Wobbulator for Filter Characterization
Iulian Rosu YO3DAC / VA3IUL - http://www.qsl.net/va3iul/
Tuning the filters shape can be optimized by means of this simple and inexpensive unit which enables the filter response to be displayed on an oscilloscope screen.
A wobbulator is no more than a signal generator, the frequency of which is repeatedly swept over the passband of the filter under test.
There are three options to sweep the frequency:
1. Frequency variation is controlled by the timebase or horizontal output of the oscilloscope with which the unit is used. In this way the sweep is internally synchronized, and the oscilloscope displays the response curve of the filter.
2. Frequency variation is controlled by an external Sweeper which provides also the synchronization signal for the oscilloscope.
3. Frequency variation is manual tuned using a potentiometer. In this way frequency reading at specific points on the shape of the filter can be made.
Our circuit is using two VCO’s, one for 455kHz filters and one for 10.7MHz filters. The 10.7MHz VCO can be easily changed for measuring 10MHz or 9MHz filters.
To the existing two VCO’s can be added also a 145MHz VCO which is useful for visualizing the characteristic of the 2m band filters, notches, or duplexers used in repeaters.
The first VCO is used for 455kHz filter characterization and uses a JFET BF245A and two BB152 high capacitance ratio varactors (D1 and D2). The DC voltage need for sweeping the frequency is coming from sweeper which is made with 3 BJTs, Q5, Q6 and Q7. The span of the swept frequency is 20kHz, but this can be adjusted for narrower or wider bandwidth filters.
Placing the switch S1 in Manual Sweep mode, the frequency of the VCO can be measured for various positions of P3. The difference between two frequency readings at both ends of the potentiometer P3 represents the frequency span.
The sweeper provides also the synchronization signal for oscilloscope (X-horizontal input). The waveforms of the sweeper are shown below.
A second VCO is used for 10.7MHz filter characterization. The span of the swept frequency is also 20kHz, but can be adjusted for other filter bandwidths.
Each VCO is followed by a buffer amplifier which raises the oscillator signal up to a level easily detected by two Germanium diodes D6 and D7. Tr1 and Tr2 are ferrite can IF resonant transformers.
The detected signal is amplified by the op-amp U1 LM358, and its output is connected to the Y-vertical input of the oscilloscope.
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