2N3819 MW mixer circuit problem - UK Vintage Radio Repair and Restoration Discussion Forum

Jolly 7 · 13th Mar 2019, 12:38 pm

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 ?

Terry_VK5TM · 13th Mar 2019, 12:51 pm

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?

Radio Wrangler · 13th Mar 2019, 12:53 pm

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.

David

MrBungle · 13th Mar 2019, 1:24 pm

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.

Terry_VK5TM · 13th Mar 2019, 1:33 pm

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.

Jolly 7 · 14th Mar 2019, 5:06 pm

Quote:

Originally Posted by MrBungle

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.

Terry_VK5TM · 6th Apr 2019, 5:49 am

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

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

13th Mar 2019, 12:38 pm	#1
Jolly 7 Octode Join Date: Feb 2018 Location: Southampton, Hampshire, UK. Posts: 1,061	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 ? Attached Thumbnails

13th Mar 2019, 12:51 pm	#2
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	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? __________________ Terry VK5TM https://www.vk5tm.com/

13th Mar 2019, 12:53 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	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. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

13th Mar 2019, 1:24 pm	#4
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	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.

13th Mar 2019, 1:33 pm	#5
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	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. Attached Thumbnails __________________ Terry VK5TM https://www.vk5tm.com/

6th Apr 2019, 5:49 am	#7
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	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 __________________ Terry VK5TM https://www.vk5tm.com/