Friday, May 5, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.12

Let's Build That 11.5 MHz Crystal Filter!


Sunday May 7, 2017

 I have completed the first round of crystal measurements and have found several groupings of crystals that have potential for a filter. One group has nine crystals and my next step is to re-measure all 9 to verify that indeed that all have the same specifications. I used different colors to highlight the crystals with the same specs. See the table below:
Stay tuned for the next round of measurements.
Pete N6QW

See video at end added today on measuring the loaded crystal frequency --yes being done with a Raspberry Pi3 and a SoftRock V6.3 SDR radio.
My bag of 25 crystals arrived (May 4th) and in case you are wondering what I bought here is the Mouser Part Number (See below) The one thing nice about the manufacturers specifications is that the ESR (Equivalent Series Resistance) is provided and that number is 35 Ohms.(That could be good or bad but at least I know what they say it is,)




It is amazing how crystal filters are suddenly at the top of everyone's list. The man himself, W7ZOI, Wes Hayward on May 1, 2017 just released an exciting document that details the "innards" of our beloved crystals as used in filters. You can find that document here  Essentially Wes describes the crystal model and how today's crystals present some issues when we try to build crystal filters. Download this and keep it under your pillow at night.
In Transceiver Architecture 2.10, I mentioned that there are some excellent you tube videos on the subject of homebrewing crystal filters and w0qe, Larry Benko, in his part 13 and 14 has taken up that task. Part 13 deals with some of the front end analytical work and Part 14 gets to the nuts and bolts.

One tool used in Part 13 was the use of Elsie (free Student Version from Tonne Software) to simulate a crystal filter. The "free version" will only let you simulate a 4 crystal filter but that is good enough for our purposes. The professional version lets you add many more crystals.  Elsie will not design a filter for you --BUT it will let you "twizzle" the parameters so that you can visualize how changing parameters impact the shape of the filter curve and the SWR. Interestingly the Elsie parameters match closely what you will finally need for a good filter. The Zin/out looks like less than 200 Ohms and the coupling caps are in the range of around 100 PF.
Using the Elsie Software (with help from Jim Tonne, the developer) I was able to simulate what a crystal filter might look like using my 11.5 MHz filter frequency. If I could match this model in practice, we would have a pretty good second filter for our DifX Dual Conversion Transceiver. It is really interesting to see how changing the bandwidth and Zin/out materially affects the shape of the curve. My latest run at this suggests that the Zin/out should be about 100 Ohms and the bandwidth 2.8 KHz which of course will place me in the league of the enhanced SSB guys. But those numbers are not the final values --just data.The combined plot below shows the filter shape and the SWR. Don't overlook the SWR as that impacts the filter capability to pass signals without distortion!

In Part 14 Larry, presents a plan on how to go about actually building a filter and it starts out with "get a bag of lots of crystals" and then "measure them and record the results in an Excel Spreadsheet" -- hmmm sound familiar?

The Elsie Software presents an idealized filter with crystals that are exactly alike (you'd like to see to 10 decimal places).  But that is not the real world as the Elsie software only captures just a few decimal places-- like out of the box maybe two or three places. As filter builders we in essence TUNE each crystal so that it will match the Elsie model. This is not a five minute job! But it can be done. In Part 14 Larry, show how he kept changing parameters and how that fine tuned the filter. I suspect the first time through the process will be like being 7 years old and waiting for Christmas and it is only June.

My first step is to identify and then  measure each crystal and finally record the results. Fortunately long ago I built a G3URR oscillator and will use my Raspberry Pi SDR radio (with Quisk) as the measurement tool or I might use my Omnia with HDSDR --so I have some choice here. Or my new Rigol Scope that has a built in frequency counter.

But having just run some tests using the Rigol Scope  "counter function option" has shown that will not work. The reason is that the scope (or my lack of knowledge on how to have this do something different) only takes you to 4 decimal places. I ran a few crystal into to the G3URRR test oscillator and the scope counter read 11.4998. This is not enough resolution! They tutorials all sing the same song. If you use an external oscillator method to excite the crystal for measuring Lm and Cm then you need to have a generator with a 1 Hz resolution. So you will need to have something as good to measure the crystal response.

My friend KV4QB, DuWayne was so kind to send me an SNA Jr. board and it will produce the 1 Hz resolution for the DDS generator. I have an Omnia SDR and with HRSDR can read the signals to 1 Hz. Keep in mind we want no more than a 50 Hz spread for a SSB filter and as I found out no more than 10 Hz for a CW filter -- the Rigol scope will work to get you into the ball park but will not tell you the seat number.

w0qe suggested several actual crystal filter design programs (including the AADE one I mentioned). But did indicate several that were more user friendly and so I will look to using one of the ones suggested.

For now I will have to take a short respite while I measure the crystals and record the results. Stay tuned. Yesterday using my handy Brother tape label machine I punched out 25 numbers and will start first by affixing the numbers to each of the crystals. Then I will need to clear a spot off of my work bench so I can do the measuring. I may get a chance to video the crystal measuring process as that may be helpful t those wanting to build the 11.5 MHz filter.

Too many projects and not enough time --but I will continue to work on the LM373 transceiver and keep you posted on its progress.

Pete N6QW


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