Oscillators – Automatic frequency stabilization using a phase or frequency... – Particular error voltage control
Reexamination Certificate
2001-07-18
2003-07-22
Pascal, Robert (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Particular error voltage control
C331S011000, C331S034000, C331S175000
Reexamination Certificate
active
06597247
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the sector of radio signal transmission and more precisely to a method and a relative circuit adapted to transfer the angle modulation of an intermediate frequency to a microwave carrier using a PLL which locked band is automatically adapted to the bandwidth of the intermediate frequency signal. The invention will be hereafter described for the use in telecommunication equipment such as for example in small capacity digital radio links according to a manufacturing sector the applicant is involved in, but it is however understood that the same invention may be advantageously applied in other electronic engineering sectors.
BACKGROUND ART
In radio signal transmitters is universally employed the heterodyne technique that foresees a first conversion at intermediate frequency IF of the baseband signal, the filtering, and a further radio frequency conversion in the specific transmission band of the preselected channel. Analogously, in reception the syntonized channel is first of all converted into intermediate frequency IF, filtered, and then converted in baseband by demodulation. The advantages of this technique are well known. The frequency conversion, both up and down, is carried out transmitting the signal to be converted to a device presenting a not linear transfer characteristic, for example a diode, and piloting conductiont of said device by means of sinusoidal pump signal generated by a local oscillator at a frequency differing from that of the center-band of the radio frequency channel for ±the value of the intermediate frequency. Practically the pump signal is generated by a local Voltage Controlled Oscillator or VCO, inserted in a Phase Locked Loop or (PLL), which is reached by a reference signal provided with great stability in frequency. The functioning of the PLL structures is well-known by those skilled in the art, but it is however opportune to underline the fact such structures comprise generally a frequency divider applied at the output of the VCO to enable the use of reference clock signals with absolutely lower frequencies compared to that of the signal generated by the VCO.
A fundamental problem arising for a company manufacturing professional transmission systems as for example those used in the stations of the sections connected by radio links, is that to achieve excellent functioning performances maintaining however the production cost low. A first difficulty in this sense is given by the necessity to be obliged to employ microwave mixers. A second limitation is given by the difficulty in realising the PLL for the piloting of the aforesaid mixers with very high frequencies, which are in small capacity digital radio links in the range of 15 GHz.
A first solution realised by the applicant to reduce the manufacturing costs at the expenses of an imperceptible reduction of the performances, consists in the use of standard glass reinforced FR4 printed circuits as microwave substrate. Such solution is disclosed in International Application no. PCT/EP98/04028 in the name of the same applicant. The object of the above-mentioned patent application is in fact a microwave VCO realised in planar technology, which layout is obtained on the same fibreglass substrate on which are realised circuits with lower frequency for the control of the VCO. Said new approach has it made possible to reduce the high production costs, thanks to the construction simplification of the circuit modules.
The applicant has recently accomplished a further step forward succeeding to eliminate the mixer used in the transmitters to convert the intermediate frequency signal to microwave. This has been possible by using the VCO, no more to generate the pumping signal, clearly useless if there is no mixer, but rather to generate directly the transmission signal. The innovation has not required outstanding circuit variations in the PLL generating the local oscillator signal, as it has been sufficient enough to replace the previous reference signal for the phase comparator by the intermediate frequency signal to be transmitted. In this way the phase modulation the intermediate frequency signal is affected by will be microwave directly transferred from the VCO of the PLL.
This second solution, that makes use of the embodiment advantages of the first solution, is described in the article entitled “FR4 PCB MODULATION TRANSFER MW VCO UP TO 16 GHZ”, authors C. Buoli, G. Mora and L. A. Cervi, published after the Proceedings of the 1997 27th European Microwave Conference. Part 2. Jerusalem, Sep. 8-12, 1997.
FIG. 1
shows a circuit scheme of a PLL
1
realising both known solutions, even if only the second will be considered. The PLL
1
is usefully employed in the low capacity digital microwave radio system transmitters.
With reference to
FIG. 1
it is possible to state that the PLL
1
comprises it comprehends a VCO
2
, essentially set up by an active element
3
electronically piloted by means of a varactor
4
. The signal coming out of the VCO
2
is sent to a microwave buffer
5
placed uphill of a directional coupler
6
distributing the input signal between two output gates. A first of these signals in output from the coupler
6
is sent to a balanced buffer
7
in such a way to obtain in output a transmission radio frequency RF with a right power level and a good degree of isolation. A second signal Si is sent dispatched, by means of a further microwave buffer
8
to a sampling phase detector
9
, or SPD which is at its turn fed by a sinusoidal signal S
2
generated at a suitable frequency UHF. Said signal is advantageously obtained by means of a VCO at low phase noise. At the output of the phase detector
9
, a signal S
3
is present, copy of the signal RF translated at intermediate frequency, that reaches a first input of a phase comparator
10
set up by a mixer, at which second input a reference signal S
4
arrives, correspondent to an intermediate frequency signal obtained by a modulated phase signal from digital data. At the output of the phase comparator
10
there is therefore an error signal S
5
for the control of the VCO
2
. This signal passes first of all a loop broad band filter
11
, useful to eliminate undesired spurious radiation produced in the PLL
1
, the latter being set up by the VCO
2
, the phase detector
9
, the phase comparator
10
, and the loop filter
11
. It is important to note the fact that the larger the lock band of PLL
1
is, the higher is its capacity to maintain the lock even if there are sudden phase variations in the modulated reference signal. The loop gain necessary for the good functioning of the PLL is guaranteed by the syntonisation capacity of the oscillator VCO
2
(about 150 MHz/VOLT) and by the sensitiveness of the phase comparator
10
(300/2&pgr; mV/rad). In order to enable the locking of PLL
1
to the reference signal during the lighting phase or after a possible out of lock condition of this one, the insertion of an automatic research device of the locking is foreseen on the way connecting the phase comparator
10
to the loop filter
11
. For simplicity reasons said known device is not shown in the figure. Regarding the phase detector
9
, one of its possible embodiments will be described in details discussing FIG.
2
.
Compared to the already known art where the PLL was used to lock at an exact reference the pump frequency of the microwave mixer, the same PLL will now be used in a different way to transfer directly to microwave the phase modulation the intermediate frequency reference carrier is affected by. Therefore the distinctive characteristic is evident between both known solutions just illustrated; it consists in the fact that while in the first case the mixer transfers (converts) in frequency a modulated signal, regenerating it to microwave through a beat, in the second case the PLL copies in the microwave carrier generated by itself the phase modulation of the reference signal, and therefore without the need to transfer the reference signal by means of a beat.
The generality
Buoli Carlo
Falasco Gianluigi
Micheli Paolo
Birch & Stewart Kolasch & Birch, LLP
Glenn Kimberly E
Pascal Robert
Siemens Mobile Communications S.p.A.
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