FREQUENCY ALIGNMENT CIRCUIT AND SYNTHESISER THEREFOR

Abstract: An alignment circuit (Fig. 3) of the type including a local oscillator (13) and one or more tuning filters (5, 9) each controlled by varactor diodes (D1 to D4) and in which a common high-voltage tuning signal (Vc) is applied to the high-voltage side of these diodes (D1 to D4). For this, a synthesiser circuit (3') is provided to apply low-voltage correction signals to the opposite side of respective ones of these diodes (D1 to D4), signals that are specific to the different characteristics (Fig. 4) of the diodes (D1 to D4). The synthesis circuit (Fig. 5) thus may comprise a shift register (41) and digital-to-analogue converter (39) for generating voltage correction signals (V1 to V3) in response to digitised data supplied from a pre-programmed memory (45) under the control of a system microprocessor (43). This circuitry may be combined with high-frequency synthesis circuits on a common integrated chip.

Description: FREQUENCY ALIGNMENT CIRCUIT AND SYNTHESIZER THEREFOR Technical Field The present invention concerns improvements in or relating to frequency alignment circuits and to synthesizers utilised therein, especially, but not exclusively, television (TV) channel selection/tuner circuits and digitally programmable frequency synthesizer therefor. At present most TV tuners use digital frequency synthesis to generate a local oscillator varactor control voltage for obtaining a selected local oscillator (LO) frequency. The digitally programmable frequency synthesizer used is normally part of a phase or frequency locked loop and will usually incorporate a high frequency prescaler. The synthesizer is frequently required to function up to frequencies of the order 1.3GHz and the present trend is to use a single chip implimentation in conjunction with external high voltage components. It is usual to incorporate a preselection filter and image rejection filters in the circuits aforesaid, and varactors (varicap diodes) also are used for aligning the frequency characteristics of these r.f .-filters. Background Art In TV tuner circuits, used currently, a common varactor control voltage is applied to each of the varactor diodes, ie. the filter diodes and the local oscillator diode. It is important that the varactor- tuned r.f.-filters and local oscillator should have good mutual frequency tracking. Because of the spread of the characteristics of tuned circuit components, the varactor diodes in particular, time consuming and costly manual alignment is requisite to achieve this required tracking and appropriate frequency ranging. The varactor diodes, therefore, must undergo a complex selection procedure so to obtain matching within acceptable close tolerance - eg. 2%. Even when such procedures are adopted, tuner performance is in general less than optimum. A solution to the problem, just mentioned, has been reported recently - cf. "Computer Controlled Television Channel Selection and Tuner Alignment" by D.Dumont and A.Neelen, Digest of Technical Papers, 1985 IEEE International Conference on Consumer Electronics, pp.78-79 (June 5, 1985) . The solution, thus reported, is to use not a single common varactor control voltage, but to use a corrected high-level control voltage specific to each varactor diode. The set of control voltages aforesaid are derived from a synthesizer generated varactor line voltage, corrections being applied to compensate for the disparate impedance - voltage characteristics of the individual varactor diodes. In this case, therefore, a wider tolerance on diode characteristics is acceptable and coarse.component selection only therefore is requisite. According to the implimentation reported, an additional semiconductor chip is utilised to perform the generation of voltage corrections and the high-level voltage signal processing necessary for producing the set of corrected control voltages (cf. Fig.3 of the paper above-cited) . It is thus noted that two semiconductor chips, at least, are utilised - one chip optimised for high- frequency low-voltage synthesizer functions, and another chip optimised to handle high-voltage correction functions. For optimum integration of the synthesizer system, it would be necessary to integrate these aforesaid high voltage outputs and associated addition circuitry with the high-frequency synthesis circuitry. This is disadvantageous since high-frequency synthesis circuitry components usually have inherent low breakdown voltage. For a typical silicon semiconductor component, this breakdown voltage would be approximately 10V, a voltage much lower than that required by the high voltage processing circuitry (c.30V). Techniques are known for designing a simple high voltage output stage which could in turn be serviced by the synthesis loop. However, techniques for high voltage voltage processing would involve complex design, and any reliable method would most like require inordinately high numbers of transistors, and would thus utilise significantly large areas of semiconductor material. Disclosure of the Invention The present invention is intended as an alternative and advantageous solution to the problem aforesaid. The need for separate semiconductor chip structures, or for inordinately complex circuitry on a single chip, is obviated. The solution to this problem, as disclosed herein, is to apply a common varactor control voltage to the high- voltage-_side of each varactor, whilst applying a varactor specific low-level correction voltage to the low-voltage side of each varactor. It is noted that only low-level corrective voltage outputs are requisite, and since these voltages are inherently within the voltage handling capability of high-frequency synthesizer processing circuit components, both synthesizer function and correction function circuitry may be implimented using a single common semiconductor chip. In accordance with a first aspect of the invention, therefore, there is provided a frequency alignment circuit of the type comprising:- an r. ' f. mixer; a local oscillator, the output of which is connected to an input of the mixer; a pre-mixer r.f. tuned filter, responsive to input signal, the output of which is connected to another input of the mixer; a set of varactor diodes, one of which is connected to the local oscillator, at least one other of which is connected to said filter; and, a synthesizer circuit, responsive to and connected to receive a sample of the local oscillator output signal, this synthesizer circuit having a varactor control voltage line output, this output being connected to the high-voltage side of each said varactor diode, characterised by;- varactor-specific low-level correction voltage generation circuitry having a plurality of corresponding outputs, each said output being connected to the low-voltage side of each corresponding varactor diode, respectively. The invention combines all the advantages of automatic frequency alignment with the further evident advantage of possible single chip implimentation, but at the same time avoids high-complexity, high-voltage voltage-processing circuitry. The advantages of automatic alignment are:- improved noise figure performance; greater constancy in gain and group delay; as also, increased selectivity and better interference rejection. In accordance with a second aspect of the invention there is provided a synthesizer semiconductor chip for use in the frequency alignment circuit aforesaid, wherein said chip incorporates an high-frequency synthesizer circuit having a varactor control voltage line output; and in addition thereto and integrated therewith, a low level, compatible, varactor-correction voltage generation circuit and plural outputs associated therewith. It should also be noted that the varactor control voltage line output aforesaid may also utilise external high voltage components, and such design is not precluded from the scope of this invention. Brief Introduction of the Drawings In the drawings accompanying this specification:- Figures 1 and 2 are block circuit diagrams, in part simplified and schematic, of a standard frequency alignment circuit, and, a known improved variant thereof, respectively; Figure 3 is a block circuit diagram of a frequency alignment circuit, a circuit modified in accordance with the present invention; Figure 4 is an illustrative graph depicting the impedance-voltage characteristics of a pair of disparate varactor diodes; and. Figure 5 is a block circuit diagram of a single chip synthesizer, also modified in accordance with the present invention. Description of Preferred Embodiment So that the invention may be better understood, an embodiment thereof will now be described, by way of example only, and with particular reference to the accompanying drawings. A standard frequency alignment/TV tuner circuit is shown in Figure 1. This comprises a tuner component assembly 1, a synthesizer 3 and a set of tuning control varactor diodes O to D each of which is connected to a corresponding part of the assembly 1. The assembly is itself comprised of several components, namely: a preselection filter 5, a pre-amplifier 1 , a band-pass image-rejection filter 9, a r.f. mixer 11, local oscillator 13 which together with the band-pass filter 9 is connected to respective inputs of the mixer 11, an IF output filter 15 connected to the output of the mixer 11 and an IF-stage pre-amplifier 17. Varactor diode Oχ, the two varactor diodes D2 and D3, and the varactor diode D are, as shown, connected to the preselection filter 5, the band-pass filter 9 and the local oscillator 13, respectively. A sample local-oscillator r.f. voltage signal is fed to the input of the synthesizer 3 and this latter, in response to this signal, generates a varactor-line control voltage V c . This voltage V c is applied to the high-voltage side of each varactor diode D to D , in turn via isolating impedances Rx to R4. The low-voltage side of each varactor diode Oχ to D4 is connected to a common earth line V EE. As discussed above, each of the varactor diodes O to D4 must be selected to be within a very close tolerance. In the known improved variant, shown in Figure 2, a correction circuit 19, implimented on an additional, discrete, voltage-optimised, semiconductor chip, is interposed between the synthesizer 3 and each of the varactor diodes O to D4• As in the previous system circuit, these diodes D- j _ to D4 are each connected to a common earth-line V EE. The correction circuit 19 is responsive to the varactor line voltage V c and generates varactor-specif c corrected voltages V c χ to V C 4 for each of the diodes Oχ to D . These corrected voltage signals V c - j _ to V C 4 are applied, as shown, to the high-voltage side of each of the diodes Oχ to D 4 . In the inventive variant, shown in Figure 3, the synthesizer processing circuitry and also corrective circuitry are implimented using a common system chip 3'. This chip 3' has an input for inserting sampled local oscillator signal, and, as is conventional, the synthesizer processing circuitry generates a varactor-line control voltage V c . The corrective circuitry, however, generates low-level varactor-specific corrective voltages V to V4. The control voltage V c is applied to each varactor diode Oχ to D4, whilst at the same time the corrective low-level voltages. are applied to the diodes O to D4. These are applied to the low-voltage side, in each case, between each diode Oχ to D4 and corresponding capacitors 0χ to C4 each of which is tied to a common earth-line V EE. The low-level corrective voltages V j _ to V3 for the filter varactor diodes O to D3 are pre-determined to compensate for differences in the diode impedance-voltage characteristics - See Figure 4. In the graph (Fig.4) curves are shown comparing the characteristics for the local-oscillator varactor diode D4 and one of the filter diodes D**_. It can be seen that there is a disparity between these two characteristics. The varactor diodes Oχ to D3 are here selected to have a broad tolerance - ie. their characteristics lie within + V m (say) of the local oscillator varactor D 4 voltage V c . In the implimentation discussed here a bias voltage V4 is applied to the local oscillator varactor diode D4:- v 4 =+ v m At a given value of capacitance C v and control voltage V c , the same capacitance can be attained for each varactor diode D^ to D3 by applying an error correction voltage V. This error voltage Vx is shown referenced to the local-oscillator varactor diode D4 characteristic. A typical implimentation of common system chip 3' is shown in Figure 5. The synthesis circuitry, part of this chip 3', is of phase-locked looped (PLL) design. (It is to be noted that other designs of synthesizer circuit can be adopted - eg. freguency-locked loop (FLL) - and are not precluded from the scope of this invention.) This circuitry comprises a reference oscillator 21, a referenc divider 23, a phase comparator 25, a pre-amplifier 27 and prescaler 29 and programmable divider 31, all arranged in well-known configuration. The output of the comparator 2 feeds a charge pump 33 and output loop-filter comprised o an amplifier 35 and associated filter 37. Each error voltage Δ_Vχ to Δ v 3 is generated by means of a digital analogue converter (DAC) 39. The programmable divider 31 and DAC 39= are each controlled by means of a common n-bit shift register word store 41, which in turn is governed b a microprocessor 43 and memory 45 (usually external). Th chip 3* also includes a generator 47 for the local oscillator diode D 4 shift voltage V4. Whilst the prescaler 29 has been described as part o the chip 3', it may also be implimented as an external component. Thus for example, a high-frequency bipolar- process component prescaler may be used in conjunction with a low-frequency (eg. MOS/CMOS/NMOS) process chip. During system assembly, the varactor diode characteristics are determined under computer test and data defining the error voltages &V to ΔV3 (V c ) is loaded from the computer into the system memory 45 to provide a look-up table for error voltage generation. r 10

Claims: CLAIMS 1. A frequency alignment circuit of the type comprising:- an r.f. mixer; a local oscillator, the output of which is connected to an input of the mixer; a pre-mixer r.f. tuned filter, responsive to input signal, the output of which is connected to another input of the mixer; a set of varactor diodes, one of which is connected to the local oscillator, at least one other of which is connected to said filter; and, a synthesiser circuit, responsive to and connected to receive a sample of the local oscillator output signal, this synthesiser circuit having a varactor control voltage line output, this output being connected to the high-voltage side of each said varactor diode, characterised by:- varactor-specific low-level correction voltage generation circuitry having a plurality of corresponding outputs, each said output being connected to the low-voltage side of each corresponding varactor diode, respectively. 2 , A frequency alignment circuit, as claimed in claim 1, wherein the correction voltage generation circuitry comprises:- storage means for holding error correction data words; and digital-to-analogue conversion means, responsive to said storage means, to produce the error correction voltages that are applied to the low voltage side of the corresponding varactor diodes. 3. A frequency alignment circuit, as claimed in claim 2, wherein said storage means is provided by a shift register. 4. A frequency alignment circuit, as claimed in claim 3, together with:- a microprocessor, co-operative with said shift register, for loading the correction data words therein; and a memory, co-operative with this microprocessor, for containing a set of correction data words therefor. 5. A frequency alignment circuit, as claimed in claim 4, wherein the memory is pre-programmed and contains thus correction data words specific to the characteristics of the circuit varactor diodes. 6. A synthesiser semiconductor chip for use in the frequency alignment circuit, as claimed in claim 1, wherein said chip incorporates an high-frequency synthesiser circuit having a varactor control voltage line output; and in addition thereto and integrated therewith, a low level, compatible, varactor-correction voltage generation circuit and plural outputs associated therewith. 7. A synthesiser semiconductor chip, as claimed in claim 6, wherein said voltage generation circuit co prises:- storage means for holding error correction data words, and digital to analogue conversion means, responsive to said storage means, to produce error correction voltages. 8. A synthesiser semiconductor chip, as claimed in claim 7, wherein said storage means is provided by a shift register. 9. A synthesiser semiconductor chip, as claimed in claim 8, wherein the high frequency synthesiser circuit includes a digitally programmable divider, which latter is co-operative with the shift-register, this register being adapted for holding control data therefor in addition to said error correction data words.

Inventors: COWLEY NICHOLAS PAUL, LAWTON RODNEY JAMES, COWLEY, NICHOLAS, PAUL, LAWTON, RODNEY, JAMES
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1986
Main classification: H03J3
IPC codes: H03J3