Saturday, February 18, 2012

Airband BITX AM demodulator

It's all in one.... the airband RF front-end with BITX components (2SC2570) and the AM demodulator from a 10Mhz IF, also using BITX components.

I used a mix of 2n2222 and 2n3904... whatever came first out of the transistors box...

It's a first test, I'm thinking in swapping the Ge detector diodes by 1N4148 as used on other parts of the original schematic and removing the 2 crystal filter after the first mixer by just one main filter with 6Khz bandwidth. In this schematic topology the VFO needs to be very accurate since the signal get's rapidly outside the filters pass-band. That's why I'm using the Si570 VFO. The 2 crystal in the first filter were not matched, they were some spare crystals from the batch used in the BITX crystal filter.
Of course this is the first working version so use at your own risk, I just plubished it for future reference or case someone needs ideas... It took some testing to get to this although other versions I built in the process still worked but for bigger input signals and not the small ones I was getting in the front-end. In fact one of the detectors I build and with just one RF amplifier was getting the local FM station quite well......

Here's the front-end:
Another future idea is to get a crystal hundred kiloherts near the 10Mhz IF, add another mixing an then detect AM on the kiloherts range. But this adds the difficulty of finding crystals around 9.8, 9.9Mhz, at least for the BITX intended audience.

Have a nice weekend.

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