Sunday, January 16, 2011

500 KHz bandpass filter and preamp

I made a bandfilter for 500 KHz. I have also added a preamp as it turned out the filter has 6 db attenuation.
The preamp has to compensate for the loss. It also has to compensate for the loss in the mixer.
It seems to work fine but I have to test the noise behavior. Gain is 6db, the -6db band width is 10 KHz.

I have several 455 KHz If transformers. They can be tuned to 500 KHz easily. However matching to 50 Ohm is difficult. I also tried filters using FT50-43 cores but that was a bad idea. The -6db bandwidth was 100 KHz.  Matching to 50 Ohm was OK though. So I use one FT50-43 filter to match the impedance of the IF transformers.

Alternatively I could have used air coils. Then the filter would be bulky.

The transistor get a bit hot I will keep an eye on it. I may have to use a heat sink. 

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