The present invention relates to a circuit arrangement for generating and stably amplifying broadband rf signals, the arrangement including in a single or multicircuit design, one or a plurality of active semiconductor elements and one or a plurality of tuning devices which are arranged in a rectangular waveguide designed for the operating frequency range and operated below the cutoff frequency.
In order to generate and amplify rf signals with semiconductor elements there are available, in addition to bipolar transistors for frequencies up to about 5 GHz mainly FET transistors as well as Gunn and IMPATT diodes for higher frequency ranges.
If, for example, more than 1000 voice channels or other broadband signals are to be transmitted in frequency division multiplex systems with less than 1 W output power together with high stage gain (20 dB) in a frequency range greater than 5 GHz, injection synchronized IMPATT (impact avalanche and transit time) oscillators are employed. To meet these requirements, it is necessary to provide synchronizing bandwidths of more than 100 MHz in order to assure distortion-free transmission. The relationship between synchronizing bandwidth B.sub.sync, the ratio of input and output power (stage gain) P.sub.in /P.sub.out, the frequency of the idling oscillator f.sub.0 and the external quality factor Q.sub.e, are given for the injection synchronized oscillator by the known equation ##EQU1##
Gunn and IMPATT diodes are also used in broadband reflection amplifiers. The principal structure, which is the same for the injection synchronized oscillator or the reflection amplifier, repectively, is shown in FIG. 1 in the circuit arrangement 1 connected to one terminal of the circulator. This circuit arrangement 1 comprises one or a plurality of active semiconductor elements--e.g. Gunn or IMPATT diodes.
Basic equivalent circuit diagrams of IMPATT and Gunn diodes with housing are shown in FIGS. 2a and 2b. In FIG. 2a, network 2 constitutes the equivalent circuit for the IMPATT diode with negative resistance 3, and network 4 constitutes the equivalent valent circuit for the diode housing (see M.S. Gupta: "Large Signal Equivalent Circuit for IMPATT Diode Characterization and its Application to Amplifiers", IEEE Trans. Microwave Theory Tech. Vo. MTT-21, November 1973, pages 689-694).
FIG. 2b shows the equivalent circuit 5 for the Gunn diode with the negative resistance 6 (see Gunn Diode Circuit Handbook, Microwave Association/HB 9000/February 1971, page 8) and the equivalent circuit 7 for the housing. The input impedance of the two equivalent circuits is assumed to be Z.sub.D =R.sub.D +jX.sub.D, where R.sub.D takes on a negative value because of the negative resistance of the IMPATT or Gunn diode, respectively. The reactance X.sub.D may be capacitive as well as inductive.
In the region where the amplifier stage is to have a stable gain G according to the known equation ##EQU2## where R.sub.L is the load resistance, a circuit arrangement must be provided in the area of the active semiconductor elements for compensating the reactance X.sub.D by the reactive component of the load X.sub.L (X.sub.L =-X.sub.D). Difficulties exist principally in carrying out the compensation over a broad frequency band since, on the one hand, the effective resistance of the diode may be very low, for example, the resistance of a typical IMPATT diode is .ltoreq..theta.0.8 .OMEGA., and the inductive or capacitive reactive component may be relatively high (20.OMEGA.-40.OMEGA.), which corresponds to a high diode Q of about 20. Moreover, the diodes with their housing constitute multicircuit arrangements as shown in FIGS. 2a and 2b, whose impedance additionally changes considerably due to fluctuations in manufacture and the selection of the operating point or the degree of modulation (the difference between small and large signal operation).
HP Application Note 935 3/1972 describes injection synchronized oscillators and reflection amplifiers which employ coaxial or coaxial waveguide arrangements in which the compensation of the diode reactance is usually effected in a single circuit. Multicircuit coaxial and stripline circuits are also known in which, however, no sufficient matching possibilities are given to sufficiently handle changes in frequency, and fluctuations in semiconductors due to fabrication. These arrangements attain external Q's which do not meet the requirements of broadband compensation of the diode reactance.
A broadband microwave oscillator has been disclosed in German patent application P 27 10 164.0. This oscillator includes a waveguide operated below the cutoff frequency and having a longitudinal slit in the narrow side of the waveguide through which the energy is coupled out into a coupling waveguide whose cross section permits the existence of the coupled-out fundamental mode. In the vicinity of the region where the energy is coupled out into the coupling waveguide there is disposed the active semiconductor device and at least one additional variable capacitance. Although it is possible with the aid of these arrangements to achieve a smaller external Q than with the prior art solutions, an external Q has not been realized which is less than the Q of the semiconductor element. To transmit, for example, 1800 calls with an injection synchronized oscillator having a stage gain of 20 dB, it is necessary, according to the relationship between synchronizing bandwidth, external Q and stage gain (see the equation above) to have an external quality factor of Q.sub.e .ltoreq.10, which then lies far below the inherent Q (.perspectiveto.20) of, for example, an IMPATT diode. The above circuit arrangement can also be realized only with difficulty in multiple circuit arrangements and is difficult to tune in that case.