RF adapter for oscilloscope. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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A simple device, the circuit of which is shown in Fig. 1, makes it possible to observe a high-frequency oscillation form on the screen of a low-frequency oscilloscope.
The prefix is essentially a direct conversion receiver. converting the original high-frequency oscillations into a relatively low intermediate frequency, the value of which lies within the bandwidth of the oscilloscope. The local oscillator of the attachment is made on transistors V1 (generator itself), V3 (buffer amplifier) and V4 (emitter follower). It does not have any design features. It should only be noted that the use of two buffer stages in combination with a resistive generator RF voltage attenuator (resistor R3 - input resistance of the stage on transistor V3) makes it possible to avoid distortion of the oscillogram of the signal under study due to parasitic frequency modulation of the generator frequency by this signal.
The ratings of the frequency-determining elements indicated in the diagram correspond to a local oscillator frequency of 25 MHz, which allows. for example, to observe on the screen of an oscilloscope with a bandwidth of up to 5 MHz the form of high-frequency oscillations of signals with a frequency of 20 ... 30 MHz. Mixer U1 is a conventional diode ring mixer, its circuit is shown in fig. 2.
The signal under study is fed through a high-frequency broadband transformer T1 n through an additional resistive attenuator to the input of the mixer.
When setting up the device, it is necessary to take its amplitude characteristic from the input signal and thereby find the maximum value of the signal under study that can be applied to the attachment. With an Ul type SRA1 mixer, this level (at pin 1) reached -3 dBm, i.e. 160 mV (50 Ω mixer input impedance),
The T1 transformer is made on an FT-37-75 ferrite ring with an outer diameter of 9,6 mm. The primary winding is the central core of the coaxial cable, passed through the ring, and the secondary contains 31 turns and is made with a wire with a diameter of 0,3 mm. It is evenly spaced around the perimeter of the ring. Such a transformer attenuates the investigated signal by about 30 dB.
The total attenuation of the signal under study (taking into account the resistive attenuator) is 50 dB, which makes it possible, for example, to analyze the signal of amateur station transmitters with a power of up to 50 W. The bandwidth of the transformer is from 0,5 to 100 MHz.
The loss in the mixer is about 10 dB, so the maximum signal level entering the oscilloscope will be (depending on the parameters of a particular mixer instance) 20 ... 50 mV. therefore, the oscilloscope must have the appropriate sensitivity.
Domestic semiconductor devices can be used in the device: KP303V {VI}. KD503B (V2 in Fig. 1), KT325, KT355, KT368 (V3. V4) and KD503B (VI-V4 in Fig. 2). Transformers T1. T'2 (Fig. 2) of the annular mixer can be made on ferrite rings of size K10x x5x3 with a magnetic permeability of 50...100. The winding data can be borrowed from the description of a similar unit of the Radio-76 transceiver, published in Radio 1976, N 6, 7. The same magnetic circuit can also be used for the T1 transformer in fig. 1.
Literature
- QST (USA), February 1982
Publication: N. Bolshakov, rf.atnn.ru
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