Preamplifier for RF Sweep Generator

The RF sweep frequency generator (‘wobbu-lator’) published in the October 2008 difficulty of Elektor has a receiver option that enables the instrument for use as an instantaneous conversion receiver. This receiver does alternatively have a noise floor of best –80 dBm, which in reality will need to have been –-107 dBm to obtain a sensitivity of 1 µV. So, for a just proper receiver sommore acquire is required. A wideband amplifie however, generates a number of additional noisas well and as a consequence is not going to resuin a lot of an growth.  As an test, the author developed a make a selectionive receiver with a bandwidth of about four MHz. Because a achieve of as a minimum 35 dB is required, the preamplifier consists of two amplifying elements. 

The enter amplifier is designed round a dual gate MOSFET, sort BF982. This element produces moderately little noise but pro-vides various achieve. The output stage makes use of a BFR91A for some further achieve. Preamplifiers where both the gate and the drain are tuned steadily fight with comments by means of their  inside capacitance. Here, the drain circuit has a rather low impedance, which prevents this from taking place. In the prototype that used to be tested, the input and output are located at proper perspectives with appreciate to one another to stop inductive coupling (see photo). Despite the high achieve, the amplifier used to be completely steady even with none defensive.  The two air-cored coils in the circuit each encompass four flips and have an interior  diameter  of  6 mm,  constructed from 1-mm diameter silvered copper wire and with a tap after 1 flip.

Circuit diagram :

Preamplifier for RF Sweep Generator Circuit Diagram

 

The amplifier is principally intended for the one hundred forty four MHz beginner band, however with different coils can also be used for the FM broadcast band, as an instance. FM detection is finished via tuning near the brink of the IF filter. At an offset of 15 kHz this is only a few dB lower than on the centre of the pass-band, so that damping just isn't no longericeable. The measured sensitivity within the 2 m band used to be about 1 µV (6 dB).A good antenna always make a contributions to the reception, after all. A wideband (scanner) outside antenna will provide excellent end results. Adding this wobbulator/receiver possibility ends up in a nice reveal receiver. By environment the scan frequencies of the spectrum analyser to a hundred and forty four and 146 MHz (or 148 MHz the place applicable), any signal within this vary is in an instant visible. When a signal is detected it's basically a case of clicking the scan stop button and then clicking on the sign within the display window the usage of the right mouse howeverton. 

After this, the receiver switches instantly to this frequency and that you presumably can be ready to take heed to the signal. You can due to this fact resume the scanning so that that you could be ready to proceed to look for different alerts. For narrowband FM detection you want to make a choice the FMN howeverton in the window for the receiver and this then gives the required offset for the threshold detection at 25 kHz bandwidth. This value is adjustable by the use of the ‘setting’ menu (default is 12,500 Hz) and might be modified experimentally for best possible end results. To energy the circuit you will must use a 9-V battery. It can additionally be possible to power the amplifier immediately from the RF sweep generator, if output capacitor C6 is changed with a link; in the ‘options’ menu you're going to then must make a choice the choice ‘use probe’.

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