(21) If you add a microprocessor chip to augment some of the BITX
functions, it might be advisable to use the 10 MHZ BFO signal to clock
your uP. This could help avoid unwanted 'birdies' from the uP oscillator
getting into your BITX circuitry.
From a nice list of BITX mods:
http://kambing.ui.ac.id/onnopurbo/orari-diklat/teknik/homebrew/bitx20/modifications/LIST%20(9-24-04).txt
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
Using a very nice M0XPD AD9860 shield from Kanga-UK and software by Richard AD7C I put together another little DDS VFO. This one went together without a lot of hair pulling. I put the LCD, brightness control and rotary controller on one piece of vero board -- this will also serve as the front panel.
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
Hi Bill,
I wanted to forward to you a slightly edited email I sent to one of our podcast listener’s as I think this is a really good example of some “Tribal Knowledge”.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Hi OM,
Ham radio publications are much like the Internet –all that is published must be scrutinized. One such publication had a beautiful QRP to QRO amp using a 3CX800A7. A few watts in and 800 Watts out. My daughter was in FM radio broadcasting and I told her to introduce herself to the station engineer and to look out for any pulls – then the floodgates opened –about a week later I had a 3CX800A7.
So I started to work on the amp. The circuits just didn’t make sense and parts were missing that would make it work correctly. I contacted the author and here is what he shared. He built the amp but never did get to proof the final article. In fact he sent me his notes and sketches which were correct. Thus I could have never built that amp using just the article. I built it for one band, 20 Meters. It has a tuned input and a Pi-L output so is quite excellent on harmonic reduction.
Oh BTW this amp caused me to learn about PIC Microcontrollers. You cannot hit the 3CX800A7 with HV voltage until the cathode is warmed up (must be a female tube). That time delay is 3 minutes. I could not find a suitable time delay relay with a 3 minute delay that didn’t cost an arm and a leg. So that is when I thought about using the PIC16F84 as a time delay relay. Later I ported that over to a 12F675. The cost was less than $10. In the 3CX1500A7 amp ( another free tube) I have two microcontrollers in there. One is used for a three second step start on the filaments (don’t want to shock the filaments with inrush current) and the second for the 3 minute delay before starting the HV step start sequence.
There was only one problem – I was worried about the cooling of the tube so I made sure there was plenty of air which I dump into a very small sub-chassis and the exhaust is out through the tube. Well “Dah” large volume in and small port for exhaust and you have a jet engine sitting on the desk top. Man it was loud – I would wear headphones when it was working. When I built the 3CX1500A7 amp I used a larger plenum so not as loud.
There are several key points I want to make about tribal knowledge:
- Turn off the soldering iron and spend a good deal of time “noodling” over the circuit so that it is clear what each function will do and that all the wires connect to something.
- Contact the author and ask lots of questions and you might be surprised to find out the published design is not what was designed!
- Look for uncommon solutions to build problems. Many times parts used in the articles are so unique that only one exists in the whole world and it is installed in the authors unit.
- The amp was built in 2000 and the use of the PIC16F84 as a time delay for an amp was a bit leading edge but don’t be afraid to employ some advanced technology into your projects. [Today an Arduino built into the amp could do time delay, temperature control of blowers, SWR sensing, grid trip protection and even warm your coffee.]
- In the Big AMP I figured out how to keep the blower going for about 1 minute after the amp is turned off to “cool down” the tube. The point here is to think about not just the amp itself but refinements to make that $600 tube last for a very long time thus filament current inrush protection and tube cool down.
- Don’t forget SAFETY – there is 2000 Volts @ 1 amp running around chassis–it is an electric chair sitting on your desk top. I included a microswitch that when the top cover is off it de-energizes the HV circuits. See if you can spot it in the photos.
73’s
Pete N6QW
PS The amp really does exist –see below.
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
This weekend I put the Arduino/AD9850 Direct Digital Synthesis device into a box this weekend. It is sort of evolving into a general purpose HF signal generator and/or VFO. It is really kind of neat that this little collection of boards can generate RF across that range, with accurate digital readout. Thanks again to Richard Visokey, AD7C, for the circuit and the code. As you can see, my cabinet making skills will never land me a job on the Discovery Channel, but I'm kind of pleased with the box. I picked up the wood panels from a hobby/craft shop.
I left a lot of space in the box. I envision an amplifier taking the output from its current .4 milliwatts up to around 10 milliwatts, followed by step attenuators (pads).
Here is the other end. Of course, I could have just taken the ATMega chip out and avoided putting the whole Arduino board inside the box, but I'll leave that exciting digital adventure for a future project.
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
Bill:
I think I’ve finally earned the right to contact you. I have a really severe case of the knack. I did not realize it until I discovered amateur radio. My thinking is that there is no hope for those afflicted with the knack. I believe that amateur radio helps those afflicted to deal with the condition. It alleviates the symptoms.
For years I wandered through the technical wilderness dabbling in physics (I have a M.Sc. in Nuclear Physics), aircraft (I was a tow pilot at a gliding club for several years), high power rockets (www.napas.net), electronics and amateur radio. The most enjoyable part of radio is not the QSOs it’s the building and the satisfaction of a working device.
I am relatively new to amateur radio but I’ve always had a passion for all things electronic (and technical). I am a self-taught electronics geek and have been playing around with digital electronics (PIC microcontroller) for some time know. I starting building altimeters for high power rockets that has the capability of setting off onboard charges to separate the rocket. When your rocket reaches 10,000 feet, you cannot open a chute at apogee because it’s going to drift too far and you need to open the rocket at apogee (no chute) and then open the main chute at about 1000 feet to minimize drift. I routinely travel to the US to attend LDRS (large dangerous rocket ships).
Anyway I had to get my amateur ticket for onboard video camera and trackers (beacons). Once I got my license and got into radio, it was like a drug!!
I am now home brewing everything in my shack.
When I came across Soldersmoke that was like a super drug. I downloaded and listened to EVERY episode (seriously!). I followed you and Mike’s journey and I learned soooo much. I was devastated when I found out about Mike.
You inspired me to build my own transmitter. Earlier this year I built a 20m CW transmitter for my rocket that will eventually send telemetry (in CW from a PIC microcontroller) completely from scratch – no kits – no one’s design. Your early episodes also pushed me to learn LTSpice which I used extensively to model transmitter design of others as well as my own. I’m thinking of calling the transmitter “kaputnik”.
My design is based on a Chinese AD9850 DDS module which generates a square wave followed by a class E amp (with a tuned circuit). Puts out 1 watt of power with harmonics down by about 40db. The reason I used this module with that I can easily change frequency (with mod to tuned circuit) – after all the DDS module is programmable. All I need to do now is clean up key clicks because the carrier is turning on too fast. I playing around with slowly increasing the bias on the mosfet to allow the power output to increase slowly. Any advice/tips/tricks would be appreciated.
Next project is a remote antenna switch. However, you have me close to tackling a SSB transceiver.
Anyway, keep up the great work and I’m looking forward to listening to another 150 episodes!!
Take care and “stay thirsty” my friend.
73
Dave Rajnauth, VE3OOI
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
.png)
