Demodulator tuned by use of a test signal

Details Demodulator tuned by use of a test signal Demodulator tuned by use of a test signal

1999-03-23

2001-02-13

Grimm, Siegfried H. (Department: 2817)

Demodulators

Pulse or interrupted continuous wave demodulator

C329S319000, C375S324000, C375S340000, C375S343000, C455S214000, C455S312000

Reexamination Certificate

active

06188275

ABSTRACT:

The present invention relates to demodulator circuits for demodulating frequency modulated signals.
DESCRIPTION OF THE RELATED ART
Referring to
FIG. 1
of the accompanying drawings, a frequency modulated radio frequency (RF) signal is conventionally received by a receiver
1
from an antenna
2
, and processed by the receiver
1
to produce an FM signal at an intermediate frequency which is lower than the RF carrier frequency. The IF modulated signal is then filtered by an IF band pass filter
4
and amplitude limited to a constant amplitude by a hard limiter
5
. The constant amplitude signal is then fed to a detector
6
for demodulation by multiplying the signal by its time derivative. This operation makes the product amplitude proportional to both the signal's amplitude and angular frequency (intermediate frequency IF plus FM frequency deviation). Since the FM IF signal has a constant amplitude, due to the hard limiter
5
, the product signal has an amplitude proportional to the frequency deviation and the modulation signal can easily be recovered after a low-pass filter removes the signal components at multiples of the IF frequency.
Prior to detection, the FM signal has to be filtered by means of an IF filter such that adjacent channels and other out-of-channel interference is suppressed.
IF filters and FM-detectors in use today often employ passive resonator components that are trimmed, either at production or by exploiting signal properties. When the resonators are pre-tuned, they are tuned to the nominal IF frequency. FM detectors often exploit signal properties (e.g. zero mean of the detected signal) to automatically compensate for detuning. This detuning may be due to resonators being off their nominal value or the received IF signal being offset in frequency. The latter may be due to a combination of frequency offsets in the local and remote reference oscillators.
Adjusting the IF strip (IF filter and FM detector) to the nominal IF frequency does not guarantee a DC free baseband signal, even if the baseband signal nominally has a zero mean. Skewing the detector, of a properly tuned IF strip, to minimize its DC offset will compensate for local and remote reference frequency differences. This will improve-sensitivity somewhat but if the linear region of the detector is not very wide, signal-dependent offsets will occur and the detector may in fact be desensitized. The detector output amplitude is smallest for alternating ones and zeroes and much larger for contiguous blocks of ones or zeroes because of inter-symbol interference in digital radio receivers. When the detector Q is high, such blocks of ones or zeroes may cause the detector to compress the signal. This compression will be asymmetric if the detector resonator or threshold is detuned to compensate DC offsets in other blocks resulting in a signal dependent offset. Similarly, in analog radio receivers signal distortion will increase when the detector is used to compensate for frequency offsets.
An FM detector with a very wide linear range will inevitably have low sensitivity and result in very small and noisy output signals. Thus, the detector sensitivity will be a compromise between signal distortion and noise.
SUMMARY OF THE PRESENT INVENTION
According to the present invention, there is provided a demodulator circuit for demodulating a frequency modulated input signal, the circuit comprising:
filter means for receiving an incoming input signal and for providing a filtered output signal;
detector means for receiving the filtered output signal, and for producing a demodulated output signal therefrom;
tuning means which are operable to introduce a test signal into the demodulator circuit in the absence of an incoming input signal, and to vary the frequency response characteristics of at least one of the filter means and detector means in response to the test signal.


REFERENCES:
patent: 5524289 (1996-06-01), Koblitz et al.
patent: 2845483 (1979-05-01), None
patent: 0473373 (1992-03-01), None
patent: 0574083 (1993-12-01), None
patent: 0753938 (1997-01-01), None
patent: 2214742 (1989-09-01), None
patent: WO86/02505 (1986-04-01), None
patent: WO92/13389 (1992-08-01), None

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