Narrowband sweep source. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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A circuit containing a low-frequency oscillator and a balanced modulator can produce a sweep frequency of 10,7 MHz ± 20 kHz, which is convenient when setting up the intermediate frequency stages in a standard FM receiver. A narrow-band sweep source is preferable in cases where the frequency response of the stage under test is observed on the oscilloscope screen: the image is stable, which is impossible when using a wide-band sweep generator. The frequency sweep range of the described circuit is 2,5 times narrower than that of a commercially available swept frequency generator. This reduces the spurious frequency modulation to a level where it has no noticeable effect.
As can be seen from fig. 1, a 10,05 MHz signal from a crystal oscillator is mixed with a 650 kHz center frequency signal from a low frequency sweeper. The mixer outputs a signal with an average frequency of 10,7 MHz, which can be changed within ±20 kHz by tuning the 650-kHz oscillator. This method of sweeping is preferable to tuning the high frequency oscillator as it gives better frequency stability.
Rice. 1 (click to enlarge)
To tune the swept frequency generator, a varactor is used, to which a sinusoidal control signal of 2 V eff is applied. at a frequency of 10 Hz. The frequency of the control signal can be increased, but if it exceeds 100 Hz. the settling time of the circuit under test can create limitations in observing its frequency response. Reducing the amplitude of the sinusoidal signal will narrow the frequency sweep range, but in fact this effect will be negligible, since the usual amplitude of the sinusoidal signal is quite sufficient to control the varactor.
At the output of the balanced mixer, a frequency signal of 10,7 ± 0,020 MHz operates. Other frequency components generated during the modulation process (mainly the fundamental harmonics) can make it difficult to obtain a stable image on the oscilloscope screen. A bandpass filter of 10,7 MHz suppresses these components, after which the signal is applied to the circuit under test (Fig. 2).
Fig. 2
In the stages of the intermediate frequency amplifier (which, in fact, are tested), the amplitude of the output voltage is a function of the frequency of the input signal. If it is necessary to oscilloscope the frequency response of the stage with sufficient accuracy, then the output voltage must be converted to a DC signal. This conversion is performed by the amplitude value detector, which consists of a rectifier and an integrating circuit; in this case, a commercially available XD-3A unit from Telonic was used for this. Next, the received DC signal is fed to the input of the vertical amplifier of the oscilloscope, and a sinusoidal voltage is applied to the input of the horizontal amplifier, which controls the oscillator.
As a result, the frequency response of the tested stage can be observed on the oscilloscope screen. The image is stable and fairly accurate because the spurious frequency modulation from the narrowband sweep generator is minimal and therefore the detector response does not change with each sweep cycle.
Author: J. Isbell; Publication: N. Bolshakov, rf.atnn.ru
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