JPH01290318A - Frequency correction circuit - Google Patents

Abstract

PURPOSE:To always the oscillated frequency of a local oscillation circuit correctly and to improve the frequency stability of a mobile equipment by using a frequency equal to an intermediate frequency and inputting a signal whose amplitude is nearly constant synchronously therewith. CONSTITUTION:A frequency converting circuit 1 converts the received frequency into an intermediate frequency. The intermediate frequency is amplified by an intermediate frequency amplifier circuit 2. The signal whose amplitude is nearly constant is generated by a carrier recovery circuit 8 synchronously with the carrier of the signal. The frequency of the signal whose amplitude is nearly constant is measured by a frequency counter circuit 5 synchronously with the carrier. A local oscillation frequency adjustment circuit 6 compares a predetermined frequency with a measured value of the frequency counter circuit 5 and controls the oscillated frequency of the local oscillation circuit 9. The local oscillation circuit 9 supplies the local oscillation frequency to the frequency conversion circuit 1.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a frequency calibration circuit that calibrates the oscillation frequency of a local oscillation circuit using received waves with high frequency accuracy and reduces excavated frequency errors of the local oscillation circuit. It can be used, for example, in communication equipment for mobile bodies.

(Prior Art) In recent years, with the increasing demand for wireless communications, it has become essential to use higher radio frequencies and narrower occupied frequencies. Accordingly, it is necessary to make the oscillation frequency of a local oscillation circuit used in a frequency heterodyne wireless device highly stable. The excavation frequency of the local oscillator circuit fluctuates due to temperature fluctuations and aging. In order to compensate for this frequency fluctuation, a frequency calibration circuit as shown in the configuration of FIG. 8 has conventionally been used. This frequency calibration circuit consists of frequency conversion circuit 1, intermediate frequency (
IF) Consists of a signal amplification circuit 2, a frequency counter circuit 5, a detection circuit 3, a local excavation frequency adjustment circuit 6, and a local oscillation circuit 7. Drift compensation of the local oscillation frequency is performed based on the received wave with high frequency accuracy. In this configuration, when the oscillation frequency of the local oscillation circuit 7 fluctuates, the intermediate frequency similarly fluctuates. The frequency counter circuit 5 receives the IF multiplied signal and measures its frequency. The local oscillation circuit 7 can adjust the oscillation frequency using an external frequency control voltage.

The local oscillation frequency adjustment circuit 6 adjusts the frequency control voltage of the local oscillation circuit 7 so that the difference between the measurement result of the frequency counter circuit 5 and a preset intermediate frequency becomes zero. With such a configuration, it is possible to compensate for drift in the local oscillation frequency.

(Problems to be Solved by the Invention) However, if the received wave is a digitally modulated wave that does not have a constant amplitude, it becomes difficult for the frequency counter circuit 5 to measure the frequency. Figure 9 shows a band-limited two-phase digitally modulated wave (B
It represents the IF multiplied signal of PSK wave). Such a modulated wave has a zero crossing point where the amplitude becomes zero. For this reason,
If this IP multiplied signal is directly measured using a frequency counter circuit, a measurement error will occur. Also, if the received input level drops significantly, incorrect frequency measurement results will occur. Since the local oscillation frequency adjustment circuit adjusts the frequency control voltage of the local oscillation circuit based on an erroneous frequency measurement result, an error occurs in the local oscillation frequency.

An object of the present invention is to relate to a frequency calibration circuit that can accurately measure the IP signal frequency and accurately compensate for oscillation frequency fluctuations of a local oscillation circuit even when the amplitude of a digitally modulated wave fluctuates and the reception input level significantly decreases. It is something.

(Means for Solving the Problems) The features of the present invention for achieving the above object include a frequency conversion circuit that converts a reception frequency into an intermediate frequency, and a frequency calibration circuit that is capable of controlling an excavation frequency and that uses a local excavation frequency. a frequency counter circuit that measures the frequency of the intermediate frequency; a local excavation frequency adjustment circuit that controls the local oscillation circuit according to the measurement result of the frequency counter circuit; a carrier wave regeneration circuit provided between the output and the input of the frequency counter circuit, and inputting a signal having a substantially constant amplitude at a frequency equal to the intermediate frequency and having a substantially constant amplitude to the frequency counter circuit in synchronization therewith. , in a frequency calibration circuit that controls the oscillation frequency of the local oscillation circuit so that the local oscillation frequency adjustment circuit compares a predetermined frequency value and the measured value of the frequency counter circuit, and makes the difference between the two zero. .

Another feature of the present invention is that the above configuration further includes a level detection circuit that detects the level of the received signal, a carrier synchronization detection circuit that detects that the output of the carrier regeneration circuit is synchronized with the received signal, and the level detection circuit that detects the level of the received signal. Upon receiving the output of the detection circuit and the output of the carrier synchronization detection circuit, if a certain specific condition is satisfied, the frequency counter continues to operate, and under other conditions, the frequency counter continues operating the frequency counter immediately before satisfying the condition. The frequency calibration circuit includes a counter control circuit that controls the frequency counter to stop operating while holding the measurement result.

(Function) In the above configuration, the input to the frequency counter circuit is
A carrier wave regeneration circuit applies a signal whose amplitude is constant regardless of the amplitude of the modulated wave. Therefore, the frequency measurement by the frequency counter circuit is accurate and the problems of the prior art are solved.

Furthermore, when carrier wave regeneration is impossible, the frequency counter circuit stops operating and holds the immediately previous measurement result, so that no large error occurs in the local oscillation frequency.

In the conventional technology, the IP multiplied signal was measured as it was, which was affected by the modulation level, whereas the present invention completely solves this problem.

(Embodiment) 1.1゛- A first embodiment of the present invention is shown in FIG. The frequency calibration circuit of the present invention includes a frequency conversion circuit 1, an IP signal amplification circuit 2,
It consists of a carrier wave regeneration circuit 8, a frequency counter circuit 5, a local oscillation frequency adjustment circuit 6, and a local oscillation circuit 9. If the oscillation frequency of the local oscillation circuit is fL and the reception frequency is f, then the output of the frequency conversion circuit 1, that is, the intermediate frequency fiF, is f
rr = f, f+, (1
). The oscillation frequency of the local oscillation circuit 9 varies depending on temperature fluctuations,
Changes from fLO to f due to aging, etc., and f+, = f
It is expressed as bo<'+α+αADJ) (2). However, α is a rate of change due to temperature fluctuations, aging, etc., and αADJ is an adjustment rate that allows the frequency to be varied by controlling fIIJ from the outside. The intermediate frequency flF is ftp = f, -L, o (1+α+a AOJ)
(3) becomes. Furthermore, the carrier wave regeneration circuit 8 operates so that the output frequency becomes equal to flP. Frequency counter circuit 5 measures the output frequency of carrier wave recovery circuit 8.

Assuming that the reference time for opening and closing the gate of the frequency counter circuit 5 is t8, the measurement result (number of counts C) of the frequency counter circuit 5 is as follows: C= (f, ft, o(1+α+a Al)J)
) t-(4). The local oscillation frequency adjustment circuit 6 is C
is input and compared with a preset constant value C8Ea. At this time, if C<C,Ea, αADJ is decreased, and if C>Csr:r, αADJ is increased.

C5Er= (f, -fLo) ts+:,, (5)
ToshiEs=jS! ! If it is near, the oscillation frequency of the local oscillation circuit 9 is adjusted so that it always becomes fbo.

In addition to the function of calibrating the local oscillation frequency, the present invention can calibrate the local oscillation frequency regardless of the IF multiplier signal modulation state. The carrier wave regeneration circuit 8 outputs a frequency synchronized with the carrier wave of the input IF signal even if the modulated wave has a zero crossing point as shown in FIG. 9 of the IF multiplied signal. The frequency counter circuit 5 counts the output of the carrier wave regeneration circuit 8 as a clock input. In the conventional configuration, since the IF multiplied signal is used as the clock input of the frequency counter circuit 5, if the IF multiplied signal has a zero cross point, the frequency count circuit 5 will cause a measurement error. On the other hand, with this configuration, the frequency count circuit 5 does not produce measurement errors. Therefore, the local oscillation frequency can be accurately calibrated regardless of the IF multiplication signal modulation state.

For oscillation purposes, the local oscillation circuit used in the frequency calibration circuit of the present invention must have the ability to adjust the output frequency using an external frequency control signal. A frequency synthesizer using a phase-locked loop is generally used as the local oscillator circuit. FIG. 2 shows an example of a local oscillation circuit using this frequency synthesizer. In this configuration, the output frequency fL is as follows, where M is the frequency division number of the reference frequency divider 12, N is the frequency division number of the feedback frequency divider 16, and fR is the output frequency of the reference frequency oscillation circuit 11. (N/M)f*
(6) becomes. A crystal oscillation circuit is used as the reference frequency oscillation circuit 11, and if this oscillation frequency fluctuates due to temperature fluctuations and aging, an error will occur in fL. fR can be expressed as f*=fRo(1+α)(7) where α is the rate of change due to temperature fluctuations and aging. In order to adjust the output frequency, by adding a varactor diode to the crystal oscillation circuit used as the reference frequency oscillation circuit 11, the oscillation frequency can be easily adjusted by applying a voltage. This voltage is defined as the frequency control voltage. If the adjustment rate is αADJ, then fR is fR=fRo(1+α+αADJ) (
s) Therefore, fL= (87M)fRo(1+α+αADJ)
(9) Furthermore, if fLo = (87M)fRo, then L, = fLo(1+α+αADJ)
(10), resulting in a frequency adjustable local oscillation circuit as shown in equation (2).

As explained in (2), the frequency counter circuit 5 requires the time reference t8 in order to accurately measure the frequency. As this time reference, the reference frequency oscillation circuit 11 of Koki can be used. However, the oscillation frequency has an error as shown in equation (7). For this reason, as explained in ■, j, =
tsET, resulting in an error in frequency calibration. However, if the frequency synthesizer output frequency fL is sufficiently higher than the oscillation frequency of the reference oscillation circuit, the measurement error of the frequency counter circuit 5 can be made sufficiently small. Furthermore, by repeating this series of controls a plurality of times, the error of the reference frequency oscillation circuit 11 becomes smaller, so the measurement error of the frequency counter circuit 5 also becomes smaller, and the error converges to zero.

Furthermore, there is a method shown in FIG. 3 as a method for adjusting the output frequency of the frequency synthesizer. In this configuration, the oscillation frequency of the reference frequency oscillation circuit 11 is not adjusted externally, but instead the frequency division numbers of the two frequency dividers are changed. If the frequency division number of the reference frequency divider circuit 12 is M, and the frequency division number of the feedback frequency divider circuit 16 is N, then from equations (6) and <7), the output frequency f
L is fL= (87M) fno (1+ a)
(11). To adjust fL, M and N
are changed to μ and ν, respectively. At this time, fL is fL=(ν/μ)L+o (i+α) (12
). The frequency division number control circuit 18 inputs the desired adjustment frequency amount as adjustment data, calculates the μ value to be set and the μ value, and applies the data to each frequency division circuit 12.16. In this way, fL can be adjusted by changing the frequency dividing numbers of the two frequency dividing circuits.

In the embodiment shown in FIG. 1, when the reception level drops significantly, the carrier wave regeneration circuit 8 becomes unable to reproduce the carrier wave, causing an error in its output frequency. Therefore, the local oscillation circuit 9
is controlled to the wrong frequency. The embodiment shown in FIG. 4 is a frequency calibration circuit to prevent this.

The embodiment of FIG. 4 has a level detection circuit 2 in addition to the embodiment of FIG.
0, a carrier synchronization detection circuit 21 and a counter control circuit 22 are added. Level detection circuit 20 monitors the IP signal level. When the IF signal level decreases, the level detection circuit 20 detects the IF signal level at which the carrier wave regeneration circuit 8 cannot reproduce the carrier wave, and sends the detection signal to the counter control circuit 22. When the counter control circuit 22 receives this detection signal, it sends a holding signal to the frequency counter circuit 5 for holding the frequency measurement result (count result) immediately before the IF signal level drops. Further, when the IF signal level increases to a level that allows carrier wave regeneration, the carrier wave synchronization detection circuit 21 detects that the output of the carrier wave regeneration circuit 8 is synchronized with the carrier wave frequency of the input IF (No. 3), and converts the detected signal into a counter. The counter control circuit 22 receives this detection signal and sends a count holding release signal to the frequency counter circuit 5.

The following methods are available for the carrier synchronization detection circuit 21 to detect carrier synchronization. The carrier synchronization detection circuit 21 receives a modulated wave modulated with a specific signal string having an arbitrary predetermined length, and demodulates this modulated wave using the output of the carrier regeneration circuit 8. Furthermore, the carrier synchronization detection circuit 21 detects that carrier synchronization has been completed and that the carrier wave has been regenerated, depending on whether the demodulated signal sequence matches the originally determined signal sequence.

In a general wireless system, the frequency fluctuation time of the local oscillator circuit is much longer than the time during which the receiving input level is decreasing. Therefore, by detecting the reception input level and the completion of synchronization of carrier wave regeneration as described above, the oscillation frequency of the local oscillation circuit can be accurately calibrated.

As carrier wave regeneration circuits, there are multiplier circuits (for example,
Sanpo Tosho Digital Communication Series, Digital Wireless Communication). - As an example, the configuration of a multiplier circuit for four-phase phase modulated waves is shown in FIG. The IP multiplier signal at the terminal 23 is multiplied by 4 by the 4-multiplier circuit 24 and passed through the filter 25 as a reference signal, and the VCO (29), the 4-multiplier circuit 28,
A phase locked loop (PLL) consisting of a phase comparison circuit 26 and a loop filter 27 is configured. With this configuration, the VCO (29) is phase-synchronized with the carrier wave of the input IP signal. The oscillation frequency of this VCO (29) is output to the output terminal 31.
input to the frequency counter circuit via. Note that 30 is a detection circuit that outputs a demodulated output to an output terminal 32.

In addition to multiplication type circuits, there are inverse modulation type circuits as carrier wave regeneration circuits (for example, Sanpo Tosho Digital Communication Series, Digital Wireless Communication). - As an example, the configuration of an inverse modulation type detection circuit for four-phase phase modulated waves is shown in FIG.

The modulation signal from the IF low signal at the terminal 23 is demodulated by the detection circuit 30 . The four-phase phase modulation circuit 34 performs inverse modulation on the demodulated modulation signal using the modulated wave input and the IF low signal via the delay circuit 33. Using the output of this four-phase phase modulation circuit 34 as a reference signal, the VCO (29) and the loop filter 27
, a phase comparison circuit 26, and a delay circuit 33 constitute a phase locked loop (PLL). With this configuration, the VCO
(29) is phase-locked to the input IF multiplied signal carrier wave.

The oscillation frequency of this VCO (29) is applied to the frequency counter circuit via the output terminal 31 as a reproduced carrier wave output.

Note that the demodulated output is given to the output terminal 32 from the detection circuit 30.

In addition to the circuits described above, there are Costas type circuits as carrier wave regeneration circuits (for example, Sanpo Tosho Digital Communication Series, Digital Wireless Communication). - As an example, a Costas type circuit for a four-phase phase modulated wave is shown in FIG. 35 is a synchronous detection circuit, 36 is a multiplication circuit, 37 is a difference circuit, and 38 is a summation circuit. The circuit of this configuration uses the demodulated modulated signal to
A control voltage for the VCO (29) that does not depend on this modulation signal is generated, and the oscillation frequency of the VCO (29) is phase-synchronized with the IF low signal carrier frequency. The oscillation frequency of V(:0(29)) is measured with a frequency counter circuit.

(Effects of the Invention) The configuration of the present invention makes it possible to always correctly calibrate the oscillation frequency of the local excavation circuit even when the received wave is a modulated wave having a zero-crossing point or when the reception level has significantly decreased. Furthermore, if there is a base station with a highly stable frequency source, such as a car phone system, and a mobile device, whose frequency stability is not very good compared to the base station, if this configuration is applied to the mobile device side, the frequency stability of the mobile device can be stabilized. can significantly improve the degree of

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of the present invention, and FIG. 2 shows an example of a local oscillation circuit that can adjust the output frequency. Figure 3 shows four rows of local oscillator circuits that can adjust the output frequency. Fig. 4 shows a second embodiment of the present invention, Fig. 5 shows an example of a detection circuit, Fig. 6 shows another example of a detection circuit, Fig. 7 shows another example of a detection circuit, and Fig. 8 shows a conventional detection circuit. Frequency Calibration Circuit, Figure 9 shows a band-limited two-phase phase modulated wave. l...Frequency conversion circuit, 2...IF amplifier circuit, 3...
Detection circuit, 4. Demodulation output terminal, 5. Frequency counter circuit, 6. Local oscillation frequency adjustment circuit, 7. Local oscillation circuit, 8. Carrier wave regeneration circuit, 9. Local oscillation of frequency control terminal bone. Circuit, 10... Control terminal, 11... Reference frequency oscillation circuit, 12... Reference frequency divider circuit, 13... Phase comparison circuit, 14... Loop filter, 15
...Voltage controlled oscillation circuit, I6...Feedback frequency divider circuit, 17...Local oscillation output terminal, I8...Frequency division number control circuit, 19...Adjustment data terminal, 20...Level detection circuit, 21... Carrier wave synchronization detection circuit, 22.. Counter control circuit, 23.. IF signal input, 24.. 4 multiplier circuit, 25.
・Filter, 26.. Phase comparison circuit, 27.. Loop filter, 28.. 4 multiplier circuit, 29.. Voltage control detection circuit (VCO), 30.. Detection circuit, 3I.. Regenerated carrier wave output terminal, 32.・Demodulation output terminal, 33...delay circuit, 34...4
Phase phase modulation circuit, 35... Synchronous detection circuit, 36... Multiplier circuit, 37... Difference circuit, 38... Sum circuit.

Claims (2)

[Claims] (1) A frequency conversion circuit that converts a received frequency to an intermediate frequency, a local oscillation circuit that is capable of controlling an oscillation frequency and provides a local oscillation frequency to the frequency conversion circuit, and a frequency counter circuit that measures the frequency of the intermediate frequency. , a local oscillation frequency adjustment circuit that controls the local oscillation circuit according to the measurement result of the frequency counter circuit, and the local oscillation frequency adjustment circuit compares a predetermined frequency value with the measurement value of the frequency counter circuit, In a frequency calibration circuit that controls the oscillation frequency of the local oscillation circuit so that the difference between the two is zero, a carrier regeneration circuit is provided between the output of the frequency conversion circuit and the input of the frequency counter circuit, and an intermediate A frequency calibration circuit characterized in that a signal having a frequency equal to the frequency and having a substantially constant amplitude is input to the frequency counter circuit in synchronization with the frequency. (2) The frequency calibration circuit according to claim 1, further comprising a level detection circuit for detecting the level of the received signal, a carrier synchronization detection circuit for detecting that the output of the carrier wave regeneration circuit is synchronized with the received signal, and the level detection circuit for detecting the level of the received signal. Upon receiving the output of the circuit and the output of the carrier synchronization detection circuit, if a certain condition is satisfied, the frequency counter continues to operate, and under other conditions, the operation of the frequency counter immediately before satisfying the condition is continued. A frequency calibration circuit comprising a counter control circuit that controls a frequency counter to stop operating while holding measurement results.