10/28/2018 ~ Questions about Si5351
This is not an Arduino/Si5351 but my Pasta dish!
Those are fresh cherry tomatoes in the sauce with
fresh Basil, loads of Garlic and the key ingredient
Virgin Olive Oil and of course Angel Hair Pasta
With the publication of the Sudden Transceiver project in the GQRP SPRAT #178 (for those who have received their SPRAT --some of us are still waiting) and are undertaking the project a couple of questions have arisen. One specific question is about "How do I build the Si5351 Oscillator and connect to the display". The Sudden project on my website has a section on building and wiring the Arduino and Display but for someone undertaking this as a first project I am sure it leaves a lot questions. Thus today's posting.
Firstly, having built quite a few rigs with the Si5351/Arduino (think close to three dozen) purely by accident you find some things that work well and some that are absolutely never do that again. So here are some "experience points" I will share with you.
- The are many variants of the Arduino including the Mega 2560, the Uno R2, the Nano R3 and the Pro-mini. My shack has transceivers containing at least one of these boards. Typically we are always trying to make our rigs compact and so the larger boards like the Mega 2560 and the Uno R3 are like Brown Shoes with a Tuxedo --not a good fit because of their size. The Mega 2560 however is a jewel when you need 10X the program space of the smaller units and a lot more IO then is when you need the mega. The Uno just is a weird shape and size. That leaves the Nano and the Pro-Mini. The Pro-Mini is physically smaller mainly because the USB to Serial functionality is not on the board. Thus when you program the Pro-Mini you need a special cable that has the USB to Serial interface built in. The Pro-Mini also lacks the 3.3 Volt on board regulator. So if you want to use the Pro-Mini with a color TFT that requires 3.3 VDC you need to add an external 3.3 VDC three terminal regulator (or a hokey voltage divider resistor bank). You now have added more space requirements. The Nano by far offers a the most capability in the smallest package. Having not only all of the features of the Uno R3 at about 25% of the size. Thus I have standardized on the Nano as the controller of choice. Incidentally it does have the onboard USB to Serial interface and the 3.3 VDC regulator.
- Boards. I have more or less standardized using perforated boards as a basis for the constructing the Arduino and Si5351. Now I have a standard layout which I use all of the time. Certainly you could come up with PC board and have a bunch made for you in China and that has been done by many. But small size standard perforated boards with solder pads can be found inexpensively from many suppliers. Some come with four mounting holes already installed in the corners. An important feature of my layout is that the arrangement enables me to gain access to the Arduino USB to Serial connector from the side of the chassis. You will always want to tweak the program and thus being able to easily plug into the controller is a real plus.
- The method used on my boards is to use sockets for the Arduino and the Si5351. The socket for the Arduino is two rows of 15 Pins each. From each socket side I then install a row of pin headers and make the connection from the socket pin to the pin header using #30 bare tinned wire (use wire wrap wire with the insulation removed. This arrangement now affords me access to all of the Arduino pins and I can simply install premade jumper wires to the pins. As an added step I also use an additional set of pin headers with plugs for items like the encoders. A four pin header enable connection to the two encoder pins (A/B) the Ground and the step rate push button. I use a two pin header to bring power to the board and from there is a 9 VDC 1amp regulator supplies all the power for the Arduino/Si5351 with the Arduino Nano having both 5 VDC and 3.3 VDC onboard regulators which further supply power for the Si5351 and the Color TFT. In one case where space was a real premium, I bypassed the socket and wire wrapped directly to the pins of the Arduino. I smoked the Arduino and had to completely rebuild the whole board. Had I used a socket it was pull out the old plug in the new.
- In one Arduino/Si5351 I made the whole shebang an integral assembly where the holes used to mount the Color TFT to the panel instead of nuts on the bolts had aluminum pillars mounted to them and then I mounted a sub-chassis where I installed the Arduino/Si5351 so that everything essentially is mounted to the font panel -- that was not a plan but a solution to a space problem with lack of space on the base plate to do this. It makes servicing terrible and violates my DFMA approach (Design For Manufacturing and Assembly.
You get the idea -- the perf board that seems to work is about 2 X 3inches.
10/25/2018 ~ What Filter Frequency?
Bill, N2CQR on his SolderSmoke Blog several days ago had a link to this blog and a question was asked about what filter frequencies are used for homebrew rigs and was it simply a matter of what you had in the junk box?
I posted an answer on the SS Blog to see this blog for the answer. Well the answer is both a YES and NO. So you have this surplus filter or a bag of crystals and want to build a rig and so you are off and running. Add to that what may be the availability of the filter surplus or how cheaply you can get the bag of crystals. BUT there are other factors and I will outline those now.
- You have this really neat Collins Mechanical Filter at 455 kHz that was given to you by a friend. Now is where the design topology comes into play. If you have chosen a single conversion approach that Collins Filter will top out at anything beyond 75 Meters because of the image problem with the low IF frequency. Using the Collins Filter will most likely dictate a dual conversion design. So it is more than just the filter.
- In my junk box I have a pair of 16 MHz Crystal Filters, one for USB and the other for LSB. The approach used with individual filters is that you hold the BFO frequency constant and switch the appropriate filter in line depending on what side band you select. Well these filters are over 40 years old and with aging have drifted so that the BFO that works with one filter will not be correct for the other. If you are using a fixed crystal BFO that is a problem. Since I use an Arduino/Si5351 when I diode steer the correct filter I also input to the Arduino to shift slightly the BFO signal. But that takes a lot of fiddling to get that to work properly. This is compounded by the higher IF frequency (especially a problem if you are homebrewing a filter). Most crystals are specified as having a frequency stability of 50 PPM. which means at 16 MHz there could be a stability issue of 800 Hz. If you use a filter at 4 MHz then the stability issue is only 200 Hz. Even commercial filters have this issue and older ones in particular may have drifted off the Center Frequency.
- IF frequencies and ham bands. One of the most common crystal filters of old was at 9 MHz and when the major ham bands were 80, 40, 20, 15 and 10 not an issue. But with the introduction of 17 Meters and single conversion that means the LO would be in the 17 Meter band. 9 MHz IF and 9.150 LO = 18.150 MHz. the second harmonic of the LO would be 18.3 MHz which is hard to filter out of LPF. Some manufacturers (Ten Tec) used an LO of 27 MHz so that on a down mix it would give the 17 Meter band but there were some concerns about this being a really good solution.
- Cheap filters may be cheap because they are bad and just because you paid $80 on eBay for a filter it may be bad too. So you are somewhat at the mercy of Caveat Emptor.
- There is a band of filter frequencies that seem to prevail. Some literature suggests that a range of suitable crystal filters (Collins are mechanical filters) is 2 to 12 MHz. Of the ones I homebrewed I found my best filters were either 4.9152 MHz or 5.185 MHz. Several rigs of mine which appeared in QRP Quarterly used these filter frequencies. This range has greater stability and avoids the 17 Meter problem. If you check the Elecraft K2 specifications you will find a 4.9152 MHz filter hidden in there.
- In the days of old, the dark ages (read Analog VFO's), the LO frequency was an issue as well. Typically to achieve frequency stability and reduce (I didn't say eliminate) drift the VFO operated in the 4 to 6 MHz range. To operate the various bands, the VFO signal was mixed with a heterodyne crystal oscillator signal to reach the need injection frequency ranges. That in itself now introduce a whole range of issues.
So there are some SAFE Intermediate Frequencies and some that are not. The filter you have in the junk box may not work well in an all band rig but may work fine in a single band rig provided the band selected is compatible with the IF used.
The New Heathkit Filter Based SSB Transceiver is Alive and Working on 40 Meters!
This Video Shows the Detail of the New Rig!