Update to Yesterdays update - Double sided PCB and Giga Hertz frequencies.

Well I was getting very weird results yesterday.......

........ but today I now understand them better!


I am used to using high frequency that is in this case  > 1 Ghz kit! I am not however used to building it! Well I have learned a new lesson for me!  Here am I trying to build a low pass filer for around 150 Mhz, I didn't think I needed to learn construction for >1 GHz, Wrong!

The filters I was building were exactly as planned and I was using double sided board with the lower side being a pure ground plane. However there behavior above 3-400 Mhz was wrong, and there behaviour at 1.2Ghz was just silly (2db attenuation when I expeced over 40db down!).

What I had not thought was that my Top layer ground fill was contiguous but greatly narrowed in places by the other components. I had made a series of resonators working in the Gigahertz ranges. I had built tuned slots and all sorts of things that were just not as they should be.

These boards are self designed and self fabricated. I had forgotten to put any vias between the top and bottom layers except at the ends where the SMA sockets connected the sides to each other.

Pick up drill, drill a number of via holes in sensible places. Insert copper wire in vias solder both ends trim off excess wire and re test.

Now my board is behaving with all below -40db above the 300Mhz level right up to 2.7Ghz. Most below 65db down and I could probably get the hump at 1.4G down if I added some more vias.


Lesson learned I may think I'm playing with 2M radio, that doesn't stop my boards playing up above 1Ghz. Grounding is critical watch how well things are grounded, and use vias!

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Peter

Update on Crystal measurement

I am making progress.  I have been playing with a resistive fitting to do the impedance matching and following that by a small two transistor post amp to bring the signal back into a measurable range.

It's looking promising.

I am also experimenting with producing PCB's by toner transfer and that is looking promising as well using SMT components. I have started on building filters with SMT components and that is also promising but I have a lot to learn. I have built a 148Mhz low pass filer and it isn't quite right yet. It's come down to 140 Mhz. I think that's due to the components I have being only 5% and almost all the capacitors on the high side (or is that just my SMD device measuring tools?) Of course stray capacitance is present in all PCB's (and stray inductance as well!) I'm not yet good at compensating for that.

The worst part is I have a lot to learn about using SMD Inductors! The filer I built looked usable up to about 240Mhz then the attenuation rises and above 500Mhz I would rather not comment about but at some frequencies between 500Mhz and 2.7G (As high as my measuring gear go's) the attenuation was under 10db! As I say I have a lot to learn.

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Peter