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|Spectrum analyser 8MHz IF Filter|
|Written by Hans Summers|
|Wednesday, 16 September 2009 22:10|
I also tried cascading two of the BC109-transistor-plus-2-filters circuits but found the results unsatisfactory. The picture to the left shows the unused transistor module before removing the 2 filters for use in the AD603 circuit.
The circuit of the 2nd IF Filter at 8Mhz is shown below. Other IF frequencies would present no problem provided all the ceramic resonators are the same and the 2nd local oscillator frequency is adjusted accordingly. I believe ceramic resonators usually have input/output impedences of approximately 500 ohms so approximately correct termination is provided using 470 ohm resistors. If desired, the two rightermost ceramic filters and AD603 can be left out of the circuit. What we have here are the wild wanderings of the mind of a madman with too many identical ceramic filters to hand and most probably too little knowledge of how to use them properly. But the result works.
Again, note the extensive use of inductor and capacitor filtering of all DC supply and control signals entering the shielded enclosure, vergeing perhaps on the paranoia. A firm believer in the theory that you can never have too much decoupling and shielding.
Shown here is an analyser trace from 0 - 70MHz of the signal generator set to 30MHz and using input attenuation of 26dB. On the left, the IF gain is set to minimum (10dB), and on the right it is maximum (50dB). The improvement in sensitivity is only about 7 or 8 dB. Though the noise floor appears as a continuous band in the photograph (right) it is in fact a continuously fluctuating strip of tiny peaks constantly appearing and dissappearing.
With the 2nd filter in line between the 2nd mixer and logarithmic amplifier modules, the spectrum analyser will now show good selectivity (sharp peaks) and greater sensitivity than the first module hook-up made on completion of the 2nd mixer.