Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
2000-04-25
2004-05-18
Trinh, Sonny (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S327000, C455S296000, C455S326000, C333S203000, C333S116000, C333S026000
Reexamination Certificate
active
06738611
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of equipment for telecommunications where a frequency conversion of signals transmitted, or received is required, and more in particular to an image rejection sub-harmonic frequency converter realized in microstrip, particularly adapted to be used in mobile communication equipment.
BACKGROUND ART
The conversion around a desired frequency is known to be obtained through “multiplication” of a signal occupying a useful band to be converted and a sinusoidal signal of local oscillator OL at f
OL
frequency duly determined. The multiplication is made allowing the two signals to reach the ends of one or more non-linear devices, called mixers, such as for instance Schottky diodes, and drawing the desired conversion products from the same. From the analytical point of view, the non-linear conductance of the diodes can be developed in series of powers of values of the input current. Considering at the beginning said current as a superimposition of a pair of sinusoidal components of f
OL
and f
RF
frequencies, in the case of down conversion, or of f
OL
f
IF
frequencies, in the case of up conversion, it shall be obtained on the load, in addition to the input components, also new components generated by the diodes themselves at frequencies expressed by all possible combinations of two integer numbers m, n independent between them, mf
OL
±nf
RF
in the first case, or mf
OL
±nf
IF
in the second case, respectively. The extension to the cases in which the current of the diodes includes all the components belonging to the whole band of RF and IF signals is immediate.
As it can be noticed in both the cases, the converted signal includes, in addition to the desired useful signal (in general obtained through m=n=1), an infinite quantity of mf
OL
terms and of relevant conversion products of growing order that result symmetrically arranged, due to the sign ±, around the mf
OL
frequencies.
In the case of transmission it is essential to filter the conversion products, placed outside the useful band since the same represent out-of-band spurious emissions, severely prohibited by the international regulations. In microwave applications, only one of the two side bands is generally transmitted to the double purpose of saving transmission power and reduce the band occupation. The other band, called image, being symmetrical versus the frequency mf
OL
, is usually suppressed like the signal of local oscillator at the frequency mf
OL
that is suppressed for the same reasons. It is of particular importance that one part at least of said suppression occurs before the converted signal enters the power amplification stages placed downstream the conversion stage (generally capable to equitably process both the side bands), to avoid linearity losses in the operation of the mixers and distortions caused by saturation of the power terminals.
As for reception, the signal IF at intermediate frequency is generated by the superimposition of the conversion products of the useful and image band of the RF signal. In the very frequent case in which the RF signal consists of a plurality of adjacent channels transmitted to unique side band, the conversion at intermediate frequency of the image band is an undesired effect, so that it is necessary to eliminate the image band through appropriate radiofrequency band pass filtering before the conversion, So, it is attained the purpose to avoid that the signal, or the noise present in the image band, are they too converted in the intermediate frequency band, causing superimposition or easing the worsening of the receiver characteristics.
The converters mentioned above belong to a first type, called “double side band” and according to what said above they require a radiofrequency image filter whose realization is always complex, if one wants to maintain the useful signal unchanged, due to the closeness existing in all the embodiments of transceivers, between the two mf
OL
and f
RF
, frequencies, both much higher than the f
IF
.
Concerning the problem of the suppression of the components at frequency mf
OL
generated by the mixer, a first possible method is that to extend the rejection band of the image filter up to including again the relevant mf
OL
frequency components. However, should the mixer belong to a mo-demodulator of the orthogonal type (I, Q), performing the direct conversion from base band to radiofrequency, and vice versa, this approach is no more possible to be actualised since the image band does not exist and the mf
OL
component is at band centre of the RF signal.
A second known method to suppress the mf
OL
frequency components generated by the mixer, valid also for mo-demodulators, is to use two mixers, instead of one, and a balun to combine in counterphase the above mentioned components of each mixer. The main drawback of this solution is the difficult realization of a broad band operating balun, particularly at the highest frequencies; the maximum rejection that can be obtained is 30 dB approximately on 10% relative bands.
A third method, also known, to suppress the mf
OL
frequency components generated by the mixer, is to use a simple or double balanced structure. The two configurations are respectively obtained though a pair of diodes in antiparallel, or through a four-diode link enabling a better balancing. The rejection degree is as much higher as the physical characteristics of diodes result equal among them. Through an adequate driving of the diodes in these configurations, it is possible for instance to obtain the global transconductance including only the even order harmonics of the frequency f
OL
. In this case, conversion products are of the 2 mf
OL
±nf
RF
kind only, for the down converter, or 2 mf
OL
±nf
IF
for the up converter, lacking for what said above, the terms 2 mf
OL
in the converted signal. This means that, in the case of reception, applying a local oscillator signal OL at f
OL
frequency to the balanced mixer, a signal at frequency 2 mf
OL
+nf
RF
is converted at intermediate frequency f
IF
with the same conversion losses that would be obtained with use of a non-balanced mixer driven at the frequency 2 mf
OL
. Since we consider of practical interest, at least to the purposes of the invention that shall be described, only the lowest conversion order, obtained coinciding with m=n=1, we can consider the above mentioned converters as devices able to operate a conversion with local oscillator at f
OL
=½(f
RF
+f
IF
) frequency, or at halved frequency compared to that of local oscillators operating with the conventional converters and for this reason they are called also sub-harmonic converters. Their use involves different advantages, among which:
capability to operate at the highest frequencies with less expensive local oscillators, because as the frequency increases it is difficult to implement at low cost stable local oscillators capable to output the power required for the good operation of the mixers;
a high rejection degree of the residual components at
2
f
OL
frequency in the converted signal, said rejection being due only to the balancing degree of the structure, that is by the coupling degree of the physical parameters of the diodes used in the mixer, rather than on the response in frequency of external networks;
a less difficult filtering of the components at the frequency f
OL
, considering the higher distance between f
OL
and the band of the useful signal at radiofrequency.
There is a second type of frequency converters differing from the type of converters mentioned up to now, due to the fact that the converters belonging to the same do not require the image filter placed at the port of the radiofrequency signal. This result is due to the adoption of a particular circuit configuration of hybrids that enables to obtain, or to use, only one of the two side bands of the RF signal. For this reasons, the converters of the second type are called “single side band”, or image reje
Siemens Mobile Communications S.p.A
Trinh Sonny
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