SU107435A1 - Low Frequency Spectrum Analyzer - Google Patents

Description

Spectrum analyzers are widely used to investigate the harmonic composition of periodic signals, as well as the spectral density of p.

Spectrum analyzers are known, based on various selekghia principles. In some cases, a filter set is used, or an unrecoverable, iis filter. Such a snozd is inconvenient, since, firstly, the passband of the filters does not remain constant when the resopanity frequency is changed, and secondly, it is not possible to maintain a high resolution over the entire frequency spectrum.

High selectivity can be obtained using a synchronous detector, however, due to the fact that it reacts not only to the main (resonant) frequency, but also to odd harmonics, prilgenenie synchronous detector would require the introduction of a bandpass filter at its input Yun; their ashtaratura and its setting.

A widespread heterodes .ininn method of obtaining selection, allowing you, -1y highlight the same bandwidth in all

range of frequencies studied using one filter.

The main difficulty in the development of spectrum analyzers, based on the heterodyne method of selection, is inherent in creating a quartz fic. Ordinary quartz (millstones, in which quartz is neutralized and the capacitance is 1): the different types of windings of the differential transformer are notoriously difficult to fulfill, since the symmetry of the windings of the transzormator prior to 5 are very stringent requirements.

The proposed low-frequency spectrum analyzer, based on the use of a heterodyne selection method, is made, similarly to well-known devices of this type, as an attachment designed to work with a sound generator n with a lamp voltmeter and containing a frequency converter with a ring modulator and a two-layer quartz filter).

The peculiarity of the proposed analyzer, which ensures the simplification of nacTpoiiKH, is that each link of the filter is assembled by a single-tube inverse scheme, in

one shoulder of which is included quartz, and the other is a semi-variable capacitor.

The drawing shows a circuit diagram of the proposed low frequency spectrum analyzer.

The input signal through the decade voltage divider at the resistances Rj-R is fed to the input of the cathode follower L, which matches the signal source with the input of the pre-filter PF. The filter evenly passes frequencies from O to 8.5 kg ,. The maximum attenuation of the prefilter is at a resonance frequency of a quartz filter {.12.5 / SHO.

Thus, the quartz filter is protected from a direct impact on it of the frequency f ' which could have an effect when measured at all frequencies. The second cathode follower L. serves to match the output impedance of the PF filter to the modulator input.

The modulator M is assembled according to a ring circuit, which, as is well known, contains the minimum number of combination frequencies at the output and drastically reduces the possibility of frequency and reference frequency entering the input of a quartz filter. The bridge is balanced using potentiometers Nb and Re, the last of which is included in the gap of the two primary transformer half windings T.

A quartz filter consists of two links assembled in a single-tube inverse pattern. In the cathode shoulders of the lamp (L -first link and ./G-second link) are included quartz Kv and Kv-2, respectively, in their anode shoulders, semi-variable capacitors C, and respectively Cz (with a capacity of 15 pf}, neutralizing the parasitic quartz capacitance. the frequency of the quartz in the first and second links of the filter differs by, 5 kHz, which results in obtaining a frequency response with steep fronts and a flat top, with an equivalent bandwidth of 4 Hz.

By accurately neutralizing the attenuation of the signal in the non-transmission region with respect to the resonant frequency, at least 50 dB per filter link is obtained.

A quartz filter assembled according to an inverse scheme is much easier to set up than common filters with a differential transformer. Between the quartz filter and the indicator input, a lamp voltmeter ./Zj, a resonant amplifier RU is switched on, compensating for the attenuation of the signal in the preceding blocks.

As a reference oscillator (local oscillator), a sound generator with a higher frequency accuracy, such as the ZG-2A type, can be used. The anode circuit is powered by unstabilized DC voltage. B.

A switch (7 on two positions) and 2 (graduation-operation) in position 1 (graduation) allows you to accurately set the zero frequency of the local oscillator (2.5 Hz), from which the frequency count starts.

Subject invention

A low-frequency spectrum analyzer based on the use of a heterodyne selection method and made in the form of an attachment designed to work with a sound generator and a lamp voltmeter and contains a frequency converter with a ring modulator and a two-layer quartz filter, different in that, in order to simplify tuning Each filter link is assembled in a single-tube inverse scheme, in one arm of which is included quartz and in the other half-variable capacitor.