Demodulators – Frequency modulation demodulator – Having specific distortion – noise or other interference...
Reexamination Certificate
1999-03-23
2001-07-10
Grimm, Siegfried H. (Department: 2817)
Demodulators
Frequency modulation demodulator
Having specific distortion, noise or other interference...
C329S325000, C455S214000
Reexamination Certificate
active
06259315
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 the signal's amplitude and angular frequency (intermediate frequency IF plus FM frequency deviation). Since the IF FM 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.
Integration of FM detectors into semiconductor devices requires the use of accurate delay elements or filters with well controlled phase characteristics to generate the time-derivative approximation or else excessive DC offsets will occur.
FM demodulators in use today often employ passive resonator components that are trimmed, either at production or by exploiting signal properties. In analogue voice systems the FM-signal is DC free and the integrated output of the FM-detector should equal zero. Any DC value corresponds to a detuning and by proper feedback, the FM detector can be adjusted to minimize the DC component. For TDMA-based digital systems the signal does not have a zero mean but a DC-free preamble may be added to the signal packet and the detector adjusted on this preamble.
Another approach, when the FM-signal frequency (i.e. the last IF) is high compared to the base-band signal, is use of a detector based on a digital delay line as differentiator. This delay line may be clocked by an accurate clock and hence result in a detector with an inherently low DC offset.
A third approach is to use analogue to digital (A/D) conversion of the signal and to perform the FM detection in a digital signal processor (DSP) or other digital circuit.
A digital radio based on a TDMA scheme may use data packets that are not guaranteed to have a zero mean. Also, if the packet preamble is not DC free it will be difficult to use the signal for tuning the FM detector or adjusting its DC offset.
When the radio uses a low IF compared to the symbol rate (for example 3 MHz and 1 Msym/s) or when the modulation index is small the use of a digital delay line may be impractical.
Offsets in the detector circuit elements, especially in multipliers when signal levels are small, may also contribute to the over-all DC offset. Hence, even if the phase characteristics of each building block is properly adjusted the detector may exhibit a non-zero DC offset.
Furthermore, if all DC offset compensation is done by detuning the resonator, the operating point may no longer be symmetrically located on the detector's (S shaped) characteristic and significant signal-pattern dependent offset may appear. That is, because of inter-symbol interference, the detector output amplitude is smallest for alternating ones and zeroes and much larger for contiguous blocks of ones or zeroes. 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 is detuned to compensate DC offsets in other detector blocks resulting in a signal dependent offset.
SUMMARY OF THE PRESENT INVENTION
According to one aspect of the present invention, there is provided a demodulator circuit for demodulating a frequency modulated input signal, the circuit comprising:
detector means which are operable to produce a demodulated signal from an incoming frequency modulated signal;
tuning means connected to the detector means and operable to vary the frequency response characteristics of the detector means; and
auxiliary detector means connected to receive a reference frequency signal and to provide an auxiliary tuning signal to said detector means, on the basis of detection of said reference frequency signal.
REFERENCES:
patent: 4356567 (1982-10-01), Eguchi et al.
patent: 4426727 (1984-01-01), Hamada
patent: 4458207 (1984-07-01), Favreau et al.
patent: 5524289 (1996-06-01), Koblitz et al.
patent: 2 214 742 (1989-09-01), None
patent: 2 283 871 (1995-05-01), None
patent: WO 95/09480 (1995-04-01), None
Haartsen Jacobus
Mattisson Sven
Burns Doane Swecker & Mathis L.L.P.
Grimm Siegfried H.
Telefonaktiebolaget LM Ericsson (publ)
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