The 2018/2019 TechNight Radio Project
The 2018/2019 TechNight Radio Project
Below is ongoing updated information about the radio project.
At the end of the design info is RESULTS from building and testing sections of the design.
First, the latest block diagram:
Here's the latest Schematic:
Schematic Notes:
The schematic and PC board are done with EasyEDA software, my favorite free eCAD system.
Version 0.2:
First, I renumbered all the parts, so you can't talk about "R27" in vers 0.1 and have
it be the same part in Ver 0.2. I may do that again before I do the board, but then of course I have to freeze
it so it stays matching the designators that will be printed on the PC board.
Also in this Ver 0.2, I got rid of the AD603 IF amp and went with a discrete string of gain.
This is more flexible and educational, and will run nicely on 5V. The AGC is just brute force PIN diodes
to mismatch the stages to lose gain.
Termination on the filter is lossy, but good resistive termination
for the filter, which needs a good matched load to keep it's flatness.
The gain string is kind of interesting. First is a Cascode pair, which has good reverse isolation, so that
the PIN mismatch doesn't get back to the filter and mess it up, and because Cascode stages are cool and have several
advantages, like sharing the power supply current with two stages that are in series DC wise.
Next is a pair of stages that are also in DC series, but are just cascaded common-emitter stages otherwise.
Also just cool, but you can only get away with this at the beginning of the chain where signals are small and
don't run into the low rails of these two stages (like 2V each). Then a more classic common emitter gain stage,
before going into the BFO mixer. There are also tuned circuits distributed throughout this gain string, to narrow
the bandwidth which reduces total noise power (each stage creates its own broadband noise), and to help stability
by limiting the gain to only 10.7 MHz.
The AGC source is very simple and probably won't work like this,
but it rectifies the IF signal at the end of the chain, and produces a positive DC level relative to the signal
level, and will drive the 6 PIN diodes controlling the radio gain if the signal gets too high. This stops the radio
from saturating with strong signals, but reduces the gain so that weak signals are lost if they are close to a
strong one (if both are inside the crystal filter bandwidth). We'll see how well it works.
After the mixer, the
"audio" signal will be as wide as the crystal filter is, and I intend to use a 30 KHz wide filter in mine, because
I happen to have a real good one like that. So I put an audio filter that rolls off at 40 KHz after the mixer,
kind of an "anti-aliasing" filter for the next stage, which is the audio digitizer plugged into the Raspberry Pi.
If we get a good one it will handle 40 KHz and I can look at that much spectrum at once. BUT, any strong signal
in that bandwidth will bother the weak ones, so you might want a narrower filter. Bob's friend Sandie's dog knows
the name of this filter: "roof rooof!". Now we'll see if anyone read this far.
Version 0.3:
Most of the changes from last time are clean-up and changes to make the board easier to lay out. Like note the wire
crossover at U18 and T3. Doing this on the schematic un-crossed the traces on the board. There are also a lot of
bypass capacitors added, note the pile of them at the top. Also, the power supply is split in 3: +5V, +5VR, and +5VT.
Oscillators and DDS's run all the time, even though only one is used in TX. Would have to reload it if I powered it
down. Might put it to sleep though. Or it can be turned off in software.
Also, I renumbered it again, since it was getting messy with all the changes. I gotta quit doing that, especially now
with the board laid out, as renumbering in EasyEDA can mess up the board. So C81 isn't C81 anymore.
I also want to point out how nice it has been using EasyEDA. I'm sold on it. They released a new version just last
week with some new additions and fixes. It's not perfect for sure, but completely usable. Amazing to me that it is
free, it's better than most of the good stuff I've used over the years. And the most intuitive CAD system I've ever used.
If you want to learn it, you should talk to me first. There are about 5 things I would tell you that would save you lots
of startup time. Mostly it is the misuse of words due to translation to English. This is really confusing. Like a
"Library" is not a library, it is a part in the library. If you hit "New Library", it brings up the part editor.
Version 0.4:
Lots of changes here for the board layout. Bypass caps added, pinouts changed for easier layout (like P1 connector pin
assignments, all changed for direct board traces). Pull-down resistors were added to all the digital inputs, bad to have
them floating if not connected. No real electronic design changes to the radio.
Here's the final PC board layout:
PC Board Notes:
This is the final board as it was ordered. This image is actually NOT from the layout software, but from a Gerber file viewer, that looks at the final artwork files you send to the board manufacturer. I made the top layer green and the silkscreen white so it looks almost exactly like the boards we're going to get.
When the board started, it was a blank rectangle with about 300 parts piled up on the side, had to move each part one at a time, then get the layout worked out, looking back and forth between board and schematic. I'm very happy with how it turned out, almost completely single sided, despite a 4-layer board. A few traces on the back side, but none with RF on them. This is possible due to power and ground NOT being on the top or bottom, so still need 4 layers. Note this top side is filled with copper in any open space. There are small via holes all over this board not shown here, connecting ground from top to 2nd layer which is solid ground, and through to the ground fill on the bottom layer. Grounding on an RF board is important, and this will have lots of it.
The front end is in upper left, then going right across the top, mixer, quad amp, filter (upper right). Down the right side is the IF strip, then bottom right is the BFO mixer, then a wideband audio amp and filter. Right now the audio output is 2 pads to solder wires to. The transmitter is in the lower left. The crystal osc reference and DDS chips are in the middle, with data connections on back side over to the main connector on the left. Not such a simple radio, with 271 parts.
The board size is 7" wide by 3.6" vertical, just picked this somewhat randomly, adjusting a little as I went. Seems to be a good size for this prototype, but a final version should be smaller. At that point we'll think about a box that we can make it fit into.
Here's the latest Gain Distribution diagram (nothing new):
For the gain section, green cells are inputs entered. Each row represents a different input level to the radio, and a different IF Gain setting. Red cells are signals that are too large (greater than the max listed for that stage). This represents an overload condition. Yellow is a signal that is probably too small, not enough for the ADC to get much info from.
Note that for a -10 dBm input, regardless of AGC or IF Gain, there is an overload at the first mixer. The radio could be built with a higher power mixer and LO to handle this. Or a lower gain RF amp, which might hurt weak signal sensitivity.
Below the gain section is now a Noise Figure section. This analyzes how the noise and losses and gains of the path affect the sensitivity of the receiver. Unlike the gain, which flows from left to right, showing the effect of gains and losses on the signal as it flows through the radio, this section starts on the right and works itself to the input of the radio, showing how sensitive the radio is given the current stage settings.
At the meeting I showed this analysis with a 3dB attenuator after the mixer and before the filter. The change I made to the design is to add some gain here, but in a special way, that serves the purpose of the attenuator that was there: To provide a good impedance termination for the mixer and the filter. With the new gain, the noise figure at the input goes down from 9dB to 1.7 dB, a great improvement. And now the AD603 IF amps do not set the noise floor of the radio. However, the radio now saturates at smaller signals. We might have to add AGC to this new IF amp, despite it only having 10dB of gain.
Here's a list of the sections that will be addressed separately:
We will discuss and design each of these sections at TechNight.
Here's the goals for this radio:
1) Cheap - less than $50 not including enclosure or peripherals like AC power supply, keyboard, and screen. Just a PC board, no box. [going to be closer to $80]
2) Pretty good quality - totally usable, not contest grade, but a level below that.
3) Small and low power, for portable operation, so it can be used at home or out camping. It would be surface mount technology, but you won't have to solder any of that, we'd get someone to solder the small stuff.
4) It would use a Raspberry Pi as its controller, so that it can run FT8 (and all the WSJT modes). No external computer required. We will write the controller software for this radio, the "front panel".
5) Hardware not too complicated, maybe only one band. I am thinking 6 meters, but it will be buildable to other bands from 160 thru 2 meters. One circuit board for everything, with maybe 250mW output from TX, use an external power amp on a separate PCB with TR switch and LPF.
6) Decent power out on TX, maybe 20 Watts (with PA).
7) Maybe it is FT8 and CW only, meaning it does not have to be linear. No SSB (but maybe FM).
8) Useful for local TechNight contests, like maybe a worked-all-attendees, worked all of these radios, or worked all zipcodes of members. Something to get people on and using the radio and having fun with it.
All specs are subject to change as we discuss and price things out.
RESULTS
Here are results of building and testing sections of the radio.New PCB
In Dec 2019, I went to build up the DDS section of the board, and realized the pinout on the single-gate ICs is backwards, in 6 out of 7 of the parts. How did I manage this one? The part is built correctly in the CAD library, but being a single gate, the schematic symbol had a power and ground pin added to an Exclusive-OR looking symbol, and which power lead was which was not marked. When I rotated the part on the schematic, that reveresed power and ground, yet I assumed the bottom one was still ground. The part symbol now has markings on it.April 5, 2020 - CoronaVirus Era
I've done a lot of work with the quadrature IF amp. There are several issues here, and I'm not done with it. In March, when the decision to cancel TechNight was made due to the coronavirus, I pretty much stopped working on the radio for a few weeks. I started working 100% from home on Friday the 13th of March, and I had some pressing work things to do. I'm pretty happy to continue to have a paying job in these times, so work got some pretty high priority. Still, since I'm home 100% it's hard to separate work and play.