Transceiver Architecture ~ 2.17
[For more info on the LM373 SSB Transceiver see http://www.n6qw.com/LM373.html)
The Crystal Filter Build Out
5/25/2017 ~ Where "Noodling" pays big dividends.
Yesterday I made a "linear layout" of the six pole crystal filter and the result was a series of 11 pads where the crystals and caps would be installed and that assembly was about 6 inches long. This is an unwieldy dimension to handle inside of a compact radio. Then I spotted a photo of an 8 pole filter as used in the Elecraft rigs and saw how they had compacted the build. So that led me to think a smaller footprint. I now have come up with a compact version that get the footprint down to something like 2 (or 3)" X 4" inches --much more accommodating. I also had concerns about signal leakage between the sections of the compact layout and think that can be fixed by the addition of a shield between the sections. See my "noodling" sketch below. (I are not an artist and barely an engineer!)
I have already created the design for the compact board in G Simple and have the dxf file ready to cut a board on my CNC Mill. I will add a photo of the cut board when I am done.
Noteworthy I have received an email about my capacitor selection process and a link to a video about polarity of capacitors and the stability of capacitors. Hopefully the extra $$$ I spent for the 1% caps and the NPO criteria will abate some of these well founded concerns. I will also exercise caution in NOT applying too much heat to the caps during the soldering process.
The caps arrived today and I also measured the 150 PF NPO caps I had in stock. One of values needed (4 places) was 151.2 PF. Using my AADE LC meter, I measured 13 caps and here are the results.
The results of this test shows that of the 9 caps we have nailed down 5 of them. The two 115.4 PF I think will be asy to achieve with the 100 PF NPO I bought and the 15 PF trimmer. The 430 PF that were to be the basis of the 437.1 PF are so small I can hardly see them so I may have to invoke a work around.
I also designed the PC board and it is about 5 inches long and 3 inches wide. There may be a way to compact this but for this 1st run it will be linear. Hope to cut the board tomorrow and start soldering in crystals and caps.
I have been awaiting the delivery of some 1% caps and they will be arriving in the next couple of days so in the meantime I will flesh out the circuit board for the crystal filter. So let us examine what is required. My Dishal filter for the 6 Crystal Filter has a total of nine capacitors and they are as follows:
- 2 X 151.2 PF
- 2 X 437.1 PF
- 2 X 115.4 PF
- 2 X 151.2 PF
- 1 X 156.8 PF
Noteworthy is that we really only have 4 values of capacitors including 4 caps of the same value. The precision caps I ordered are 100 PF, and 430 PF and these are 1% and NPO AND Surface Mount. I have a bag of 150 PF NPO caps which I will measure to find values all below 150 PF. (Hopefully finding four that are all below 150 PF but close in value. I have an AADE LC meter and that is my measurement tool.
The PC Board will require a total of 11 Pads to accommodate the crystals and the caps. To get me exactly on the values needed I will make the pads large so that there room for the 15PF air variable trimmer caps in addition to the fixed value ones and the crystals. Playing with these parts before I cut any pads is critical to the final assembly. Thus 437.1PF = 430 PF + 0-15 PF Trimmer
I will add to this posting with the layout of the crystal board after I get it laid out.