I thought I'd both seen and worked on the SA/NE602/612 Mixer, crystal filter and LM386 AF amp design before for both TX and RX and I forgot where I used this first, then I remembered after gathering some bits together for the October GQRP Rally at Rishworth that Drew Diamond VK3XU designed a TCVR using exactly this. The circuit is above, with some filtering. It is dated 1994 which predates both of the others by 10 years!
In 2003 when I was M3KTA so for those that don't know that is a novice licencee in UK I did a talk at local radio club as all newcombers did and mine was about filters. The maths scares me silly and I could never calculate things correctly so I had homebrewed a VNA, the IV3HEV design had just been published and I obtained a load of sample chips from AD and TI to build it. I also needed a homebrew radio that showed all the filters I wanted to talk about and I could easily build and demo in the talk and the Drew Diamond design was it. On the night I had both a finished TCVR connected up to an antenna and the filter modules cloned on a 'fake' homebrew bench in the lecture. I explained to an audience that definitely knew heaps more than I did about filters and then proceeded to demonstrate the alignment etc using the VNA real time (projected the PC display onto a white board, next to a web camera display showing what I was altering) to show the filter response. The use of the 'real' TCVR with antenna meant I also was able to show some live feedback as the effect of the filters.
I remember some of the members suddenly perked up a bit and a lively discussion of the build and use of the VNA, that cost me maybe £10 to build ($15) started up, especially involving those that used big professional bits of kit with huge price tags. I also went on to show the N2PK VNA kit I had planned on using but was missing a few bits and I ended up ordering 4 sets of components for club members to build their own.
Obviously everyone knows the MiniVNA from IW3HEV now from WiMo etc but this was his first design.
With hindsight showed I was totally niave about amateur radio although I knew what I wanted to show in the talk so built the VNA and the TCVR and spare modules with no thought as to the apparent magnitide of what I had done.
Anyway, I won the club award for a talk that year hihi.
But I KNEW I'd seen the design and use somewhere else before.
Comment on the post.... Just made up an Express PCB mask and etched it, got to drill it yet for the RX side of this, with a modification where I added varicaps (Will start with MVAM109 but could use red LED) and a potentiometer which will make the crystal filter width adjustable. I have a few different IF crystal selections too and might start with 4.9152MHz and as I will use a FCC-1/FCC-2 I should be able to make this a little more frequency agile by adding a bypass to the BPF.
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