1. Field of the Invention
The present invention relates to a frequency modulating device feasible for, e.g. the modulation of radio frequency, and a modulated frequency measuring method feasible for, e.g. the adjustment of a modulation frequency to be set in the frequency modulating device.
2. Description of the Background Art
Generally, a frequency modulating circuit includes a frequency divider, a first and a second band-pass filter, a first and a second frequency-to-voltage converter or detector, and a controller, as taught in, e.g. Japanese patent laid-open publication No. 2000-131359. In the frequency modulating circuit, a signal modulated by a frequency modulator is fed to the frequency divider on a branch path. The frequency divider divides the frequency of the input signal by a number N and delivers the resulting signal to the first and second band-pass filters.
The first band-pass filter divides a frequency equal to the central frequency of its pass band by N, i.e. frequency/N, to output it as a carrier frequency for a frequency modulated signal. The second band-pass filter passes only the frequency of a modulated wave (first sideband) appearing around the frequency which is equal to 1/N of the carrier frequency.
The outputs of the first and second band-pass filters are input to the first and second frequency-to-voltage converters, respectively. The first frequency-to-voltage converter converts the amplitude of the input carrier to a corresponding DC voltage and feeds the DC voltage to the controller. Likewise, the second frequency-to-voltage converter outputs a DC voltage corresponding to the amplitude of the input modulated wave and feeds the DC voltage to the controller. The controller produces a modulation index from a ratio between the two input voltage signals and generates a control signal such that the modulation index coincides with a number equal to 1/N of a preselected value. The first and second band-pass filters, first and second frequency-to-voltage converters and controller are the major components that contribute to the measurement of a modulation factor.
The two output voltage signals mentioned above are respectively proportional to the carrier frequency not subjected to frequency division and the modulation wave frequency not subjected to frequency division. A modulated frequency can be produced from a difference between the two output voltage signals. The frequency modulating circuit therefore compares the difference between the voltage signals with a preselected reference voltage for thereby adjusting the modulation frequency.
The conventional frequency modulating circuit has the following problems left unsolved. Generally, when the circuit is integrated into a semiconductor device, the first and second band-pass filters occupy a substantial area each on a semiconductor chip, making it difficult to reduce the total area to be allocated to the circuit by integration. However, when priority is given to circuit integration and the reduction of the total area, a circuit for the measurement of a modulation factor may not be integrated but excluded from the frequency modulating circuit integrated on the chip. This successfully reduces the total area to be occupied by the frequency modulating circuit on the chip.
However, measuring with a conventional tester a modulation factor of a frequency modulating circuit integrated on a semiconductor wafer including no measuring circuit is far higher in cost than measuring it with a measuring circuit included in the frequency modulating circuit. This is particularly true when it comes to a radio circuit that directly deals with a high-frequency signal. More specifically, a transmitter system included in a radio circuit is usually used to measure a modulated frequency and sends its output to a spectrum analyzer, allowing the spectrum analyzer to directly measure a modulated frequency. When a high-frequency signal is measured on the wafer including no measuring circuit, the entire route extending from a contact pad provided in the integrated circuit to a tester via a probe and a probe card must be provided with a special measure for protecting a high-frequency signal against disturbance or noise. Such a measuring system needs disproportionate costs.