Scott KQ4AOP put a comment on a recent SolderSmoke Blog post that I found especially encouraging and apprportiate. He was writing about his experience building the High School Direct Conversion receiver.
Scott wrote:
"This was my first receiver build and, it was great fun. When you finish the build and prove you are able to tune through the band, you are welcomed into the secret society! The build is the initiation. I am happy to print and ship the PTO if needed."
The 3d printed form for the tuning inductor is often a show-stopper for prospective builders. Scott offers to print out a form for you, and send it to you.
Scott's mailing address is on his QRZ page. His e-mail address is: streez55@gmail.com
Thanks Scott!
Here is a post I did early this year on Scott's receiver:
The folks over at Ham Radio Workbench have graciously accepted the challenge issued in our last podcast: that they scratch-build homebrew the 40 meter Direct Conversion receiver that Dean KK4DAS and I designed for local high school students. We want to help our brothers over at HRWB. For example, we may be able to supply a few of the 3D printed PTO coil forms. Here is some background information on the project.
Design Decisions in the Direct
Conversion Receiver
Why did we do it this way?
In thinking about how to design this receiver, we had to
make early design decisions on almost every stage. Here are some of our key considerations.
VARIABLE FREQUENCY OSCILLATOR:
Normally we might have used a variable capacitor to change
the frequency of this oscillator. But
variable capacitors are now expensive and hard-to-source. Our friend Farhan in Hyderabad used a simple
variable inductor for this purpose in his “Daylight Again” transceiver. The
coil form for this part could be 3D printed.
A metallic screw would vary the inductance as it is screwed in and out
of the coil.
We also decided to use the same simple Colpitts oscillator
circuit used by Farhan in his own high school direct conversion receiver. This circuit is unusual in that the feedback
capacitors are also the frequency determining elements (along with the variable
inductor). This simplified the circuit
and reduced the parts count, and proved to be remarkably stable.
For the VFO buffer we used the simple JFET buffer from
Farhan’s Daylight Again design.
Based on suggestions from other radio amateurs, we developed a simple frequency readout based on the position of the end of the tuning screw (how far in or out?).
We selected the 40 Meter band for this receiver because we
thought it would be easier to get the VFO stable on this frequency, and because
Farhan had built his receiver for 40 meters.
MIXER:
At first we hoped to use a simple singly-balanced mixer
using two diodes and a single trifilar transformer. But we found unacceptably high levels of AM
breakthrough (mostly from Radio Marti on 7335 kHz) when using this
circuit. So we switched to a diode
ring. This required two more diodes and
an additional trifilar transformer. We
believed the students would have great difficulty building and installing two
trifilar transformers so early in their building experience. So we used transformers that had been wound
in Hyderabad by a women’s collective employed by Farhan, and developed
a scheme for fool-proof installation of these transformers.
We also found that the mixer needed a diplexer at its output
– this would provide a 50 ohm termination at all frequencies and would result
in much cleaner action by the mixer and greatly reduced AM breakthrough from
Radio Marti. We used the same circuit
used by Roy Lewellen W7EL in his Optimized Transceiver circuit.
BANDPASS FILTER:
This was the simplest board in the project but it required
the students to wind two coils on toroidal cores. A simple dual-tuned circuit design would be
sufficient. We used component values
from the QRP Labs website. We showed
them how to wind the coils, and made a video about the technique. Students used a simple Vector Network
Analyzer (Nano VNA) to tune the filter.
AUDIO AMPLIFIER:
We had to make several design decisions here. First, we rejected the idea of using an IC
amplifier like the ubiquitous LM-386. We wanted this to be a completely analog
and discrete component experience. Then we rejected the idea of using a
push-pull output circuit. While this
would have eliminated the need for an audio output transformer, it would have
resulted in a more complicated circuit. In the end we opted for three simple
RC-coupled common-emitter amplifiers with an audio output transformer. There was no feedback in these circuits. We found there is a lot of gain (hFe)
variation in the 2N3904 transistors that we used. Care needs to be exercised in making sure
that transistors of moderate (but not too high) gain are used.
This AF amplifier chain probably presented a 1500 ohm
impedance to the mixer (instead of the desired 50 ohms), but we think this
problem may have largely been taken care of by the diplexer.
We found some very small (one square inch) speakers that
could be easily used in this circuit.
ANTENNA:
While the students could use a wide variety of antennas, we
recommended a simple ¼ wave antenna with a ¼ wave counterpoise. We thought that this antenna – of only 33
feet in length would provide good performance with low complexity, and would be
well suited to the “upper floor bedrooms” from which many of the students would be listening. Also, this antenna would not require the use
of coaxial cable or an impedance matching transformer. We made a video on how
to build and use this antenna.
POWER SUPPLY:
We opted for the use of 9 volt batteries. This proved to be a safer and wiser choice that
limited the kind of mayhem that could occur should a variable voltage supply be
used.
Details on the receiver can be found here:
https://hackaday.io/project/190327-high-schoolers-build-a-radio-receiver
May 29, 2023
So around the time we were building 40 meter direct conversion receivers, someone else was doing something similar. His product (above) is a lot nicer than ours. He has an S-meter and a digital frequency readout. But like ours, his is built on a wooden board. FB OM.
If you want to see what a direct conversion receiver can do, watch his video (above).
I was really amazed to see him use a modified VFO from a Kenwood TS-820. Not long ago Pete N6QW spotted one of these on e-bay and recommended that I buy it. As with the Yaesu FT-101 VFOs, we bought it for the gears and reduction drives but ended up with the entire VFO circuit. I now have one on my shelf, ready to go. TRGHS.
We are not sure who Chappy Happy is, but "Tezukuri" means "hand-made" in Japanese. The writing in the video descriptions are in Japanese, then Chinese.
Here is the YouTube channel. Amazing stuff here: https://www.youtube.com/@chappyhappy3675 He is clearly a ham. He even works on an old S-38. Who is this guy?
This is just so cool. Scott KQ4AOP has successfully homebrewed a ham radio receiver. He used the circuit Dean and I developed (with a lot of input from Farhan and others) for the High School receiver project. But Scott has had more success than any of our students. And I think he has had -- in a certain sense -- more success than any of us. After all, how many of us can say -- as Scott can -- that he used a homebrew receiver that he made to listen -- for the very first time -- to amateur radio signals? Scott writes: "Those first sounds were my first time ever hearing any Amateur Radio first hand!"
In the video above you can watch Scott tune the entire 40 meter band and a bit beyond. You hear CW at the low end. Then FT-8. Then SSB. Up just above the top of the band I think you can hear our old nemesis Radio Marti. And this powerful broadcaster is NOT breaking through on the rest of the band. FB Scott. Congratulations.
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Bill,
Bill and Dean - Thank you for sharing and documenting this receiver. I greatly appreciate you publishing the circuit, class notes, and build videos. That got me 75% to completion.
I feel blessed that both of you chipped in and encouraged me through the troubleshooting to finally getting the receiver to start “breathing RF”.
Those first sounds were my first time ever hearing any Amateur Radio first hand!
Ianis S51DX in Slovenia was the first call sign heard. Some peaking and tweaking remains to be done, but the receiver is working.
Congratulations to Scott KQ4AOP who got his Direct Conversion receiver working yesterday, And congratulations to Armand WA1UQO who got his regen receiver working. I think all of us are following Farhan's advice and are taking some time to just listen to the receivers we have built ourselves.
Dear Bill,
In our talks at the local high school we have frequently mentioned Ashhar Farhan, his Cubesat experience, and his use of a direct conversion receivers teach electronics to students in Hyderabad, India. So we were really please to have the opportunity to bring Farhan himself in to speak to the students we have been working with.
Here is the audio of Farhan's talk:
http://soldersmoke.com/FarhanTJ.mp3
The acoustics in the room are not great, so you may have to listen carefully, but it is worth it. Farhan dispenses a lot of tribal knowledge and wisdom about satellites and about the value of homebrewing simple radio equipment.
We soon had the students come forward with two projects that were ready for final testing. Sure enough we found problems with both. The solutions provided a lot of educational fun.
The first group had not yet built the diplexer -- we advised them to skip over the diplexer for the moment -- just connect the output of the mixer to the input of the AF amplifier. We can't let the perfect be the enemy of the good! Build the diplexer later, but for now, get the receiver going. They did, and a few minutes later they were receiving signals from Mike KD4MM's transmitter (on the other side of the lab).
Then a second group came forward. We put a San Jian frequency counter on the PTO output. Uh oh. Trouble. Gibberish! A wonderful troubleshooting session ensued. With the student, we found that the signal was good at the output of the oscillator transistor, but NOT at the output of the buffer. At first we suspected that the buffer was bad, but it was not. Then we lifted the connection to the mixer and suddenly the buffer output was good. So the problem was in the mixer! When we disconnected the input transformer of the mixer from the diode ring, THE PROBLEM WAS STILL THERE. So the problem was clearly in the input transformer. Dean gave us a replacement transformer. Soon all was right with the rig, and this group joined the ranks of the successfully completed receivers.
I think that seeing that two groups had finished helped motivate the others. Our announcement that successful completion would lead to a "Certificate of Completion" also helped. But most of all, I think the natural desire to finish the job and avoid the "Shelf of Shame" was pushing the students forward.
Other news:
-- Our stage-by-stage award program continued. Last time we awarded "The Torry" for the first successful bandpass filter; this time we awarded "The Audy" for the first successful audio transformers.
-- We told the students that their work has been entered in a Hack-A-Day contest. Most of the info and files on the project can be found on the Hack-A-Day site. Check it out:
https://hackaday.io/project/190327-high-schoolers-build-a-radio-receiver
-- We also told the students about Walter KA4KXX's very generous offer of a reward for the first students to check into the Florida Sunrise net. (We had to make it clear that this offer is completely extracurricular and unconnected in any way from the school .) The students were clearly intrigued. Sunrise Net may get some new check-ins!
-- We provided instructions on how to build a simple 1/4 wave reception antenna. We also did a video.
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This week we had two sessions of about two hours each. We introduced the final board: the audio amplifier. Many of the students began work on this amplifier; others were catching up with work on boards presented earlier.
The AF amp is their most challenging board: It used 14 Manhattan pads and about 26 components. We warned the students that amplifiers often aspire to be oscillators. We told them to pay attention to layout, and to keep their leads short.
By the end of Friday, two groups had completed the build of the AF amplifier board.
We think there are about 13 receivers in production. Some are near completion, others will need more work.
On Thursday of next week those teams that have completed all four boards will put the circuits together and will test the entire system. They will then add all needed front and back panels and socketry.
We really want the students to complete as many of these receivers as possible. Exam season and the end of the school year is approaching, so we have to get this done. We will remind students that they don't want to that person who ALMOST finished a project! We will urge them to GET IT DONE! They can tweak it and mod it later. This kind of tweaking and modification is part of the homebrew experience.
We have been presenting awards to the students who are first to complete each stage: The winners of the PTO board competition got a copy of SolderSmoke: Global Adventures in Wireless Electronics. Those who won the mixer competition got a W1REX Hamfest Buddy transmitter. Thanks Rex! And this week we presented an award to the students who were the first to complete their bandpass filter. You've heard of the Tony, the Emmy and the Grammy? Well, we presented "The Torry" (from Toroidal). The trophy was made from a toroidal winding tool made in Alaska by KL7FLR. I explained to the students who had made it. Thanks Paul!
Here is the problem:
For the capacitive element in the LC circuit we have
essentially two 660 pF caps in series.
This results in a total capacitance of 330 pf. I measured 362 pF.
To get a resonant frequency of 7.0 MHz with 362 pF we need
1.428 uH.
To get 1.428 uH on the PTO coil form we need about 21 turns
of wire.
21 turns on our coil form yields 1.440 uH and resonates with
362 pf at 6.9708 MHz
That’s pretty close to what we need, but the problem
arises when we screw in the brass tuning screw. This reduces the inductance and raises the frequency. Putting the screw
all the way in reduces the inductance to 1.138 uH resulting in a resonant
frequency of 7.8414 MHz. So with a coil
this large (that we must use if we want to tune down to 7.0 MHz) we end up with
a tuning range that is far too large. We
only need 7.0 to 7.3. In effect, this
means that we end up using only a small portion of the tuning range: We can turn the screw approximately 34 times,
but only 6 turns keep us within the range of 7 to 7.3 MHz (the 40 meter
band). There is about 50 kHz per turn of
the dial. This makes tuning
difficult. It becomes more difficult to
separate stations and tune them in. It
would be better if we could tune across the band using more turns of the dial. At least 15 turns of the dial would be
nice: That would mean about 20 kHz per
turn. But how can we do this?
Just using a steel screw slows the tuning rate down. In a normal PTO we increase the inductance
(and reduce the frequency) by gradually introducing a ferrous material that
increases the inductance of the coil, pushing the frequency of oscillation
down. But our brass screw is
non-ferrous. This means that putting it
into the core does not change the permeability of the coil. The permeability of brass is the same as that
of air.
What does happen, however, is that introducing the brass screw
into the coil causes currents to flow in the screw. These are called eddy currents. In effect they become shorted secondary coils.
And they have the effect of lowering the
inductance of the coil – this is why the frequency of the oscillator increases
as we screw in the brass screw.
When you use a steel screw you get both effects: As you
screw it in, eddy currents flow in the screw, reducing the inductance and
increasing the frequency of oscillation.
But you are also introducing ferrous material – this pushes in the
opposite direction, increasing induction and lowering the frequency of
oscillation. I think the eddy current
effect dominates, but the increase in permeability pushes in the opposite
direction. This means that with a steel
screw you have to use more turns to cover the same frequency range. And that is what we want.
For example, using the same coil, with screw of the same
thread pitch (the same nuts), with both screws ten turns in, one turn of the
brass screw moved the inductance .014 uH.
The same single turn of the steel screw only moved the inductance .005
uH. So just because of metallurgy, the
steel screw will lead to a lower (better) tuning rate. I used a Hillman 45479 screw
that is steel with a Zinc (anti-corrosive) coating.
But there is more: steel
screws are also available with tighter (#28) thread pitches. The Hillman 45479 uses this tighter thread pitch. This too means that more turns are needed to
move through the same tuning range. Again, that is what we want.
I found that using a steel screw with #28 thread pitch allowed
for the coverage of the 40 meter band in approximately 11 turns of the dial. That is much better than what we got with the
brass screw: About 27 kHz per turn
instead of the 50 kHz per turn that we got with brass. But it is not quite good enough. It would be better if we could use the
entire range of that PTO coil form.
Solution Two: Add
a fixed inductor in series with the PTO coil.
After some noodling, I decided to split up the
inductor: A portion of it would remain
fixed, the other portion would continue to be tunable.
I estimated that I was starting out with a coil of about
1.428 uH. So I just put a 1 uH choke in
series with the variable inductor and reduced the variable coil to about .428
uH (about 9 coil turns). This worked,
but it worked a bit too well! It would
not tune the entire 40 meter band. So I
figured I needed less fixed inductance and more variable inductance. I found an air-cored coil in my junk box and
cut it so that it measured about .650 uH.
I added turns to the variable coil, going to a total of 15 turns. This REALLY worked well and yielded the 26 or
27 turns to tune across 40 meters that you can see in the video.
TWEAKS:
Later, I tweaked it a bit more: With 15 turns of #22 wire on the variable inductor, a steel screw tuned from .791 uH (screw out) to .662 uH (screw in). I put one additional turn on the fixed inductor, making it .749 uH, or about 8 turns of #22 (wound tighter on a cardboard tube from a coat hanger than was the coil on the variable inductor). With these coils I could tune from 6.9772 to 7.386 MHz. That's a bit more than we need but this allows us to keep the tuning away from the ends of the coil where tuning is more likely to become non-linear. I am able to go from 7.0 to 7.3 MHz in 23 turns of the dial. And the tuning is quite linear: The first turn from 7.0 MHz moves the frequency 12 kHz. At the mid-point of 7.150 MHz, one turn of the dial moves the frequency 12 kHz. At the high end, going down from 7.3 MHz, one turn of the dial moved the frequency 11 kHz. That, for me, is VERY linear tuning. You probably will have to adjust the coils a bit (just squeezing the turns together or spreading them apart) to get the tuning range where you want it.
YMMV – Keep it simple!
Like they used to say in the commercials: Your Mileage May Vary. There are many ways of doing this. The objective is smooth tuning across the 40
meter band. I think that by varying the
pitch of the variable coil turns you could get a more linear tuning response (please let us know if you have any luck). You might also be able to get similar results
by changing the amount of capacitance in the feedback network (which is also the
frequency determining element in this simple Colpitts oscillator). But remember that simplicity and a low parts
count were also our objectives in this.
This mod adds only 1 part (the fixed inductor), requires the removal of
some turns from the main tuning cap, and perhaps the replacement of the brass
screw with a steel #28 screw and nuts.
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