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 Post subject: SDR Transceiver Idea with 2n7000
PostPosted: Jul Sun 11, 2021 12:53 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 732
Another SDR radio with mosfet switch.
It will only cover 40m band since CD4013's upper frequency is 16MHz. A 74AC74 can be used. The 2n7000 mosfet will switch just fine with either 5V or 12V square wave. Audio connections of 0 and 90 degrees can be reversed to select LSB/USB. No center tap output transformer at the drains of 2n7000 as I don't think there will be any significant carrier leakage since LO signal is flowing from gate to source and then to ground via a 1n capacitor. Add bidirectional output amplifier and linear amplifier and you've got yourself a homebrewed SDR SSB transceiver! The SSB RF output will be quite low due to the 1K audio resistors. It's necessary to keep the audio signal flowing through 2n7000 small to limit distortion. Only one mosfet is turned on at any given time. The 100ohm pot is for RF balance and ensures 50ohm impedance. It should be set in the middle for good carrier suppression.


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 Post subject: Re: SDR Transceiver Idea with 2n7000
PostPosted: Jul Sun 11, 2021 8:30 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 732
With software, sideband suppression of 70dB can be obtained.
This design alone should give 37dB suppression of the opposite sideband. All it needs is NE5532 AF interface and RF preamp at its output.


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 Post subject: Re: SDR Transceiver Idea with 2n7000
PostPosted: Jul Fri 30, 2021 7:36 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 732
The AF needs to be coupled via DC blocking capacitor to the source of 2n7000. RF bypass cap from source to ground is also needed. It should provide a clean SSB output.


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