LOOP FILTER FOR A FREQUENCY SYNTHESIZER

Abstract: According to the invention another zero has been added to the frequency response of a loop filter situated in the feedback branch of a frequency synthesizer causing thus the phase margin to grow, and a pole has been added at a frequency higher than the zero point, when the influence of the zero on the amplitude response is essentially over-ruled.

Description: Loop Filter for a Frequency Synthesizer The object of the present invention is a loop filter situ- ated in the feedback channel of a frequency synthesizer and consisting of an integrator with a zero in the fre¬ quency response in order to achieve stability, and of a low pass filter. Such a frequency synthesizer comprising a loop filter known from figure 1 is used for example in radio telephones as a local oscillator. In the syn¬ thesizer, frequency is formed in a known way according to figure 2 in a voltage-controlled oscillator the frequency of which has been locked to the frequency of a crystal oscillator used as comparison frequency. The frequencies of the voltage-controlled oscillator and the crystal os¬ cillator are conveyed via dividers to a phase comparator, from the output of which the signal based on the phase or frequency difference is obtained. Generally, at least one of the dividers is then programmable. From the output of the phase comparator, the signal goes to the loop filter, the control voltage of the oscillator being obtainable from the filter. In a known loop filter a problem is caused by the filter- ing of sidebands brought about by the comparison frequency while striving for as fast a loop as possible. Reason for the problems is that the poles (on the frequency level) of the low pass filter near the natural frequency of the loop reduce the phase margin decreasing thus the damping fac- tor. It has been tried to solve the problem by using a low enough natural loop frequency, or by using a separate acceleration coupling. However, in that case, the dimen- sioning has to be made so precise that even a small toler¬ ance variation in the components causes the setting time to grow too long, or that the sidebands rise too high. A further drawback is the complex structure of the accelera¬ tion coupling. In these known cases, the loop filter has been formed by using an integrator, as can be seen in connection with the operational amplifier V of figure 1. In the feedback chan¬ nel, resistor Rl and " condensators Cl and C2 together with input resistor Ri form an integrating factor. Further, an RC-filter is often coupled after the integrator. The object of the present invention is to remove or essen¬ tially reduce the problems and drawbacks mentioned above. In order to achieve this, the invention is characterized in that another zero has been added to the frequency re¬ sponse causing the phase margin to grow; further, a pole has been added at a frequency higher than the zero point, when the impact of the zero on the amplitude response is fundamentally overruled. For example, a bipolar active low pass filter is previously known from an article of D. Kesner: "Take the guesswork out of phase locked loop de¬ sign," EDN, Jan. 1973, pages 54-60. According to the ar¬ ticle, purpose of the active filter is to replace the RC- filtering following after the integrator in order to re- move sidebands. In the present invention, the frequency response of the integrator is changed so that the phase margin grows and the sideband filtering can be achieved by RC-filtering. A multipolar loop filter is also known from a publication of J. Gorski-Popiel: "Phase-locked Loop Frequency Syn¬ thesizer," IEEE Press, 1975, Frequency Synthesis: Techni¬ ques and Applications, pages 111-118. This filter with no zero in the frequency response contains a block (Tl), the purpose of which is to totally overrule the effect of RC- circuits in the filter output. According to the present invention, the impact of the RC- circuits on the phase is reduced at the loop cut-off fre¬ quency. Loop of a frequency synthesizer including the loop filter of the present invention becomes faster, and at the same time, it is still possible to effectively filter sidebands. Additionally, the loop can be planned as less critical in relation to the tolerance of the components. The present invention will now be described by way of an embodiment example and with reference to the accompanying drawings of which: Figure 1 is a realization of a previously known loop fil¬ ter; Figure 2 is a block diagram of the structure of a known frequency synthesizer; Figure 3 shows the construction of an embodiment of a loop filter of the present invention; and Figure 4 shows the component values of the embodiment in question. Phase margin of the frequency synthesizer mentioned in the introduction can be increased when, according to the pres¬ ent invention, a zero is added to the frequency response (transfer function on frequency level) above the natural loop frequency. Effect of the zero on the amplitude re¬ sponse is reversed with a pole situated at a frequency above the zer point. Increase in the phase margin can be exploited by dimen¬ sioning the natural loop frequency and the poles of the low pass filter closer to each other on the frequency level. In practice this means that the loop can be made much faster, and it still is possible to effectively fil- ter out sidebands. At the same time, the loop can be plan¬ ned as less critical in relation to the tolerance of the components. The desired change in the frequency response, i.e. adding of a zero, is achieved by adding two com¬ ponents to the feedback channel of a known loop filter (figure 1), as can be seen in figure 3. Instead of resis- tor Rl of figure 1, resistors R3 and R4 and condensator C3 are then in T-coupling. Response of the filter embodiment of figure 3 is 1+s(C s R 3 +C 5 R * +C,R 3+ c4R4+C 3 R3)+s 2 (CsCsR.R ^ +C^RaR * ) F(s)= s(CsRi)+s 2 (CsCRA+C- RA)+S 3 (C S C 4 C 3 R 1 R3 «) or, B 2 s 2 + B . s + B 0 F(s) = A 3 S 3 + A 2 S 2 + A X S This equation shows only the share of integrator in the loop filter. When the RC-circuits usually in use are add¬ ed, the performance number of the denominator increases correspondingly; for example, by two RC:s, as in figure 4; one before the integrator, and one after, performance number of the denominator being then five. However, per- formance number of the numenator will still be two. The response will then be in the following form: + Ao In known loop filters, zero below the loop cut-off fre- quency is generally at a frequency of a magnitude of fo/5. This zero is necessary in order to make the loop generally stable. According to the present invention, the other zero added to the frequency response lies above the loop cut-off frequency, at the most 2«fo. If the zero lies too near the cut-off frequency it has an impact on the amplitude re¬ sponse, and if it is too far, impact on the phase response is lost. Impact of the zero is overruled by a pole produced by the RC-circuits which is at a higher frequency, perhaps about one and a half times higher in comparison with the fre¬ quency of the zero. In order to filter sidebands, it is usually most advantageous to use the cut-off frequency of the low pass filter which is the geometrical average of the cut-off frequency and the reference frequency. Frequencies of the filter in the embodiment are ap- proximately: Integrator zero: Added zero: Integrator pole: Low pass filter poles: Cut-off frequency fo of the whole loop abt 15-20 kHz Idea of the loop filter of the invention is that the zero added to the response changes the phase margin exactly at the natural loop frequency, i.e. phase margin is delivered to the frequency response excactly at the right frequency by a simple coupling; a passive low pass filter can then be used in filtering sidebands. The desired frequency response achieved according to the invention by adding another zero and pole can be realized with several different filter couplings, as will undoub¬ tedly occur to a professional of the field when viewing the idea of the present invention.

Claims: Claims 1. Loop filter situated in the feedback channel of a fre¬ quency synthesizer and formed by an integrator with a zero in the frequency response in order to achieve stability, and a low pass filter, c h a r a c t e r i z e d in that another zero has been added to the frequency response causing the phase margin to grow; a pole is added at a frequency higher than the so-called zero point when the impact of the zero on amplitude response is essentially overruled. 2. A loop filter according to claim 1, c h a r a c t e r ¬ i z e d in that the added other zero is produced by addi¬ tional components of the integrator feedback channel and that the added pole is produced by an RC-circuit before and/or after the integrator. 3. A loop filter according to claims 1 or 2, c h a r a c ¬ t e r i z e d in that the added other zero is at a fre- quency the magnitude of which is one and a half times the loop cut-off frequency. 4. A loop filter according to claim 3, c h a r a c t e r ¬ i z e d in that the added pole lies at a frequency which is about one and a half times the frequency of the added zero. 5. Use of a loop filter according to claim one in a local oscillator of a radio telephone.

Inventors: SATOMAEKI ESKO, SATOMAEKI, ESKO
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MATKAPUHELIMET OY
Application year: 1990
Main classification: H03L7
IPC codes: H03H11, H03L7