiner 17. The switch 16 is operablly coupled to the controller 11 such that the controller 11 may operate the switch. An output of the signal source 13 is connected to the antenna switch 2. The antenna switch 2 is controlled by the controller 11 such that signals may be passed from the signal source 13 to the RF filter 3 and the rest of the receiver in preference to signals from the antenna 1.

The controller 11 is a microprocessor and has associated with it memory 18 in the form of solid state memory chips. The memory 18 has a number of subdivisions including random access memory and read only memory and also stores a program governing the operation of the controller.

The normal operation of the receiver will now be described in general terms, for its operation as a receiver will be readily understood by a man skilled in the art. Signals received by the antenna 1 are passed via the antenna switch 2 to the RF filter stage 3. At the RF filter stage 3, spurious signals are reduced or eliminated and the filtered RF signal is passed to the mixer 4 which converts the radio frequency signals to intermediate frequency signals which are passed to the crystal filter 6. As mentioned earlier, the crystal filter 6 filters the intermediate frequency signals and these are then passed to the receiver back-end 7 for the earlier mentioned demodulation and amplification prior to being fed to a speaker.

As will be understood by a man skilled in the art, the crystal filter 6 has a frequency response which is, in part, set by the values of the varactors 8, 9. The varactors 8, 9 have a variable capacitance which enables the controller to tune the crystal filter or to calibrate it. It is the calibration process that will now be described.

The calibration process is carried out periodically. Thus, the first step in the calibration operation is step 20 as shown in Figure 2. The controller 11 is activated in the step and the controller 11 in turn, activates the signal source 13. This step is represented by step 21. In a next step, step 22, the antenna switch 2 is activated to break the connection with the antenna 1 and to connect the signal source 13 with the RF filter 3. The controller 11 also ensures that the switch 16 is open to ensure that only signal generator 14 is connected to combiner 17. In a further step, step 23 involves the application of a wide band RF signal provided by the signal generator 14 to the RF filter 3 and then to the rest of the receiver. In the next step, step 24, the varactors 8, 9 are controlled by the controller 11 and the effect of that variation determined by the controller 11 monitoring the

RSS indication provided by the RSS indicator 10. The monitoring step is represented by step 25 in Figure 2. The controller 11 stores the RSS indication values in the memory 18 and determines at each variation whether or not the RSS is at a maximum value as represented by step 26.

If the current RSS value is not a maximum, then steps 24, 25 and 26 are repeated. If the RSS value is a maximum value then the controller 11 in step 27 activates switch 16 to connect the signal source 15 to the combiner 17.

The signal source 13 then produces a signal which is a combination of the signals provided by signal generators 14 and 15 and includes a third order intermodulation product signal. For example, the signal generator 14 generates the signal Frx + AF and the signal generator 15 generates the signal Frx + 2 * AF which Frx-is the receiver frequency, and AF-is the constant wave tone.

The combiner 17 combines the two signals and outputs to the radio receiver 19. This third order Intermodulation product signal is passed to the RF filter 3 and then to the rest of the receiver. The controller 11 now controls the varactors 8 and 9 in order to minimise the RSS. The varactors 8 and 9 are controlled in step 28 to vary the response of the crystal filter 6. The effect of this variation on the RSS is monitored as before by storing values received from the RSS indicator 10 and determine whether a current RSS value is a minimum. This is represented by step 29 and should the RSS value of the third Intermodulation product signal is not a minimum steps 28 and 29 are repeated. If a minimum value has been reached then, switch 2 is instructed by the controller 11 to switch from the signal source 13 to the antenna 1. This is represented by step 30 and the signal source is then deactivated by the controller 11 in step 31 and normal receiver operation starts in step 32.

The significance of this two-stage calibration operation will now be described with reference to Figure 3.

Figure 3 shows a graph of the variation of received signal strength against a range of varactor tune voltage applied by the controller 11 to the varactors 8, 9. Curve 40 shows the variation of the received signal strength derived from the signal provided by signal generator 14 alone. It can be seen that this wide band frequency signal produces a relatively flat or rather not sharp peaked curve. This means that for a wide range of varactor tune voltage there is only a small variation in received signal

strength. Furthermore, this variation may be to some extent"masked"by noise. Thus, a range of varactor tune voltages between the points indicated by W and X will be considered to give an acceptable crystal filter response.

However, it will be appreciated with reference to the curve 40 that this may not give an optimal linear response of the radio receiver 19 for the crystal filter 6 does have a non-linear region that extent over this range.

In the next stage, the third order intercept point or in more common words the linearity of the crystal filter 6 is considered. This is represented by the equation below showing the relationship between third order intercept point ( IP3 = POUT + IMR/2 POUT- The output power of the crystal filter in dBm.

Where the IMR is equal to IMR-Intermodulation product desired signal It can be seen with curve 41 that there is a more sharply peaked curve than the curve 40 due to the wide band frequency signal shown as curve 40. The varactor voltage will be varied between the values represented by W and X and the RSS due to the third intermodulation product will be determined. The variation made will be so as to minimise the RSS and this leads to the varactor tune voltage to be set to value Y. It will be seen that this coincides with the optimum region for a linear crystal filter response represented by labelled arrow 42. It will be seen that by use of the invention, a receiver having a more linear response is achieved than it would be the case if the crystal filter 6 was calibrated using solely the wide band frequency signal.

It can be seen from the equation above that minimised of the third intermodulation product will cause an increase in the IMR. The increase of the IMR will cause an increase of the third intercept point, thus improving the linearity of the crystal filter 6 and therefor the radio receiver 19.

In alternative embodiments of the invention, the method could be used in a factory to tune or calibrate more conventional radio receivers.

This would dispense with the need for the signal source 13 and some of the functionality provided by the controller 11. For example the radio could be test bench calibrated using an external signal source and the output of the crystal filter 6 could be displayed on a spectrum analyser. An operator could then with reference to the displayed signals calibrate the control

voltages of the varactors 8 and 9. Thus, in a visual way the region of optimum'flatness'could be selected.

In the described embodiment a receiver in accordance with the invention is provided with a signal source. This could take various forms.

For example where the receiver is a part of a transceiver it is may be possible to provide the signal source utilising stages from the transmitter section. It could also be possible to provide digitised signal sources in which the signals are held in digital form in the memory 18. These digital signals could then be used to provide signals in an appropriate form for the type of receiver involved, for example by providing a digital to analog converter.