2.4GHz ISM Spectrum Analyzer Monitor

Iulian Rosu, YO3DAC / VA3IUL, http://www.qsl.net/va3iul/

        2.4GHz ISM Band became very crowded, and a sensitive Spectrum Analyzer Monitor is necessarily to view the activity in the band.

Just plug an omnidirectional antenna to the input of the analyzer and you can see who is transmitting.

The signal from the input is amplified by the Low Noise Amplifier (Q1, BFP740, Digi-Key) and filtered by the 2.4GHz Band Pass Filter, with central frequency on 2.45GHz. The Noise Figure of the LNA is about 0.8dB and the gain is 19dB. For simplicity this stage can be replaced with a monolithic circuit ERA-5, at the expense of some degradation on the analyzer Noise Floor.

The 2.4 GHz Band Pass Filter was made in micro-strip topology and provides approximately 40dB rejection for image frequency. The micro-strip BPF could be replaced by an already made filter Helical, SAW, BAW on the same frequency. There are plenty of offers for filters on this band from different manufacturers.

The mixer MAX2682 (Digi-Key) is used to downconvert the 2.4 GHz frequency to the 500 MHz IF frequency, using the LO signal coming from the VCO (Q2, Q3, BFP420, Digi-Key). The VCO is DC supplied with a differential +/-5V power supply. Using a switch (S1) could provide for tuning a manual frequency sweep (using a 100k multi-turn potentiometer). A Low Pass Filter and a 6dB pad attenuator are placed at the output of the VCO and to the mixer input.

A 500 MHz Band Pass Filter is placed after the mixer. Is made also in micro-strip topology, or could be replaced by an existing Helical or SAW filter having the same central frequency. The IF frequency could be slightly different than 500MHz, but have to change the VCO frequency range concordantly.

Two monolithic amplifiers MAR-6 boost the IF signal to the input of the Log Amplifier AD8307 (Digi-Key). This Log Amplifier has 92dB dynamic range, good enough to accept different input levels. The DC output range of AD8307 is between 0.5V to 2.5V (25mV/dB), and an op-amp DC supplied at 12V is used to boost the DC level range, which drives the Y-input of the oscilloscope. Analog Devices recommend for gain improvement under narrow-band operation (as our case), to use a matching circuit at the input of the Log Amplifier. This was done using two caps and one inductor (pin 1 and pin 8 of AD8307).

The sweeper use a LM555 (Digi-Key) circuit in a configuration that generate a linear ramp voltage necessarily to sweep the VCO frequency. Also the circuit provides a blank signal for X-input of the oscilloscope.


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