FREQUENCY FINDING SEARCH METHOD

Principally there are two main frequency finding methods: search and non-search.

 

Non-search method allows find main frequency almost instantly but as for search method it requires some time because of RCV resetting need. Frequency finding non-search method allows highly reduce intelligence time, but such intelligence time reducing is possible because of accuracy and calculation resolution capability decreasing or devices quantity increasing. But as for search method because of high intelligence time it allows find main frequency with high accuracy and provides high calculation resolution capability.

Frequency finding search method is usually realized in so-called panoramic RCV with block-scheme of the pic. 1.

Panoramic RCV in the easiest case can be presented as a super-heterodyne resetting automatically or manually in intelligent frequencies range.

During frequency searching process RCV resetting is made with help of resetting program defining synthesizer output signal frequency.

Received signal after amplification in intermediate-frequency amplifier (IFA) and detection is sending to the indicator together with mark of synthesizer instantaneous frequency defining RCV set frequency value.

Main frequency f0 searching time ts (intelligence time) is panoramic RCV important characteristic.

Usually all working frequencies range review is made periodically with period Ts in serrated way (pic. 2). That’s why during continuous signal main frequency f0 intelligence maximum searching time ts is less than Ts. But short signal main frequency f0 intelligence is more difficult. This intelligence visual performance is shown by frequency intelligence frequency-time diagram.

As it shown at the pic. 2, continuous signal of fc frequency detecting with 100% probability as well as impulse signal detecting and frequency finding can be impossible sometimes.

In general case impulse signal detecting and frequency finding process has probabilistic character. In dependence of relation RCV resetting period Trcv and intelligent device signal impulses following period Ts there are three frequency finding search methods:

•      slow searching;

•      fast searching;

 •      searching with average speed.

 During slow searching RCV resetting period Trcv on its bandwidth is higher than intelligent device signal impulses following period Ti (pic. 2), other words Trcv>Ti .

If frequency finding can be made by one impulse slow searching provides periodic impulse signal detecting probability of Pdet=1 during RCV resetting period Trcv. Slow searching great disadvantage is high service time, low capacity and short-working intelligent device low intelligence probability.

For intelligence time ts decreasing with defined resetting range and speed (Δfres and γ) it’s necessary to widen RCV bandwidth Δfrcv. That’s why panoramic RCV with slow searching has wide bandwidth. Such RCV bandwidth Δfrcv is nearly equal to

Δfrcv=(0.1…0.01)Δfres (1), where Δfres is a resetting range (intelligent frequencies range).

 Main frequency f0 finding accuracy δf with help of such RCV is not high. This accuracy is nearly equal to RCV bandwidth half Δfrcv, other words

δfmax=0.5Δfrcv=(0.05…0.005)Δfres (2).

 RCV with slow searching sensitivity a because of great bandwidth Δfrcv can’t be high. This RCV is often made as scheme of direct amplification with resetting input circuits. Guarantied detection time tg of slow searching method is defined by RCV resetting period Trcv as tg=Trcv.

During fast searching RCV resetting period Trcv on its bandwidth is lower than intelligent device signal impulses following period Ti (pic. 2), other words Trcv

This case resetting speed γ can be really high (hundreds and thousands megahertz in microsecond). Such speed can be providing only by electronic methods [1].

Resetting speed γ is limited by allowed sensitivity a, accuracy δf and resolution capability decreasing limits during frequency finding and these limits depend on inertial resonance devices transition processes duration.

Resonance devices after signals with changing frequency influence are characterized by dynamic frequency characteristic means relation of output to input voltage relation from system own resonance frequency mismatch in case of defined resetting speed γ.

Dynamic characteristic depends on resonance system parameters (first of all on static bandwidth Δfrcv) as well as on resetting speed or external signal frequency speed changing γ. At the pic. 3 single oscillation circuit frequency characteristics family is shown [2] as the illustration of mentioned above.

Simultaneous high resetting speed γ and high frequency resolution capability providing can be successfully achieved in RCV with impulses compression [1]. Here the same increasing resolution capability principle as in wide bandwidth RLS with inter-impulse modulation is used. Except slow and fast searching, searching with average speed (probabilistic searching) is possible to use. This frequency finding search method provides the best compromise conditions between resetting speed γ and frequency finding accuracy δf in case of defined intelligence probability.

Sources.

 

1.     Вакин С. А., Шустов Л. Н.. Основы радиопротиводействия и радиотехнической разведки. М.: Сов. радио, 1968.

2.     Мартынов В. А., Селихов Ю. И.. Панорамные приемники и анализаторы спектра / под ред. Г. Д. Заварина. 2-е изд., перераб. и доп. М.: Сов. радио, 1980.

3.     Основной источник для адаптированного перевода. Радиоэлектронная борьба. Основы теории / А. И. Куприянов, Л. Н. Шустов. М.: Вузовская книга, 2011. – 800с.: ил.