JPH0130332B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for preventing the occurrence of cross spurious beats caused by a communication device having a multi-frequency conversion configuration, particularly a circuit including a PLL oscillator. [Prior art] Most conventional communication devices are of the superheterodyne type, and many of them use double or more multiplex conversion, and each transformer (hereinafter referred to as a mixer) has a local Although it is equipped with an oscillator, variable frequency oscillators are often controlled by PLL (Phase Locked Loop) for ease of frequency setting and stability. FIG. 1 shows such a double conversion receiving circuit, in which the input signal wave from the antenna 1 becomes the first intermediate frequency at the front-stage mixer 2, passes through the bandpass filter 3, and becomes the second intermediate frequency at the rear-stage mixer 4. , and is amplified and demodulated through a bandpass filter 5. The local oscillation frequency _L1 of the pre-stage mixer 2 is injected from the PLL controlled oscillation circuit 6. PLL controlled oscillator 6
oscillates the fundamental wave with a voltage controlled oscillator (hereinafter referred to as VCO), supplies its output to mixer 2, and mixes it with the frequency of fixed oscillator 7 in internal mixer 62 to _{generate L2.}
Output. In FIG. 1, the frequency of the common oscillator 7 is not only fixed by the mixer 62, but also used as the local oscillation frequency _L2 of the rear mixer 4, and the common oscillator 7 serves as a shared oscillator for the front mixer 2 and the rear mixer 4. This configuration not only saves one oscillator by sharing the common oscillator 7, but even if the oscillation frequency of the common oscillator 7 varies _somewhat , the first intermediate The change in frequency is calculated as injection frequency _L2 of rear mixer 4.
This can be canceled out by the fluctuations in the second intermediate frequency so that it does not affect the second intermediate frequency at all.The principle behind this method is well known as the drift cancel method, so I will not go into details about it, but this is the purpose of this book. This is one of the important components of the invention, so it should be specially mentioned. The output frequency of the mixer 62 is passed through a frequency divider 63, and the phase comparator 64 compares the phase with the reference frequency _R , and the positive or negative detection voltage generated according to the phase difference is passed through a low-pass filter 65 as a DC control voltage.
In addition to VCO 61, it constitutes a control loop that stabilizes the oscillation frequency, but since the oscillation frequency _L1 basically changes in steps of the reference frequency _R , _R is very small compared to _L1 . , and the output frequency of the programmable frequency divider 63 whose phase should be compared with _R is naturally a small value. On the other hand, there is a problem that the larger the frequency division ratio of the frequency divider 63 is, the lower the control loop gain and the lower the stability of the PLL loop.
It is desirable that _n be as low a frequency as possible, and there are constraints on the operable frequency of the frequency divider, so
As mixer 62 is used as a down mixer, _n is normally set to be smaller than _L1 . The cross spurious beat in the PLL circuit is most noticeable when the frequencies of _n and _L1 are in an integer ratio, so the mechanism of its occurrence will be explained below. 62 is a down mixer, so if the above integer ratio is n and _L1 > _L2 , then _n = _L1 - _L2 , and the mixer output includes _L1 , _L2 and _L1 + in addition to _n .
Frequencies such as _L2 are included, but since these are much higher frequencies than _n , they can be easily removed by a low-pass filter or band-pass filter at the mixer output section, and only _n can be extracted. In reality, harmonics of the above frequencies are generated due to the nonlinear characteristics of the mixer. This harmonic itself has a higher frequency, so there is no problem, but in the frequency state of n _n = _L1 , the nth harmonic of output _n has the same frequency as _L1 , causing interference. This becomes a zero beat and does not appear externally, but when the input frequency changes by Δ, the change in the fundamental wave output is Δ, and the change in the nth harmonic output is nΔ. Therefore, n _n →n _n ±nΔ= _L1 ±nΔ If we take the difference between this and the input _L1 ±Δ, ( _L1 ±nΔ) − ( _L1 ±Δ) = ±(n−1)Δ From this formula, we get・It can be seen that in the mixer, before and after the frequency where the input frequency is n times the output frequency, a beat frequency that is (n-1) times the difference frequency with the input frequency is generated at the output, and the frequency changes around the zero beat. Since the beats change in a crossing manner, they are generally called cross spurious beats. This beat passes through a frequency divider with _n or modulates _n and is mixed into the output of phase comparator 64, and when it is below the cutoff frequency of low-pass filter 65, it is added to VCO 61 to output _L1. It is frequency modulated and becomes a spurious component. [Problem to be Solved by the Invention] In order to suppress the generation of spurious components, if the cutoff frequency of the low-pass filter 65 is set low, the time constant increases, and the lock-up time (the time it takes for the oscillation frequency to stabilize) of the PLL controlled oscillator 6 increases. )
is long, so there are limitations in practical circuits. There are also spurious waves caused by harmonics and nonlinearity of the reference frequency _R and the phase comparator 64, but since all of them are limited to specific frequencies, it is desirable to design a design that avoids frequency relationships that generate spurious waves.
This is not possible for broadband general coverage. An object of the present invention is to provide a means for avoiding spurious generation in a frequency band including the above-mentioned cross spurious beat. [Means for solving the problem] Cross spurious and
This configuration prevents the occurrence of cross spurious beats by moving the common oscillator frequency within the frequency range of the intermediate frequency pass band between the front-stage mixer 2 and the rear-stage mixer 4 at the operating frequency that generates the beat. . [Embodiment] FIG. 1 shows a basic circuit, which will be explained starting from FIG. In the figure, at the operating frequency that causes cross spurious beats in the output of the PLL controlled oscillator 6 ,
In addition to supplying the local frequency to the internal mixer 62,
By moving the oscillation frequency of the common oscillator 7, which also supplies the local frequency _L2 to the rear mixer 4, within the frequency range of the bandpass filter 3 located between the front mixer 2 and the rear mixer 4, cross spurious beats can be eliminated. This is a communication device circuit designed to prevent this from occurring. In the PLL controlled oscillator 6 , the common oscillator 7 of the mixer 62 converts the frequency _L1 of the VCO 61 to _n .
When the frequency of is moved (hereinafter referred to as shift),
_L1 changes by the same frequency as the shifted frequency, and the VCO locks. Therefore, if the oscillation frequency of the common oscillator 7 is moved to jump over the spurious generation frequency just before reaching the frequency of _L1 that generates cross spurious inside the PLL control circuit 6 , _L1 will not contain any spurious. become.
Also, when _L1 changes, the output frequency of mixer 2 naturally changes, but the fact that the local frequency _L2 of downstream mixer 4 changes in the same direction and does not affect the output frequency of mixer 4 is due to the drift described above.・This will already become clear from the explanation of the cancel method. However, since the first intermediate frequency between mixer 2 and mixer 4 varies, the passband width of filter 3 must be added to the required bandwidth by the amount of variation. The cross beat generation range is determined by the low pass of the PLL control circuit 6 and the cutoff frequency of the filter 65, and in reality, lockup time is not a problem as long as it is about 1 kHz or more, so the frequency shift of the common oscillator 7 is 2~2kHz.
Approximately 5kHz is sufficient. Therefore, the passband width of filter 3 is 10~10 for CW and SSB signals with narrow occupied bandwidth.
15kHz, 20-30kHz is sufficient even for FM waves. To shift the frequency of the common oscillator 7, since the oscillator is usually crystal controlled, a voltage controlled variable capacitance diode 72 is inserted in parallel or in series with the oscillating crystal piece 71 as shown in Fig. 2 to change the control voltage. do it. Control methods include providing a switch that changes the voltage in the control voltage circuit and switching the switch when a cross spurious beat occurs, or automatically operating the switch at the spurious generation position of the frequency tuning mechanism. It is possible to perform either the mechanical operation described below or the electrical operation described below. Claim 2 will be explained below.
This means that the frequency data is sent to the programmable frequency divider 63 that determines the oscillation frequency of the PLL controlled oscillator 6 according to claim 1.
In the configuration supplied from Unit) 8 (Currently, many communication devices use a CPU for storage, calculation, and control, and using this function, the PLL frequency data setting is executed by interrupt operation. ) The CPU 8 stores in advance the same value as the frequency setting data for the frequency at which the cross spurious beat occurs, and stores both data values (normally, all digits are not required; (depending on the actual situation of the beat generation range, only 10kHz digits or 100kHz digits) are matched, the CPU 8 outputs a shift signal, and this is passed through a simple D/A converter 9 as a control voltage 91, for example, a voltage Control VXO
(Variable frequency crystal oscillator) In addition to the voltage controlled variable capacitance diode 72 connected in parallel or series with the crystal piece 71 of 7, by performing a predetermined frequency shift, it is possible to prevent the occurrence of cross spurious beats. This is a communication circuit. In the circuits of Figures 1 and 3, the front mixer 2 is
The PLL controlled oscillator 6 and the post mixer 4 are directly supplied from the VXO 7, but by reversing this, the pre mixer 2 is supplied to the VXO, the post mixer 4 is supplied to the PLL controlled oscillator 6.
The present invention also works even if the water is supplied from the source. Alternatively, you can insert a frequency converter into the injection circuit in the latter stage, or use PLL-controlled oscillators in both the former and later stages.
The present invention is similarly applicable to any drift cancel frequency. [Effects of the Invention] According to the present invention, one of the local oscillators of the front-stage mixer and the rear-stage mixer is a PLL oscillator, and the frequency of the same common oscillator is minutely frequency-variably controlled, making it possible to eliminate cross spurious beats. Therefore, the practical effect is great.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a communication device circuit embodying the present invention, FIG. 2 is a VXO circuit diagram, and FIG. 3 is a block diagram showing an embodiment of the present invention. 1...Antenna, 2...Pre-stage mixer, 3, 5...
... Band pass filter, 4 ... Post-stage mixer, 6
...PLL controlled oscillator, 61...VCO, 62...
Internal mixer, 63...Programmable frequency divider, 6
4... Phase comparator, 65... Low pass filter, 7... Common oscillator, 70... Transistor,
71... Oscillation crystal piece, 72... Voltage controlled variable capacitance diode, 8... CPU, 9... A/D converter,
_L1 , _L2 ...Local oscillation frequency, _R ...Reference frequency.

Claims (1)

[Claims] 1. At least one of the local oscillators of the front-stage mixer and the rear-stage mixer is configured with a PLL oscillator, and the frequency is controlled by the same common oscillator, so that fluctuations in the output frequency of the front-stage mixer due to fluctuations in the common oscillator frequency are suppressed. In a communication device configured to perform compensation using a rear-stage mixer, the common oscillator frequency must be moved within the margin frequency of the intermediate frequency passband between the front-stage mixer and the rear-stage mixer at the operating frequency that causes cross spurious beats in the PLL oscillator output. A communication device circuit characterized in that the generation of cross spurious beats is prevented by. 2. At least one of the local oscillators of the front-stage mixer and the rear-stage mixer is configured with a PLL oscillator, and the frequency is controlled by the same common oscillator, and the fluctuation in the output frequency of the front-stage mixer due to fluctuations in the common oscillator frequency is compensated for by the rear-stage mixer. In the communication device configured as above, the frequency setting data of the PLL oscillator is
The CPU outputs a frequency shift signal in a frequency band that includes the operating frequency that causes cross spurious beats in the PLL oscillator output, and if necessary, the frequency shift signal is converted into an analog voltage through a D/A converter. 2. The communication device circuit according to claim 1, wherein the occurrence of cross spurious beats is prevented by applying the voltage to a voltage controlled variable capacitance diode provided in the common oscillator circuit as a variation.