FR2494929A1 - Voltage controlled UHF transistor oscillator - has octave range output using varicap diode and resonant line in base circuit with impedance shunt across diode removing parasitic resonance - Google Patents

Abstract

The oscillator consists of a transistor (2) whose transition frequency is greater than twice the maximum oscillator frequency. The transistor is connected in common collector mode with a resonant circuit coupled to its base by a capacitor (19). The resonant circuit is a section of line (1) of predetermined characteristic impedance connected at one end to the transistor and at the other to a variable reactance formed by one or more varicap diode which is connected at its other end to ground. The diode is shunted by a second section of line (24) of significantly higher impedance whose length is chosen inversely with the maximum operating frequency. The diode serves to shift the effect of the parasitic inductance of the diode out of the operation band. The varicap diode may be two diodes connected back to back so that they can be reverse biased despite the shunt. The resonant lines and chokes of the circuit may be striplines formed by printed circuit techniques on a dielectric base, the other components being soldered in place. The circuit uses low cost commercial components, and may be controlled to oscillate over the range 1.2 GHz to 2.3 GHz and may be used for down conversion of satellite broadcast TV without switched reactances.

Description

TRANSISTOR MICROWAVE OSCILLATOR, TUNABLE BY TENSI - #
The present invention relates to a microwave oscillator
transistor (oscillating in L-band and part of S-band)
whose frequency is continuously variable depending on a
DC voltage reverse biasing at least one capacity turkey
variable.

By the expression "microwave" we must understand here,
in particular, the upper part (from 1 to 3 GHz) of the band of
ultra-high frequencies (UHF - from 300MHz to 3 GHz or
3000 MHz), that is to say the decimated wave band (whose wavelength is, for example, between 10 and 30 centimeters), called respectively S and L bands, which allows the use , without excessive bulk, of quarter-wave lines (h / 4) as resonant tuning elements.

UHF oscillators with transistors and quarter-wave lines have been described in the collection of application notes entitled "RF POWER TRANSISTOR APPLICATION NOTES" from the American company RCA, Solid State Division, in particular application note n0 AN- 6084 by HODOWANEC on pages 451-457, entitled "HIGH-POWER TRANSISTOR MJCROWAVE OSCILLATORS", dating from May 1973, or on pages 409 to 412 of the French work of
CARRASCO and LAURET entitled "FUNDAMENTAL COURSE OF
TELEVISION ", published in" EDITIONS RADIO "in 1976.

A microwave oscillator of this type can be used, for example, as a continuously tunable local oscillator for a single mixer intended to transpose a high-frequency signal which it receives, either downwards, i.e. in a band centered around an intermediate frequency of a few tens or hundreds of megahertz, in the manner of a primary or secondary radar signal receiver or as a second local oscillator for a second mixer used in a receiver intended to directly receive broadcast television signals by a stationary satellite, in a band going from 11.7 to 12.3 GHz (as described in numerous articles, such as those of FREEMAN on pages 234 to 23b of the British review "THE RADIO AND
ELECTRONIC ENGINEER ", Vol. 47, no. 5 of May 1977;
DOUVILLE in the American magazine "IEEE TRANSACTIONS ON
MICROWAVE THEORY AND TECHNIQ # JES ", Vol. MTT-25, n. 12, of December 1977; from KONISHI on pages 720 to 725 of Vol. # MTT-26, n. 10 of the previous review of month October 1978 by HAWKER on pages 27 to 35 of the British journal "IBA
TECHNICAL REVIEW "n. 11, from July 1978; and from
HALAYKO and HUCK on pages 112 to 119 of the American review "IEEE TRANSACTIONS ON CABALE TELEVISION", Vol.CATV-3, n. 3 of July 1978, for example), where a first down transposing mixer powered by a first local oscillator at fixed frequency, is used to transpose the frequency modulated carriers, from the 12 GHz band to a first intermediate frequency band located substantially in the L band (from 0.9 to 1.4 GHz approximately), for example (see also French patent application nn EN 80 09387 filed on April 25, 1980 by the company "THOMSON-BRANDT").

Another advantageous application of the microwave oscillator according to the invention is its use in a high frequency head ("tuner" in English) universal for television receiver making it possible to receive signals in all the bands
VHF and UHF allocated to the broadcasting of television signals (which range from approximately 41 to 940 MHz), without switching of reactances at the level of the local oscillator. This has been described in a French patent application no. EN 80 24970 filed on November 25, 1980 by the same applicant. This HF head has a double frequency change, the first of which allows all the VHF and UHF bands (and even those of cable television) upwards, on a first intermediate frequency located in the L band between the maximum frequency of the UHF (V) bands and the minimum frequency of the oscillator and the second of which realizes a transposition downwards, in the normalized F1 band of conventional television receivers.

 This means that the local oscillator must allow a continuous variation of its frequency over an octave, that is to say approximately from 1.2 to 2.2 GHz without any hole or notable weakening of the oscillations in this range and that by means of a DC voltage, called a chord, to constitute a microwave oscillator controllable by a voltage so as to be able to control its chord using a voltage or frequency synthesizer and / or a loop automatic frequency control (called "automatic frequency control" or "automatic fine tuning" in English).

In the aforementioned RCA Application Note, a transistor microwave oscillator of the so-called "Colpitts" type has been shown, the tuning range of which extends over 300 MHz, from about 1.1 to 1.4 GHz. It is also known to vary the tuning of a quarter-wave line (; / 4) by the variation of an adjustment capacity joining one of the ends of this line to ground. This adjustment capacity can obviously be replaced by a variable capacity diode polarized upside down.
LC / C of the variation 6 C of the capacity between its maximum and minimum values to its average value C, where, C = C max -
C min and C = (C max + C min # 12, is insufficient for the ratio
~ F / F can be equal to about 0.6 (where 1 ~ F = F - F
max min = 1 0Hz and F = max + Fmin) / 2 = 1.7 GHz approximately). Consequently, to obtain a voltage-controlled and large-slope microwave oscillator (F /. V of 80 MHz / V approximately), two diodes with variable capacitance must be connected in series with the quarter-wave line. so as to divide by two the variation C and pali four the mean value C (that is to say to multiply the ratio XC / 1: by two).

 In any case, the use of variable capacitance diodes in series with a quarter-wave line in this lower part of 1 to 3 GHz of the microwave band involves the formation of series resonant circuits which then constitute hatches or stop band filters (so-called "band-stop filters1" or parasitic rejection for # certain frequencies in this band, causing the oscillation to stop on one or more parts of the desired spectrum.

 The present invention also makes it possible to overcome this drawback and to realize a microwave oscillation on a variable frequency continuously using a tuning voltage, while using components of the "general public" type at the lowest price, without precision wound elements and without switching elements.

 According to # the invention, a microwave oscillator comprising a microwave transistor whose transition frequency is greater than and, preferably, at least equal to twice the maximum desired oscillation frequency and one of the electrodes of which is coupled to the one of the ends of a first line section of conductive material having a series inductance and a parallel capacitance per unit of length which determine its characteristic impedance, the other end of the first line section being coupled, by at least one diode to variable capacity as a function of its reverse bias voltage between maximum and minimum values, to ground, the length of this first section of line being calculated so as to form with the minimum capacity of the diode a circuit resonant at a frequency at least equal to the desired maximum oscillation frequency, characterized in that the other end of the first line section is furthermore connected to ground at by means of a sec # ond section of characteristic impedance line notably greater than that of the first, -connected in parallel with the diode (s) with variable capacitance, in order to transfer the parasitic series resonance frequencies forming a trap, outside the range of desired oscillation frequencies of the oscillator.

The invention will be better understood and other of its characteristics and advantages will appear from the following description of the accompanying drawings relating thereto, given by way of nonlimiting example, in which
- Figures la and 1-b show very schematically two classic embodiments of a microwave oscillator, in particular according to the RCA Application Note mentioned above, the second of which is at frequency controllable by voltage, due to the replacement of the capacitor tuning by a variable capacity diode
- figure 2 is a diagram illustrating a modification compared to figure 1-b, which makes it possible to increase the slope F / V of the microwave oscillator to extend its range of variation of frequency F by increasing the ratio C / C by connecting two variable capacity diodes in series
- Figure 3 shows the equivalent diagram of a variable capacity diode to illustrate the fault of the circuits of Figures 1-b and 2; and
- Figure 4 is the block diagram of an embodiment of the microwave oscillator whose frequency is controllable by a voltage continuously over about an octave (from 1.2 to 2.3 GHz), according to invention.

FIG. 1a shows a microwave oscillator with a resonant circuit in the form of a transmission line 1, one end of which is connected to the base of a bipolar microwave transistor 2, of NPN type, and the other end of which is connected to one of the armatures of a variable capacitor 3, of the type known in English as "trimmer capacitor", the other armature of which is connected to ground 4 and the capacity C3 of which can be adjusted between a few tenths and a few picofarads Line 1 is produced using an elongated conductive layer of predetermined width W and of length L less than a quarter of the wavelength (^ 9/4) of the maximum desired oscillation frequency, covering one of the haulms of a dielectric (insulating) plate, the other side of which constitutes the ground plane, is entirely covered with a conductive layer to together form an asymmetric transmission line (called "strip-line" or "microstrip" in the literature an glo
American, as, for example, in the British book by
HARVEY entitled "MICROWAVE ENGINEERING" published by
"ACADEMlC PRESS" in 1963, in particular on pages 4U7-408 and 412
415). The conductive layers are generally made in
copper e fixed on the dialectric with low losses, such as fibers
glasses impregnated with silicone or polytetrafluoroethylene or
PTFE (marketed under the registered trademark "TEFLON") whose
dielectric constants are 4.2 and 2.7 respectively
ron, by the conventional printed circuit technology described, by
example, on pages 428 to 430 of the aforementioned work of HARVEY.

The wavelength on line # g and its charac impedance
g
ZO teristics are, therefore, functions of the width W of the
line, the thickness T of the dielectric and the constant di
electric (for exact calculations see, for example, the work of
HOWE entitled "STRIPLINE CIRCUIT DESIGN" published by ARTECH
HOUSE, INC., In 1974 and many articles cited in the
bibliography of this work and that of HARVEY cited above).

The length L of the elongated layer 1 is therefore chosen
less than a quarter of the minimum value of the wavelength on
the line g min corresponding to the highest frequency of
the oscillator, to take into account the minimum capacity C3 min of the
capacitor 3 which has the effect of increasing the apparent length
of the line (see page 405 of the CARRASCO book and ai
above), so that the circuit behaves as a circuit
resonating parallel. The junction of the base of transistor 2 with the
line 1 is joined, via a first shock coil
5 whose impedance is very high at the oscillation frequency
minimum (corresponding to the maximum capacity C3 max of the conden
sector 3), at common point 6 of two resistors 7 and 8 connected
in series between step 9 of a continuous power source
(Vcc) and its negative pole connected to ground 4, which form a divider:
resistive voltage.

The emitter of transistor 2 is joined, via a second shock coil 10 and a first resistance Il (of a few or a few tens of ohms), to ground 4 and its collector is joined, d firstly via a first decoupling capacitor 12 (of a few nanofarads), to ground 4 and secondly through a second resistor 13, to the positive pole 9 (+
This second resistor 13 makes it possible to reduce the collector voltage of transistor 2 by the voltage drop caused at its terminals by the average collector current.

 It can easily be seen that the mounting of the transistor 2 constituting the active element of the oscillator is of the common collector type, which has the advantage of a high input impedance and a moderate output impedance. As a result, the adaptation requirements in the reaction loop are much less restrictive than for the common base assembly which, moreover, constitutes the most efficient assembly for producing a microwave oscillator (unlike the common transmitter assembly which is limited to frequencies significantly lower than the transition frequency fT of the UHF transistor). In the circuit of the figure 1-a, the loop of - reaction of the oscillator of the type of "Colpitts" is formed by the capacities collector-base 14 and baseemitter 15 (parasites) of the transistor 2 in its housing.

On the figure 1-b, the adjustable capacitor - 3 (of the figure 1-a) has been replaced by a variable capacity diode 30 whose anode is connected to the ground 4 and whose cathode is connected, of a firstly, to line 1 and secondly, through a resistor 16 (of several kiloshms) to a terminal 17 to which the tuning voltage is applied
VA which polarizes it backwards. This terminal 17 is connected, via a second decoupling capacitor 18, to ground 4. The variable capacitance diode 30 is chosen so as to allow a variation of the frequency such that the oscillator can supply a wave whose frequency varies continuously from 1.2 to 2.3 GHz for a variation of the tuning voltage from 2 to 28 volts, for example. This is relatively difficult to obtain by means
a single variable-capacity diode 30, available in the
trade: "general public" quality).

Due to the variable positive polarization of the cathode of the
diode 30 which is directly connected to one end of the line
1, the other end of it must be galvanically coupled
only isolated at the base of transistor 2. This coupling is carried out
here by means of a coupler capacitor # e '19 of a few tens
of picofarads. If the emitter resistance 11 exceeds a few
hundreds of ohms, it is possible to connect a parallel
third decoupling capacitor 20 (from several tens to
a few hundred or thousands of picofarads).

FIG. 2 illustrates an advantageous modification of the circuit of
the figure 1-b.

To obtain a C / C ratio, i.e. a variation of
frequency F, sufficient, two diodes have been used in FIG. 2
variable capacity 31 and 32 connected together by their cathodes, of which
the respective anodes are connected to earth 4 and to one of the
ends of line 1. The cathodes of diodes 31, 32 being
joined to terminal 17 by the bias resistor 16, the anode of
the second 32 must be galvanically connected to ground 4 to
that it can also be biased backwards by the same 'tuning voltage VA as the first 31. To this end, the other
end of line 1, at its junction with the capacitor
coupling 19 for example is brought together through a
third shock coil 21 with high reactance and resistance
zero, to ground 4. It should be noted here that double diodes with
variable capacity and common cathode are currently available
commercially available, for example type BB 204 of the Division
Semiconductors "SESCOSEM" from the French company "THOIvISON-
CSF ".

The transistor 22 which is represented in FIG. 2 is of the type
PNP, the emitter of which is in positive pole 23 from another source
direct voltage (VEE) and to which the collector is directly connected
to mass 4 (which constitutes the negative pulse of this source).

 In Figure 3, there is shown the circuit equivalent to the high frequencies of the L and S bands of a variable capacitance diode. The equivalent circuit comprises between its terminals 40 and 41 a fixed capacitor 42 which is mainly that of the box and of the conductors and which is connected in parallel with a circuit composed of a parasitic inductance 43, of a capacitor 44 which varies according to the voltage between terminals 40 and 41 and of a resistor 45 which is of low value and varies slightly with the reverse bias voltage and which results from the reverse leakage current 1R (of a few tens or hundreds of nanoamps).

 The fixed capacity 42 which can be considered to include the minimum value of the capacity of the variable capacity diode (s) (30 or 31 and 32), corresponding to their maximum reverse bias voltage, has already been taken into account in the calculation the length L of line 1 to resonate at the desired maximum frequency of the oscillator. The non-variable (fixed) term of this capacitor 42 then forms with the inductive reactance of line 1 and the capacitors 14 and 15 of transistor 2, a "Colpitts" circuit modified by "Clapp" which has a first series resonance frequency for which the oscillator stops. In addition, the equivalent inductive reactance of line 1 is added in series with the parasitic inductance 43 and the variable capacitance 44 between the base of transistor 2 (or 22) and ground 4 , to form another hatch (rejection filter), this one at a second resonant frequency variable as a function of the tuning voltage (VA). Either or both of these two resonant frequencies may, when the desired range of frequency variation or '- F / F ratio is large, fall within this F range of such so that the oscillator can only operate in several distinct portions thereof, separated from each other by stop bands or "holes" which is the opposite of what we aimed to obtain, i.e. the continuous variation of the oscillation frequency of the oscillator in a range of one octave or more in the L and S bands.

 According to the invention, a second section of line is connected in parallel with the variable capacity diode or diodes, which represents an inductive reactance making it possible to transfer the resonance frequencies of the aforementioned hatches outside the desired variation range , preferably, beyond its maximum frequency.

 Figure 4 illustrates schérn ## ically an embodiment of a microwave oscillator corllprenant this second section of line, according to the invention.

In FIG. 4, the oscillator comprises a bipolar transistor 2 of NPN type, mounted as a common collector, the transition frequency fT of which is to say the frequency for which its gain () or its direct transfer ratio of current in common emitter (hFE) becomes equal to one (O d3), is chosen to be greater # than the maximum oscillation frequency desired and, preferably, at least equal to twice this latter. Thus, to equip a microwave oscillator (UHF) having to oscillate up to 2.3 GHz, a transistor is chosen whose transition frequency fT is approximately 4,500 MHz, such as those of the BFQ 22, BFQ 63, BFR 90, bFR 91 type,
BFR 93 or BFR 96 from the SESCOSEM Semiconductor Division of THOMSON-CSF, whose typical fT is specified at 5 GHz.

The base of transistor 2 is coupled here by capacitor 19 to one of the ends of a first section of line 1 whose width
W helps to determine the characteristic impedance Z whose length L is decisive for the maximum resonant frequency (see Figure 1). The other end of this first section 1 is connected to one end of a second section of line 24 whose width B is significantly less than that W of the first section 1, so that it has a characteristic impedance Z much greater than that Za n of the latter. The other end of the second section 24 is connected to earth 4 and its electrical length is preferably chosen as a function of the desired F / F ratio so as to met. rye a low inductance in parallel with the parasitic inductance of the diodes 31, 32, its physical length being therefore less than that
L of the first section 1 and, for example, close to that of the single housing containing the two variable capacity diodes 31 and 32 with a common cathode, the anodes of which are respectively connected to the two ends of this section 24 with the shortest possible conductors.

 As there is an abrupt discontinuity of impedances between the first 1 and the second section of line 24, this constitutes the equivalent of a shock coil in series with the first section 1 which has little or no influence on the tuning thereof in the frequency range from 1.1 to at least 2.3 GHz, which is effected by means of diodes 31, 32 in series. Due to the presence of the second galvanic conductor section 23 between the anode of the second diode 32 and the ground 4, the third shock coil 21 of FIG. 2 intended to make its reverse polarization possible, becomes unnecessary here.

 It should be recalled here that the connections to ground 4 are preferably grouped at a point because a length of conductor even small represents inductive and capacitive reactances which cannot be neglected at these high frequencies.

 Thanks to the use of the two common cathode dipdes 31, 32 in series, made possible by the second section 24 which makes it possible to move the respective frequencies of the two aforementioned hatches beyond the maximum oscillation frequency, an oscillator is obtained. microwave with very large slope ss F / hV of the frequency variation as a function of the voltage with very great economy of means since a minimum of components are used and the resonant circuits as well as some of the coils can be produced shock using metal strips ("strip-line") attached to a dielectric substrate forming a printed circuit which also carries fixed on it by soldering, the other components such as conductors, resistors and transistors # (of low value capacitors which can be produced using nested structures in the form of combs).

 In the circuit of FIG. 4, two additional decoupling capacitors 25 and 26 have been added, one of which makes it possible to decouple the voltage source (VCC) and the other the polarization divider of the base 7, 8. It is possible to also stabilize the supply voltages of transistor 2 by connecting a Zener diode (not shown) in parallel with the first decoupling capacitor 12.

 As has been said previously, the oscillating circuit of the oscillator, formed by the line 1 and the diodes 31, 32 in series, can be coupled to another of the electrodes of the transistor 2 without departing from the scope of the present invention.

 The VA tuning voltage applied to terminal 17 can be supplied by switchable potentiometers, for example, using electronic switches, by conventional voltage or frequency synthesizers, as well as by automatic frequency control circuits comprising discriminators, which operate at an intermediate frequency of a few tens of MHz.

 It will also be noted that it is not compulsory to use, for the two sections of line 1 and 24, the technology of printed circuits and to replace the ribbons reported by rigid bars of rectangular section, kept apart of the metallic ground plane by means of discrete spacers made of dielectric material with low losses, or else microwave lines of symmetrical type produced by means of ribbons arranged between two parallel ground planes, possibly using the printed circuit technique (" triplates ").

 It would also be possible to replace the bipolar junction transistors with field effect transistors adapted to operate at these high frequencies. It would then be necessary to provide modifications in the reaction circuit, in the event that the internal capacities of the tra1) sistor - and parasites of the housing between drain and gate and gate and source prove to be insufficient for the production of oscillations.

Claims (6)

 1. Microwave oscillator-continuously variable frequency in a wide range whose frequency variation is controlled by voltage, comprising a microwave transistor (2, 22) (UHF; rlont one of the electrodes is decoupled to ground (4) and resonant circuit comprising a fixed reactance element constituted by a first section of line (i) of length (L) and of predetermined characteristic impedance, the useful end of the ends of which is coupled to another electrode of the transistor (2, 22), and a variable reactance element, consisting of at least one variable capacity diode (30 or 31 and 32) as a function of its reverse bias voltage (VA), joining the other end of the first section of line (i ) to earth! 4), characterized in that this other end of the first line section (2) is also connected to earth (4) by means of a second line section (24) of significantly higher characteristic impedance to that of the first section (1) and length chosen as an inverse function of the desired maximum frequency of the range, in order to transfer hatches formed by the equivalent circuit of the diode (s) (30 or 31 and 32) with the inductive reactance of the first section (1), outside of this one.  2. Oscillator according to claim 1, characterized in that the variable reactance element consists of two variable capacity diodes (31, 32) with a common cathode, the anodes of which are respectively connected to the respective ends of the second section (24 ) and whose common cathode is polarized by means of a variable tuning voltage (VA).  3. Oscillator according to one of the preceding claims, characterized in that the length of the second line section (24) is chosen so that its equivalent inductive reactance is significantly less than the undue parasitic lance from which the capacitance diodes variable (30 or 3L and 32), in order to transfer the hatches above the desired maximum oscillation frequency.  4. Oscillator according to one of the preceding claims, characterized in that the transistor (2, 22) which equips it is a bipolar transistor with junctions whose transition frequency (fT) is greater than the desired maximum oscillation frequency .  5. Oscillator according to claim 5, characterized in that this transition frequency (fut) is chosen at least equal to twice the maximum oscillation frequency.  6. Oscillator according to one of claims 4 and 5, charac terized in that the transistor (2, 22) is mounted in common collector with the resonant circuit (1, 31, 32) coupled to its bate by means of a capacitor (19).