Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/06—Receivers
  • H04B1/10—Means associated with receiver for limiting or suppressing noise or interference
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/06—Receivers
  • H04B1/10—Means associated with receiver for limiting or suppressing noise or interference
  • H04B1/1027—Means associated with receiver for limiting or suppressing noise or interference assessing signal quality or detecting noise/interference for the received signal
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
  • H03J7/00—Automatic frequency control; Automatic scanning over a band of frequencies
  • H03J7/02—Automatic frequency control
  • H03J7/04—Automatic frequency control where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant

Definitions

• the invention relates to a receiver as described in the preamble of claim 1.
• the invention further relates to a detector for use in such a receiver.
• the invention further relates to a method of scanning a frequency band as described in the preamble of claim 5.
• the receiver checks the (RF) signal level and the IF frequency of the received signal. On the basis hereof it is decided if a usable AM signal is received or not. If yes this signal is further processed and supplied to the loudspeaker(s) of the receiver. If not the frequency is changed with a predetermined value and the same checks are repeated for this new signal.
• Transmitter frequencies of AM stations are normally chosen in such a way that they cause each other minimal interference's. This however is only valid during day time. At night the ionisation of the ionosphere changes dramatically and temperature inversions occur, causing a significant improvement of propagation. This in turn results in strong reception of far away stations that can not be received during day time. Because these distant stations cannot be received under normal conditions, they can use the same transmitter frequency as nearby transmitters. This is no problem during day time but at night such a distant station can cause major interference's with a local station. The frequencies of these interference's are called beat frequencies. These interference's can reach a level that make the station unusable, in that case such a station should be skipped during auto search.
• a disadvantage of the known receiver is that after checking the signal level and the IF frequency and it is decided that they are usable, the signal is further processed and supplied to the loudspeaker(s). Whereas the interference's that are not detected can cause the supplied signal to be not suitable for reproducing the audio signal.
• An object of the invention is to provide a receiver, and a detector that have not the disadvantages of the prior art and further to improve the performance of the receiver.
• a first aspect of the invention provides a receiver as claimed in claim 1.
• a second aspect of the invention provides a detector as claimed in claim 4.
• a third aspect of the invention provides a method of scanning a frequency band as claimed in claim 5.
• the detector detects an interference the detector supplies a detection signal to the processor to resume scanning for a next station.
• the AM stations in the US are all privately owned, they use crystal-stabilised transmitters (compared to the state owned European stations (at the moment) who use an atomic clock for reference).
• the result of this is that the simultaneous reception of two US AM stations (with the same transmitting frequency) produces a beat frequency in a receiver of about 0 to 40 Hz with an amplitude of about double of normal modulation. It is this beat frequency that can be used advantageous to acquire the amount of multiple reception by the detector.
• this system provides a great improvement in AM automatic search during the evening hours.
• the detector can easily be added to an existing AM receiver, with minimal cost.
• a receiver for receiving an amplitude modulated signal with upper and lower sidebands.
• the receiver further comprises a quality detector to provide upper and lower quality signals representative of undesired noise in the demodulated upper and lower sideband audio signals and a selector responsive to the quality signals for processing upper and lower sideband audio signals to provide an audio output signal with reduced noise.
• An embodiment of a receiver according to the invention comprises the features of claim 2.
• a beat frequency of about 0 to 40 Hz can be detected using a low-pass filter with a 40 Hz cut-off frequency, followed by an amplifier/rectifier producing a DC voltage proportional to the amount of interference, as a detection signal.
• An embodiment of a method according to the invention comprises the features of claim 6.
• the operation of the method is further improved by measuring more than once with an predetermined interval there between.
• Figure 1 a block schematic example of a receiver according to the invention.
• FIG. 2 a block schematic example of an AM quality detector according to the invention.
• FIG. 1 shows a block schematic example of a radio receiver RR according to the invention, having an antenna input AI for receiving a RF signal.
• a mixer unit MIX for mixing the RF signal with a local oscillator signal Slo from an oscillator unit VCO, and supplying an IF signal.
• This IF signal is supplied to an AM unit for amplifying the IF signal and detecting an AM signal and supplying an (unfiltered) AM signal LF.
• the AM unit supplies a signal level VI and a frequency signal FIF to a processor unit PROC, which processor unit supplies a signal to the oscillator NCO.
• the processor unit PROC When the signal level VI and/or the IF signal is/are not within the predetermined range the processor unit PROC will supply a (new) voltage signal to the oscillator unit to amend the local oscillator signal, for example during an automatic search.
• An output of the AM unit is coupled to an audio unit AU for further processing the unfiltered AM signal LF, and supplying the processed signal to a loudspeaker LS.
• the processor unit further comprises means for automatic scanning (searching) a frequency band.
• means for automatic scanning (searching) a frequency band In this example this can be achieved by supplying each time a higher (or lower) voltage to the oscillator unit (here implemented as a so called voltage controlled oscillator).
• the radio receiver RR further comprises an AM quality detector AQD (see figure 2 for a detailed description) for detecting whether or not a usable AM signal is received. So performing an interference check.
• the KM quality detector receives the unfiltered AM signal LF. This is necessary because otherwise the interference's would have been filtered out (for example after the audio unit (AU).
• the receiver only stops searching when a usable station has been found. That is having an usable RF frequency, a good signal level and no interference's with a far away station
• the receiver (especially at night). This is achieved with the receiver according to the invention by using the AM quality detector which detects whether or not two (or more) signals from stations are received having (nearly) the same transmitter frequency.
• the AM quality detector detects an interference the AM quality detector supplies a detection signal AMql to the processor unit PROC to resume scanning for a next station.
• the AM stations in the US are all privately owned, they use crystal-stabilised transmitters (compared to the (at the moment) state owned European stations who use an atomic clock for reference).
• the result of this is that the reception of two US AM stations (with the same transmitting frequency) simultaneously produces a beat frequency in a receiver of about 0 to 40 Hz with an amplitude of about double of normal modulation. It is this beat frequency that can be used advantageous to acquire the amount of multiple reception by the detector.
• Figure 2 shows an example of an AM quality detector AQD2 in more detail.
• the AM quality detector receives at an input the unfiltered AM signal LF2, which signal is supplied to a low pass filter LPF2, for example with a cut-off frequency of 40 Hz for obtaining only the signal components (that is interference's) below 40 Hz. By filtering out signals above about 40 Hz it is prevented that the AM quality detector can react on (real) low frequency AM audio signals.
• This signal is amplified in an amplifier AMP2.
• the output of the amplifier is coupled with detecting means DET2, for example creating a DC level, and for detecting the value of the signal below 40 Hz, and supplying a detecting signal AMql2 to the processor unit(PROC, see figure 1), which processor unit will decide on the basis of the value of the detection signal AMql2 whether or not a usable AM signal is received or not.
• the cut-off frequency of about 40 Hz is chosen to prevent the AM quality detector from measuring normal audio signals.
• a four pole filter is preferred because a fast decrease is needed to prevent the AM quality detector from detecting low frequency audio signals.
• the AM quality detector sensitivity decreases if the signals of the two transmitters have a difference in signal strength.
• the AM quality detector supplies a detection signal as a consequence of the tuning step. The detection signal will be proportional to the signal level of the received signal.
• the AM quality detector would erroneous indicate that a disturbed, not usable signal, is received and that scanning for a (following) station should be resumed.
• this requires a link between the so called front-end and the so called IF/ AM detector (AM unit). This link is not desirable due to the risk of cross interference's between different elements of the receiver, resulting in whistles, instabilities or worse.
• the signal level (Nl) is checked and if the level is found to be above a predetermined value, the IF frequency (FIF) is checked. When the IF frequency is also found to be within the required IF window, the AM quality (AMql) is measured. Because the signal level of the tuned station is known at that point, it is possible to make a delay proportional to this level. Following this delay for example three measurements are performed with for example 20 ms interval. The average of this measurement should not be equal or higher than a given level. Due to this proportional delay the unwanted pulse (due to the received station tuned to) has enough time to decrease before the AM quality measurement (interference check) is performed. A short delay for small antenna signals and the delay is made longer as the antenna signal increases. The delay can be chosen using measurements of the shape of the unwanted pulse at various antenna inputs.
• the invention provides a receiver, a detector and a method using an AM quality detector to overcome the problem of stopping automatic scanning (searching) at unuseful
• the AM quality detector After deciding that the AM signal level is above a minimum level, and the IF frequency is within a predetermined frequency window, the AM quality detector detects whether interference is present. This interference results in a detection signal in the lower frequency range (for example 0 to 40 Hz) which detection signal is used by the processing unit to resume searching for a next station.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Noise Elimination (AREA)
• Circuits Of Receivers In General (AREA)
• Monitoring And Testing Of Transmission In General (AREA)

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Applications Claiming Priority (4)

Publications (1)

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• 1999
  • 1999-03-08 EP EP99905127A patent/EP0995273A2/de not_active Withdrawn
  • 1999-03-08 WO PCT/IB1999/000382 patent/WO1999049581A2/en not_active Ceased
  • 1999-03-08 CN CN99800343A patent/CN1273714A/zh active Pending
  • 1999-03-08 KR KR1019997010955A patent/KR20010012983A/ko not_active Withdrawn
  • 1999-03-08 JP JP54797799A patent/JP2002500850A/ja active Pending

Non-Patent Citations (1)

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