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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Mar 2019, 12:38 pm   #1
Jolly 7
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Default 2N3819 MW mixer circuit problem

Here is a jfet mixer circuit I am trying to use as a building block for an AM radio. I have built a separate local oscillator circuit that is working fine but the issue is that as soon as I connect it to the mixer circuit, the oscillator stops oscillating down from 3 MHz to zero.

Can anyone please advise what the issue could be ?
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Old 13th Mar 2019, 12:51 pm   #2
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Default Re: 2N3819 MW mixer circuit problem

If the oscillator stops working as soon as you connect it, I would suspect an oscillator problem rather than this circuit.

Can you post the oscillator schematic as well?
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Old 13th Mar 2019, 12:53 pm   #3
Radio Wrangler
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Default Re: 2N3819 MW mixer circuit problem

The local oscillator, whatever it is, will see a rather low impedance ( 220 ohms//1/Gfs) and it all depends on the oscillator circuit and its drive capabilities. You need an oscillator or buffer compatible with that load.

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Old 13th Mar 2019, 1:24 pm   #4
MrBungle
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Default Re: 2N3819 MW mixer circuit problem

Agree. stuff another 2n3819 follower after the oscillator. I usually have one as the oscillator, then a buffer 2n3819 and then a 2n3904 "power amplifier" but I'm driving diode mixers which require a bit more grunt.
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Old 13th Mar 2019, 1:33 pm   #5
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Default Re: 2N3819 MW mixer circuit problem

Here is a circuit that sounds similar to what your are trying to do.

Ignore the frequencies it is set up for, they can be changed.
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Old 14th Mar 2019, 5:06 pm   #6
Jolly 7
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Default Re: 2N3819 MW mixer circuit problem

Quote:
Originally Posted by MrBungle View Post
Agree. stuff another 2n3819 follower after the oscillator. I usually have one as the oscillator, then a buffer 2n3819 and then a 2n3904 "power amplifier" but I'm driving diode mixers which require a bit more grunt.
Thanks for the 2N3819 buffer suggestion. For the buffer circuit to work, the LO signal needs to be fed to the gate and the output frequency taken from the buffer source terminal before it is passed on to the source terminal of the 2N3819 mixer circuit, as far as I can make out.

The buffer circuit has not killed my oscillator so I seem to be heading in the right direction.

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Old 6th Apr 2019, 5:49 am   #7
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Default Re: 2N3819 MW mixer circuit problem

Here is a dual JFET mixer that may be of interest:

https://zl2ctm.blogspot.com/2017/09/...iguration.html
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