Last night was a terrible night for sleeping and my mind was in overdrive. It was a nightmare almost terror filled! No, it was not of the ilk such as the check engine light came on or that Mary Jo suddenly craved something other than the Bob's Big Boy Hamburger (pickles and ice cream). It was a schematic for a project I had seen on the internet.
So, how this all started was that I was trying to put together a project that possibly would be a future article. It would also provide fodder for several postings on this blog. I now find that I am captive to the beast -- unless there is new fresh meat on the blog each day -- the readers go elsewhere. It is like being on the hamster wheel!
Given my time constraints my plan was to take a prior project that had been published and to add to that project thus creating an entirely new project.
Using a metric of how many requests for Arduino code as the screening mechanism, it was an easy choice. The Direct Conversion Receiver project using the MC1496 has had over 200 requests for the Arduino sketch, and I have seen examples that were actually built.
Now the 2nd piece is the many reader requests for modifying many of my SSB transceiver projects to include CW. I always say that building CW into a SSB rig is very hard if you want to do it right. A better approach is to just build a CW Transceiver -- period.
Thus, an internet search to find a CW transmitter driven by an Arduino/Si5351. The MC1496 used the Si5351 and that essentially is a starting place from where I could develop the transmitter section.
Now, I should "splain" I look at the schematics 1st and if I see something that fits with what I think should be there, then I will take the time to read the documentation. If something looks hokey then I just move on to the next search item. The Google search parameter was CW transmitter + Si5351
The Arduino/Si5351 part looked OK, but it was the transmitter part that really raised the RED FLAG. The 1st transistor is an audio amp stage much like a microphone amplifier. This essentially means you have a modulated oscillator. Now that can cause some problems.
Next is the right-side transistor that has the output from CLK 0 dumped into the base which means two things -- there is no "keying" like when it is ON it is on. The second issue is that CLK 0, is more like a square wave which is being amplified by the 2N3904. Most of the time the Si5351 is dumped into a Diode Ring Mixer which likes Square Waves.
But the huge FLAG is the lack of any tank circuit and harmonic filter on the output that would limit harmonics and weird mixing products. I wonder what the signal actually sounds like. With a 9 Volt supply the output would not be huge but likely to be transmitted possibly hundreds of miles.
NOW a disclaimer is that I did not read the documentation as just looking at the schematic signaled, I should not waste any more of my precious time. In all fairness perhaps the author has addressed the concerns I highlighted.
My notional approach starting place would be something like this back of envelope sketch.
Essentially, I would take an output from CLK 2 which is 600 Hz lower in frequency from CLK 0 and CLK 2 is only enabled on transmit. This signal is fed through a Band Pass Filter to a 2N3904 Buffer stage that is keyed by a 2N2905 keyer transistor. The Vcc is raised to 13.8VDC.
To enable a smooth sequence, we would need some circuitry ahead of the 2N2905 that enables CLK 2 and Mutes the DCR. The 2N5109 is capable of producing 500 Milliwatts of RF. Don't forget the W3NQN LPF.
Now we will have the other side of the blog readers suggesting we use a second MC1496 to create a DSB transmitter and DSB Transceiver. Guys the project is a CW transceiver.
Thus, poor sleep last night wondering how many of these transmitters are on the air.
TYGNYBNT.
73's
Pete N6QW