Abstract this article proposed one kind of FM receiver broadcasting station searches the improvement program, has overcome the existing technology about the FM receiver search broadcasting station when easy to have the miscarriage of justice phenomenon insufficiency. This design simultaneously instructs and the frequency offset information using the input signal intensity judges the broadcasting station. Demonstrated in the actual test that this plan can enhance the FM receiver search broadcasting station’s accuracy effectively.
Key word FM receiver; Broadcasting station search; Frequency offset; Input signal intensity instruction
1 introduction
The frequency modulation system by characteristics and so on it strong antijamming ability in the audio frequency broadcast, the television, VHS HiFi, Laser Disc as well as the wireless communication domain obtained the widespread application. The traditional superhet frequency-modulated signal receiver, the signal demodulation needs to use Gao QZHI the intermediate frequency sound surface wave filter or the crystal filter realizes. Along with the digital signal processing technology as well as the ultra large scale integrated circuit technology’s swift development, the traditional simulation equipment is replaced gradually by the digital simulation mixing device. Literature [1] proposed the use adjustable electric capacity and the linear filter’s digital-analog mixing circuit substituted the precise exterior component which and the adjustment method in the FM demodulation used, the literature [2] proposed in one kind of use signal belt had the ideal straight characteristic FIR filter to carry on the frequency-modulated signal frequency discrimination the method, but needed the FIR filter exponent number to achieve several thousand steps, was very high to the system work clock as well as the system storage capacity’s request. The literature [3] proposed uses the sinc3 extraction filter and three point approximation differentiator uses in the FM demodulation, reduced digital IC to realize the high performance FM frequency modulated receiver’s order of complexity greatly.
In uses in the FM frequency modulated receiver which the digit realizes, how searches the channel which the broadcasting station is at is the difficult problem which accurately fast a commercial digital FM frequency modulated receiver must solve.
Nowadays the FM receiver choice broadcasting station often uses the method is: Before the frequency-modulated signal demodulation, has the limiter circuit, this limiter circuit’s electric current and the input signal intensity’s logarithm is proportional in the quite great scope. Using the received signal intensity instructed that (RSSI) the electric circuit (this electric circuit examination limiter circuit’s electric current) may obtain the input signal the intensity, thus choice broadcasting station. This method’s application for example, uses the received signal intensity instruction expression received signal in mobile communication’s hand-hold equipment the size.
In the literature [3] mentioned in the demodulation plan, after the received signal intensity may use under the digit the frequency conversion, the signal energy (I2 Q2) attributes.
Although the above method has realizes the electric circuit simple merit, but actually has the flaw which easy to misinterpret. The FM receiver signal band width is about most greatly 180KHz, but searches for the Taiwan frequency interval to divide into 50KHz, 100KHz and the 200KHz three kinds. When meets a strong broadcasting station, and its signal band width is big, with the frequency interval is when 50KHz and 100KHz search for Taiwan, the above method the signal strength which instructed in 180KHz is bigger than the hypothesis threshold (this threshold generally for most small power radio station signal strength), will misinterpret will be 2-3 broadcasting stations, the serious will influence search for Taiwan’s accuracy. Meanwhile discovered in the actual test that as a result of signal influences and so on band width’s instantaneous change and channel disturbance, the signal strength which this kind of wide frequency-modulated signal band width brings instructed all not necessarily conforms to the axial symmetry principle in the corresponding band width, is unable to use the symmetrical principle gain accurate broadcasting station frequency.
Therefore this article based on this proposed one kind of new design proposal, improved the FM receiver to search for Taiwan’s accuracy large scale, meanwhile maintained electric circuit’s succinct.
2 frequency-modulated signal orthogonal demodulation principle
Supposes the frequency-modulated signal expression is:
(1) And, a(t) indicated after receiving the channel noise and other disturbing effect, along with time variation’s frequency-modulated signal scope, m(t) is the modulation signal, ωc for signal carrier.
-like (1) will show the frequency-modulated signal and local produces the orthogonal carrier multiplication may result in:
Carry on FIR separately after sI(t) and sQ(t) low-pass filtering [4],[3] may result in:
(2)
(3) Derive separately to I(t) and Q(t) count [1] to be possible to result in:
(4)
(5) -like (2) with type (5), type (3) will multiple separately with type (4) divides (2), (3) sum of squares to be possible again to result in m(t):
(6)3 improvement programs
After Figure 1 is the improvement FM receiver work diagram, during consideration received signal intensity instruction, but also simultaneously refers to the received signal the frequency offset to decide whether is the accurate broadcasting station.
This time if receives the frequency-modulated signal carrier and local establishes the demodulation orthogonal wave has a frequency difference △w, namely is not the accurate broadcasting station, then has:
(7) (7) substitutes fore-mentioned (2) ~ (6) decode procedure, may result in:
(8) Considered tonic train signaling m(t) exchanges the signal, then the decoding output’s direct component is.
Thus, defines two thresholds to weigh searches for the Taiwan result. Threshold 1: Defines an appropriate threshold is Ka (basis input signal intensity, corresponds RSSI the examination value).
Threshold 2: Defines an appropriate threshold is Kb (basis frequency offset size, corresponding frequency offset examination value).
Defines two steps to examine searches for Taiwan:
Step 1: Examination chart 1 received signal intensity instruction examination module output (RSSI) >Ka;
Step 2: Examination frequency offset amplitude detection module output (frequency offset amplitude detection value) <Kb;
When the step 1, 2 simultaneously satisfy, this signal is normal Taiwan.
4 test results
Because the tonic train signaling frequency range was generally the 300Hz~15KHz, FM biggest frequency offset (has adjusted carrier frequency signal greatest instantaneous frequency and the carrier frequency difference of) is ±75KHz, the standard tone moudulation percentage (moudulation percentage referred to frequency-modulated signal peak value frequency offset and system biggest frequency offset percentage) is 30% and 100%, corresponding frequency offset for ±22.5KHz and ±75KHz. The FM biggest modulation frequency offset has 22.5KHz and 75KHz generally.
Considered that the crystal oscillator the error is smaller than 100PPM creates the local oscillation error frequency offset is 10KHz most greatly, then the Kb reference supposes is 10KHz. If continues by the 50KHz frequency interval to search for Taiwan, namely the local oscillation step-by-steps by 50KHz chooses the broadcasting station, nearby two searches the local oscillation frequency offset in normal Taiwan most to be few is 40KHz, its examination value is bigger than threshold Kb, will thus not choose this frequency will be normal Taiwan. Because the local oscillation frequency offset is bigger, the direct-current examination value is bigger, therefore not a normal Taiwan miscarriage of justice for many Taiwan.
In Figure 2 has unfolded the part test data. And the Ka hypothesis is the 21db, Kb reference supposes is 10KHz.
(a) 101.5~102.2Mhz RSSI and frequency offset relational graph
(b) 103.5~104.2Mhz RSSI and frequency offset relational graph
(c) 105.5~106.2Mhz RSSI and frequency offset relational graph
Figure 2 FPGA test result
First may see the RSSI instruction the actual value in has the Taiwan place relative chirp bottom to have is bigger than 6db the signal enhancement, therefore may the smooth preliminary localization broadcasting station be at frequency. But simultaneously may see in Figure 2 in the signal energy is quite strong take the accurate frequency as in the central 100Khz scope, moreover not necessarily follows symmetrical principle like a, b. This is mainly originates from the survey instantaneous disturbance and the modulation signal size. Therefore the simple seeking center of symmetry’s plan is inappropriate.
Next, Figure 2 frequency offset’s measurement result showed that in accurate broadcasting station frequency for in central 100KHz, if signal energy enough, surpasses the hypothesis threshold 21db, namely the frequency-modulated signal submergence in chirp the bottom (in Figure 2 (a),(c)), the frequency offset examination has not been able the accurate discovery 100KHz frequency offset, thus examines the most accurate broadcasting station frequency spot. Otherwise, if signal energy energy insufficient, has not surpassed the hypothesis threshold 21db (to see Figure 2(b)), the frequency-modulated signal submergence in chirp the bottom, by the step 1 may guarantee that cannot misinterpret not the correct frequency judgment will be a broadcasting station.
In the synthesis in FPGA realizes with in the ASIC chip test, confirmed this improvement program to be able the very accurate survey 100KHz frequency offset, thus has avoided when by 50Khz, the 100Khz length of stride searched for Taiwan a signal strong broadcasting station miscarriage of justice for many phenomena, achieved enhanced the chip to search for the Taiwan accurate effect.
5 conclusions
Easy to have the miscarriage of justice phenomenon insufficiency in view of the existing technology about the FM receiver search broadcasting station, this article proposed one kind simultaneously using the input signal intensity instructed (RSSI) and the frequency offset information’s FM receiver broadcasting station searched the improvement program, has overcome when by 50Khz, the 100Khz length of stride searched for Taiwan a signal strong broadcasting station miscarriage of justice for many phenomena, enhanced the chip to search for the Taiwan performance. The actual test proof, this plan can the effective improvement FM receiver search broadcasting station’s accuracy, enhance the frequency-modulation reception chip to search for the Taiwan performance.
This article mentions design proposal for tripod with two handles core communication (Shanghai) Limited company FM in radio project a part, has also contained the support which the king dangerous mountain peak, the yellow cry, Zhu Lizhen works, expresses thanks specially.
Reference
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[2] opens Ze, Wu Siliang. “the row controls the system multi-sound frequency-modulated signal the total explanation of characters to adjust the method [J].”Military and civilian dual purpose technology and product, 2004,1
[3] Bang-Sup Song and In Seop Lee. “A Digital FM Demodulator for FM, TV, and Wireless” IEEE Transactions On Circuits and Systems-II:ANALOG AND DIGITAL SIGNAL PROCESSING, VOL.42, NO.12, DECEMBER 1995
[4] R.E.Crochiere and L.R.Rabiner. “Interpolation and decimation of digital signals-A tutorial review.”Proceedings of the IEEE, March,1981,69(3):300~331
