A METHOD AND A DEVICE FOR RADIO RECEIVERS IF (INTERMEDIATE FREQUENCY) SIGNAL DETECTION

Abstract: A method and a device to detect an IF signal (F1) from a radio receiver. The device consists of a first mixer (M1) the first input of which is connected to the output of the last IF stage, and of a prior art phase tracking circuit (PSC) the input of which is also connected either directly or through an amplifier (OA) to the output of the last IF section, and the output of which is connected to the second input of the first mixer (M1). Then the mixed signal (F2) is produced by phase tracking the IF signal (F1) and the local oscillator signal (F3).

Description: A method and a device for radio receivers IF (intermediate frequency) signal detection. The object of the invention is a method and a device that uses this method for IF signal detection in a portable radio receiver that is specially used for tracking the location of a target object carrying a radio transmitter. In receivers using a pulsed RF (radio frequency) signal as a carrier wave, a so called BEAT oscillator and a mixer for IF detection has usually been used. This kind of a prior art method has a mixer in the receiver after the last IF stage. The IF signal e.g. 455 kHz and the signal from the BEAT oscillator, e.g. 454 or 456 kHz, are mixed in that mixer. The mixing result is a 1 kHz LF (low frequency) signal which can be heard. The method described is used e.g. in devices that are tracking the location of animals by using a portable receiver to track a transmitter which is attached to an animal. In this kind of devices the transmitter is small and cannot therefore be manufactured to be stable against ageing and temperature drifting. One of the disadvantages of such a prior art method is that it is very sensitive to the frequency changes in the trans¬ mitting or the receiving end. If the frequency provided by the last IF stage changes (whatever the reason of the change is), the frequency of the receiver's BEAT-oscillator has to be changed accordingly, so that the mixer output signal will remain at 1 kHz. E.g. changes in temperature will cause drifting in the electronic components in the receiver and in the transmitter which will have an effect on the frequency. In the prior art devices the frequency of the BEAT-oscil¬ lator is tuned manually with the receiver's tuning mecha¬ nism by listening to the tone. Manual tuning by listening to the tone is difficult and to be successful certain experience is needed from the user, who usually is not an expert in this field. The method according to the present invention improves the solution of this problem in an essential way. The method is characterized by using phase tracking to produce the mixing result for detection of the IF signal after the receiver's last IF stage. This is done by mixing the IF signal with a desired LF signal that is produced in the receiver itself. Now the manual tuning is not needed because the mixed signal is tracking automatically the changes in the IF frequency, and the LF signal which is detected from the IF signal by the mixed signal is remain- ing constant. Because the mixed signal is produced from the IF signal and the LO (local oscillator) signal with constant fre¬ quency it will change according to the changes in the IF signal. And the LF signal detected from the IF signal by the mixed signal will remain constant. The device which will embody the method of the present invention is also the object of the invention. The device is characterized by that it consists of a first mixer circuit, the first input of which is connected to the receiver's last IF stage output and a prior art phase tracking circuit, the input of which is connected directly or through an amplifier to the output of the last IF stage and the output of which is connected to the second input of the first mixer circuit. Then the mixed signal, used to detect the IF signal, is composed by phase tracking the IF signal with the LO signal. The following is an example where the invention is descri¬ bed by making references to the figures enclosed. The figures are Fig. 1 Principle block diagram of a prior art method Fig. 2 Block diagram describing the principles of the method of the object of the invention Fig. 3 Block diagram of the method of the embodiment of the invention In the diagram in Fig. 1 the IF signal Fl from the receiv¬ er's last IF stage, e.g. 455 kHz, and the BEAT oscillator signal F2, e.g. 454 kHz (lower sideband), or 456 kHz (upper sideband), are routed to the mixer Ml inputs. Then the mixer Ml output will provide a 1 kHz LF signal which can be heard. If e.g. the IF signal Fl changes 1 kHz upwards or it is 456 kHz and the BEAT oscillator frequency remains un¬ changed at 454 kHz, the mixer Ml output will provide a 2 kHz LF signal. The error in the signal is 100% from desir¬ ed 1 kHz. To compensate the situation the BEAT oscillator frequency must be tuned manually by listening to the tone. Fig. 2 describes the principle of the new method of the invention. In the new method of the invention, the mixed signal F2 is produced using the IF signal Fl and signal F3 generated by the LO. Then the mixed signal F2 is tracking the changes in the IF signal Fl directly and the LF signal F0 which is detected from the IF signal Fl by the mixed signal F2 in mixer Ml will remain in it's desired constant value. In practice the circuit in fig. 2 will not work because the mixed signal F2 consists of both sidebands. The method of the invention can be realized by using the circuit in fig.3. The principle of the phase shift circuit PSC in fig. 3 is priorly known from so called SSB ( single- -sideband suppressed carrier) technique where it is used in radio transceivers that send only one of the sidebands. Then the bandwidth required by one channel is reduced to the half . In fig. 3 the IF signal Fl is routed to the input of a first mixer Ml and directly or through an amplifier OA to the input of a first 90 deg phase shift circuit PS1. Signal F4 from the output of the first phase shift circuit PS1 is in phase with the IF signal Fl. Signal F4 is routed to the input of a second mixer M2 and the signal F5 from the first phase shift circuit PS1 which is 90 deg out of phase with the IF signal Fl are routed to the input of a third mixer M3. The LF signal F3 generated by a local oscillator LO is routed to the input of a second 90 deg phase shift circuit PS2. The signal F6 from the output of the second phase shift circuit PS2 is in phase with the L0- generated LF signal F3, and it is routed to the input of the second mixer M2. Signal F7 from the output of the second phase shift circuit PS2 is 90 deg. out of phase with the LF signal F3 and it is routed to the input of the third mixer circuit M3. Signals F8 and F9 from the outputs of the second M2 and the third M3 mixer are routed to the inputs of a summing amplifier SA, and the output of the summing amplifier SA is connected to the input of the first mixer Ml. In fig.3 there is a mathematical model which explains the functions of the circuitry by using trigonometric func¬ tions. Sin(xt) represents the IF signal Fl from the last IF stage. Sin(yt) represents the LF signal F3 which is generated by the local oscillator LO. At the mixer outputs Ml, M2, M3 there is the product of the mixer input signals and at the output of the summing amplifier SA there is the sum of its input signals. In fig. 3 circuitry the mixed signal F2=2cos(x-y)t is detected from the lower sideband of the modulation result of the LO signal F3=sin(yt) and the IF signal Fl=sin(xt) by using phase tracking. If in fig. 3 either the outputs of the 90 deg phase shift circuit PS2 in the local oscillator circuit or the outputs of the 90 deg phase shift circuit PSl are interchanged, the result will be that from the summing amplifier we will get the mixed signal F2=2sin(x+y)t which is the upper sideband. E.g. in a receiver using pulsed RF (radio frequency) as a carrier wave and where IF signal is detected by using the method of the invention it is possible to use either side¬ band. The way of presenting the invention in fig. 3 is not in¬ tended to limit the invention. It is also obviously pos¬ sible to use other prior art phase tracking applications well known by a professional of this branch to realize the idea of this invention.

Claims: Claims 1. A method for IF signal detection in a portable radio receiver, especially meant for tracking a target equipped with a radio transmitter, is c h a r a c t e r i z e d by that the mixed signal which is used to detect the IF signal from the receiver' s last IF stage is generated by mixing the IF signal with a desired LF signal generated in the receiver using a prior art phase tracking method. 2. A device to realize the claim 1 method is c h a r a c ¬ t e r i z e d by that it consists of a first mixer Ml the first input of which is connected to the receiver' s output in the last IF stage and a prior art phase tracking circuit PSC the input of which is also connected either directly or through an amplifier OA to the output of the last IF stage and the output of which is connected to the second input of the first mixer Ml, whereupon the mixed signal F2 is composed by phase tracking the IF signal Fl and the local oscillator signal F3.

Inventors: LUOMA ARI-MATTI, LUOMA, ARI-MATTI
Applicants: LUOMA ARI MATTI, LUOMA, ARI-MATTI
Application year: 1990
Main classification: H04B1
IPC codes: H03D7