Sunday, May 28, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.18

The Final Filter

6/3/2017 ~ A definition of Covfefe: Running the same exact test on the same piece of crap will change the outcome!
My question is why is there only about 25 dB difference is the peak response and the rejection? The bandwidth looks about 2.2 kHz but the ultimate rejection looks poor. I would feel better about the results if there was say a 45 dB difference in the peak response. The shape looks fairly flat but not within 0.2 dB as predicted by Dishal for a 127 Ohm termination. I can only reflect that what we have going here with the Dishal approach is much like the touted Gas mileage is indeed YMMV!
Pete N6QW

I have received a private email from someone who knows and they have shared that my disappointing results with the Dishal Filter possibly is linked to Covfefe. I surely hope not but this malady may be affecting a lot of electronic failures from now on and for the foreseeable future.
Shown below is a second data run which is not inconsistent with the 1st run. The results are disappointing. I wouldn't call this a very good filter.

Pete N6QW

The filter is built and this may be the last one I ever build. Talk about wasted man hours. I will now begin some characterizations tests. Getting to the precision caps was a lot of effort and without the AADE LC meter this would have been impossible. I have added a plot of the response and now need to go back and do some fine tuning  of the caps.

To use a crass common vernacular term --this was a real "ball buster"! I am not satisfied with the result. This was a lot of effort for a curve that looks no better than picking 6 close crystals and using 150 PF caps in a ladder configuration. One has to ask why go through the agony?
Pete N6QW

The above plot is with the data converted to dBm. No resemblance to the Dishal curve but we now have a baseline for "Twizzling" the caps. I am not all that hopeful that this will dramatically improve.
The BW looks like about 2.4 kHz but not enough data points to look at the 60 dB points. I only scanned about 5 kHz at every 100 Hz point. More points and maybe at 50 Hz increments would have further defined the curve. But this is a starting point.

I am so glad that my brain works while I sleep as that often keeps me out of trouble or at least may minimize some downstream trouble.
When I awoke the other morning I got a message from my brain about the effect of stray capacitance and the design of my compact board. The pads as cut are 0.5 inches by 0.5 inches and there is a pad to pad capacitance as well as a pad to ground plane capacitance. So I took my handy AADE LC meter and measured away. Boom the pad to pad capacitance is in the range of 3 PF and fairly consistent from pad to pad and the pad to ground is higher in the range of 4 PF. So those values are in parallel with what I am soldering to the board. Thus I need to re-look at the values I have chosen for the caps and this may negate the need to have trimmers installed.
It may take a day or two but this is a precision crystal filter and thus will need to rethink the install. There has also been traffic on the illuminati reflectors about capacitor Q and how this impacts the use of caps in various tuned circuits. I purchased precision MLCC 1% caps so I am not starting in a deep hole for those who will be quick to email me about the Q of capacitors. I have been advised accordingly so am aware of this concern. Thanks Greg.
Pete N6QW

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