A Modular 40 Meter

CW Transceiver with VFO

This easy-to-build transceiver costs less than $50. Add a digital display and frequency control to a popular QRP rig.

Dr Jack Purdum, W8TEE; Farrukh Zia,

K2ZIA, and Dennis Kidder, W6DQ

Way back in my Novice class license days, homebrew equipment was the rule rather than the exception. Back then, anyone whose major source of income was from mowing lawns had two choices for getting a rig: You either built it from scratch, or from a kit. With a lot of help from my

Elmer, Chuck Ziegler, W8FTQ, I built a two-tube 5 W transmitter, and later bought and built a Heathkit DX-20. There’s a tremendous feeling of accomplishment and pride when you make contacts on something that you’ve built.

Today, we can build a VFO-controlled

40 meter band CW transceiver with an

LCD display for less than $50, and the construction is pretty easy, due to the use of a modular approach. We snap prefabricated modules together in LEGO ® fashion, and end up with a viable and fun CW transceiver for 40 meters. You can easily integrate the VFO/display board presented here into virtually any other rig. Furthermore, the VFO is capable of covering 160 to 10 meters.

There’s another reason for building this kind of rig. At the Milford Amateur Radio

Club Field Day location, we always have a GOTA (Get On The Air) station set up, where members of the public can come in and make a contact. I overheard a mother talking to her enthusiastic young son: “Yes, it looks like a fun hobby, but where are you going to get the thousands of dollars it takes to buy the radio?” That’s how our hobby is perceived. So, at next year’s Field Day I’m attaching a sign to this rig that says, “Build this station for under $50.” I specifically selected a clear acrylic case (see Figure 1) so that passers-by can see how simple it is.

The Components

Table 1 shows the five major project components. Prices are taken primarily from

Internet sources, and are usually the lowest

Figure 1 — The modular transceiver in a clear acrylic housing.

Table 1

Major Construction Components

Component

Forty-9er transceiver kit

AD9850 DDS module

16 × 2 I2C LCD display

Arduino Nano V 3.0

Rotary encoder

Total cost:

Cost

$11.00

$8.00

$6.00

$3.50

$1.00

$29.50

Description

Crystal-controlled 3 W transceiver kit for

7023 MHz ( www.ebay.com)

( www.ebay.com)

Blue 1602 IIC with I2C interface SKU:

EA-010204 ( www.yourduino.com/)

Make sure it has the USB connector

Purchased in a lot of 10 for $10

This leaves room for an enclosure, antenna and power connectors, and miscellaneous parts.

(with shipping) we could find. We use the

Arduino Nano microcontroller to control the rig’s features. Make sure you get the

Nano V3.0 that has a USB connector on the board. You can download the Arduino development environment free of charge.

1

Our program source code and assembly manual are available from the QST in

Depth web page.

2 You could also use an

Arduino Uno, Mega 1280, 2560, or Teensy, if you already own one.

The Forty-9er transceiver is sold as a 3 W crystal-controlled kit that operates from a 12 V dc power source. Running the rig at 9 V halves the power output. There have been some design changes to the transceiver since it was first introduced as the NorCal QRP Club’s Forty-9er kit by

Wayne Burdick, N6KR, (of Elecraft fame) and Doug Hendricks, KI6DS (of Youkits fame).

QST ® – Devoted entirely to Amateur Radio www.arrl.org March 2016 1

Figure 2 shows the parts contained in the transceiver kit. We inventory parts by poking the components into a large foam sheet, but don’t poke the static-sensitive components, like ICs and transistors, into the sheet. This makes it easier to keep likevalue components together, which in turn makes it easier to build the kit.

The instructions enclosed with the kit are very sparse, and omit construction details.

To help with construction, we wrote an assembly instruction manual that you can download from the QST in Depth web page. Our manual includes relevant schematics, and also illustrates the modifications that are necessary to accommodate our VFO and LCD display additions.

The small (1.675 × 1 inch) AD9850 DDS signal generator board forms the heart of the VFO. There are several variants of the board available, and not all are pin-for- pin compatible. It’s best to select one that looks like the one pictured in Figure 3 if you plan to use our PCB for the VFO/

Nano. The AD9850 chip uses a 125 MHz reference oscillator and operates from a

5 V source, which ties in nicely with the

Arduino family of microcontrollers. We noticed that the chip ran a bit warm, so we dropped the supply voltage to 3.4 V using a couple of diodes in series.

We chose the Nano microcontroller because of its low cost and small footprint.

The downside is that the Nano doesn’t accept a standard Arduino plug-in shield.

Therefore, you must use prototype construction with non-pluggable shields, perf board, or the PCB we designed for the project.

3 The VFO presented here is a very stable simplified version of the multiband VFO that Dennis Kidder, W6DQ, designed.

4

Modifying the Forty-9er

The original Forty-9er transceiver is crystal controlled. It transmits and receives on a fixed frequency separated by a small offset. Figure 4 shows a partial schematic of the original Forty-9er as distributed with the kit. The receiver filter section (green boxed area) has a fixed frequency crystal filter (Y1) with a very narrow receiver bandwidth centered around 7.023 MHz.

The red boxed area shows circuit elements that relate to the oscillator.

The transmit frequency is controlled by a second 7.023 MHz crystal (Y2) con-

Figure 2 — The Forty-9er PCB and parts.

Figure 3 — The

AD9850 Direct

Digital Synthesizer

(DDS) board.

Table 2

Components Affected by the Modifications

Component Value Note

C2*

C4, C5

30 pF or 33 pF

82 pF

C6 0.01

C21*

D2

R3

R5

W1

Y1

Y2

30 pF or 33 pF

1N4001

10 kW

100 kW

50 kW trimmer pot

7.023 MHz

7.023 MHz

Replace with 82 pF capacitor

Do not install

Do not install

Replace with 0.01 mF capacitor

Replace with 4.7 V or 5.1 V Zener

(install in same orientation as D2)

Do not install

Replace with 10 kW

Replace with jumper (see schematic diagram)

Replace with LC filter (see schematic diagram and assembly manual)

Replace with 3-pin header (see schematic

diagram and assembly manual)

*Note that original schematic shows C2 and C21 values as 33 pF but the actual kit may contain 30 pF capacitors as shown in modification schematic.

2 March 2016 ARRL, the national association for Amateur Radio ® www.arrl.org

nected to the NE602 or NE612 internal oscillator circuit. We replaced the two crystal controlled circuits with a VFO that covers the entire 40 meter band. Download our assembly instruction manual and follow the schematic for the following discussion.

QS1603-Purdum04

C2

33 p

Y1

7.032 MHz

D5

1N4148

Figure 4 — Original Forty-9er frequency control schematic.

QS1603-Purdum05

C2

82 pF

1

INA

2

INB

3

GND

4

OUTA

U2

NE602

Vcc

OSCE

OSCB

OUTB

8

7

6

5

D1

1N4148

1

INA

2

INB

3

GND

4

OUTA

U2

NE602

Vcc

OSCE

OSCB

OUTB

8

7

6

5

To Q1

Lx

22μH

Cx

150 pF

C10

82p

Y2

7.023 MHz

82p

To transmit pre-amp

D1

1N4148

C12

0.1μ

From

78L08

C10

The first modification changes the receiver input filter circuit to increase its bandwidth so it spans the entire 40 meter band. Replace the receiver narrow bandwidth crystal filter (green box, see Figure 4) consisting of

C2, Y1, and C21, with an LC band-pass

L2

100μH

To DDS

VFO J2

J2

D5

1N4148

Figure 5 — Modified frequency control for the Forty-9er.

R3 10k

C4 82p

C5

82p

C9

0.1μ

J1

R5 100 k

D2

1N4001

CP1

100 μF

25 V

To Nano

TX/RX Pin

R5 10 k

D2

5.1 V

Vcc 12V

L3

FT37-43

Modifications circled in red

C6

0.01μ

C21

33p

33p

W1

From Q2

C21

From Low Pass Filter/Ant.

filter that covers the entire 40 meter band.

Diodes D1 and D5 prevent the transmitter output signal from damaging the NE602 mixer input. The LC band-pass receiver filter must prevent a dc path between diodes

D1, D5, and NE602 mixer input INA (Pin

1), as stated in the NE602 data sheet.

A digitally controlled DDS VFO provides the transmit oscillator signal over the

40 meter band, so the internal oscillator in the NE602 chip and its associated external components (C4 – C6, D2, R3, R5, W1,

Y2) are no longer needed. The KEY UP or

DOWN (RX/TX) position of switch transistor

Q2 provides the signal to shift the DDS

VFO microcontroller transmit frequency over a small offset. Because the Nano digital input can withstand a maximum voltage of only 5.5 V, we use a 5.1 V Zener diode to limit the 12 V dc signal from Q2 to

5.1 V. A lower voltage Zener diode can be used as long as the voltage presents a logiclevel high (about 3.5 V) to the Nano keying the transmitter.

Build the board following our assembly instruction manual. The manual and Table 2 show a detailed list of components that are either replaced or omitted. Figure 5 shows the modified circuit. Changes to the basic kit are minimal. The left image in

Figure 6 shows the silkscreen for the board and highlights the modified areas. The image on the right shows the actual board.

The Nano and VFO Board

We use a small PCB (see the schematic in

Figure 7) to mount the AD9850 module and the Nano. The VFO output can be taken from either J2 or J3. Our RF output power was consistently lower than 3 W using the “straight” VFO design. The VFO output from J2 benefits from transistors Q1 and Q2 that raise the peak-to-peak voltage to about 4 V (adjustable by R1) that, in turn, drives the Forty-9er to about 3 W. Use the

J3 VFO output if you prefer to run lower output power. The Nano and VFO can be built on a prototype board, perf board, or on our small PCB board mentioned earlier.

Construction

Our project case measures approximately

6.75 × 4.75 × 1.75 inches, which is larger than needed. The case selection can accommodate more hardware later on. The

VFO/Nano board is on the left side, and the Forty-9er board is on the right in Figure

1. You can see the BNC antenna connector centered on the back panel, and the

QST ® – Devoted entirely to Amateur Radio www.arrl.org March 2016 3

Figure 6 — Parts placement for modifying the board.

QS1603-Purdum07

Rotary Encoder and Switch

LCDI 2C

12V

SDA

SCL

GND

5 V

Power

Supply

Figure 7 — The

Nano and AD9850

PCB schematic.

Components and

PCB are discussed in the assembly instruction manual on the QST in

Depth web page.

24

22

20

18

16

14

12

10

8

6

4

40-9er T/R Pin

J1

23

21

19

17

15

13

5

3

11

9

7

D1

1N5817

100 μF

25 V

R7

20k

5V

12 V

16

D13

17

3V3

18

REF

19

A0

20

A1

21

A2

22

A3

23

A4

24

A5

25

A6

26

A7

27

+5V

28

RST

29

GND

30

Vin

C7

Arduino Nano R3

0.1μF

D12

D10

15

D11

14

13

D9

12

D8

11

D7

10

D6

9

D5

8

D4

7

D3

6

D2

5

GND

4

RST

3

RX0

2

TX1

1

3.6V

R8 10k

5 V

D3 1N4001

D2 1N4001

C8

Mini-360

In

REG

Out

Gnd

C5

0.1μF

C6 0.1μF

C1 0.1μF

R1

10k

12 V

100 μH

L1

C4

0.1μF

11

14

Vcc

12 W_C

13 F_U

DAT

15

RST

16

GND

17

SQ1

18

SQ2

19

SN1

20

SN2

AD9850 DDS

C2

J2

0.1μF

Vcc

D7

10

D0

D1

D2

7

D3

9

8

6

D4

5

D5

4

D6

3

2

GND

1

R6

220

Q1

2N3904

R2 R4

C3

470

R3

12k

1k5

0.1μF

R5 220

Q2

2N3904

J3

4 March 2016 ARRL, the national association for Amateur Radio ® www.arrl.org

Figure 8 — The rotary encoder.

power connector hot-glued in place on the left side of the rig. After drilling a hole for the power plug, place a blob of hot glue inside the case and slide the internal power connector in place. We held it in place by pushing a wall wart plug through the case hole and into the internal power connector until the glue was set. The headphones and key jacks on the Forty-9er board are accessible through holes drilled on the right side of the case.

We centered both the Nano/VFO and

Forty-9er boards in the case, anticipating addition of new features to the rig. In retrospect, we would mount the VFO/Nano board more towards the rear of the case to allow easier access to the USB connector on the Nano for program changes. A pushbutton power switch mounts on the left side of the case front, and the right side holds the rotary encoder. Parts placement is not critical.

The LCD Display

Our LCD display uses the I2C interface to connect the display to the microcontroller board. The I2C interface uses just two control lines and two power lines —

Pins 10, 12, 14, and 16 on J1 in Figure 7.

A small potentiometer on the back of the display controls the LCD backlight. Set it once and forget it. You could use a non-

I2C LCD display for the rig if you wish, because there are more than enough I/O pins available on the Nano. However, you will need to modify the interface from that shown in Figure 7, and you need to modify the software.

The Encoder

Figure 8 shows a KY-040 encoder. We purchased a package of 10 on www.ebay.

com for about $10. Rotary encoders are designed to send a series of pulses as you turn the shaft. By measuring the sequence of the pulse chain, you can determine whether the shaft is rotating clockwise or counterclockwise. This particular encoder sends out 20 pulses for each full rotation of the encoder shaft. This means that a “detent” marks a new pulse sequence every 18 degrees of shaft rotation. The Nano processes these pulse sequences.

A software-defined frequency tuning increment ranges from 10 Hz to 100 kHz per encoder pulse chain. Change the increment by pressing the encoder shaft. An internal switch in the KY-040 encoder signals the software to set the frequency increment value. We tied this switch to D7 of J1, as seen in Figure 7. Each press of the switch increments by a factor of 10. If you increment past 100 kHz, the increment wraps around back to 10 Hz. Because the increment values are controlled in software, you can redefine them as you wish. The

“100Hz” displayed in Figures 1 and 9 is the current frequency increment.

The encoder (Figure 8) has five pins. Two pins are the clock and data pins used by a software Interrupt Service Routine (ISR).

Rotating the encoder shaft triggers an interrupt. When the software senses the interrupt, it suspends all other activity and immediately executes the ISR code. In our case, it changes the frequency of the rig and shows the new frequency on the LCD display. We use the Nano external interrupt pins D2 and D3 for the encoder. Pins 18 and 22 of J1 are tied to the two interrupt pins.

The center encoder pin links to the encoder switch. Because the KY-

040 uses a mechanical mechanism for encoding, it is subject to bouncing.

That is, the contacts vibrate as you move from one detent to the next, and the Nano is fast enough to read each vibration as an event in the pulse chain. This can result in sending a series of false pulses to the microcontroller before it stabilizes. We needed to de-bounce the encoder.

You could use software to filter the pulses by introducing a small delay in the pulse chain until the state of the encoder has stabilized. A delay of around 250 ms should remove the false pulses. Another alternative implements a hardware de-bounce solution by connecting 0.1 mF capacitors from the clock and data lines to ground.

This option is shown in Figure 8. We wired the two capacitors directly to the encoder clock and data pins, and then to ground. In the final design we tied the center switch pin to the +5 V using the internal pull-up resistor on the pins of the Nano. Program code activates the internal pull-up resistors.

Pressing the rotary encoder shaft grounds the switch and the increment value is adjusted in the software accordingly.

The modifications to the encoder result in a smoothly operating tuning knob. You can feel the detents as you turn the encoder shaft, and you can stop without over-shooting a target frequency. We opted not to use labels on the case for aesthetic reasons.

Software

The Arduino family of microcontrollers uses three types of memory — flash,

SRAM, and EEPROM. The Nano and

Uno boards include 32 Kbytes of nonvolatile (memory state persists even after power is removed) flash memory for storing program code. The bootloader code uses about 2 Kbytes of flash memory, so you have slightly less than 30 Kbytes for your program. SRAM (Static Random

Access Memory) holds the data stored in variables as the program runs. There is

2 Kbytes of volatile (loses data when power is removed) SRAM. There is 1 Kbyte of non-volatile EEPROM (Electrically Erasable Programmable Read-Only Memory), which is slower than flash memory.

You can do quite a lot with 32 Kbytes of program space. The program code that manages the transceiver display, VFO frequency, and encoder processing uses about 9 Kbytes of flash memory. The program data consumes 575 bytes of SRAM, and 8 bytes of EEPROM. The remaining

Figure 9 — The LCD display.

QST ® – Devoted entirely to Amateur Radio www.arrl.org March 2016 5

21 Kbytes of flash memory leaves plenty of room for new or expanded features.

VFO Calibration

The AD9850 spec sheet shows an equation that explains how the frequency is determined. The equation is:

FOUT = (TW ×CLKIN)/2 32

FOUT is the desired output frequency,

CLKIN is the input clock reference frequency (here 125000000), and TW is a 32-bit integer tuning word. Let’s say that the clock is functioning perfectly for 7.050 MHz. Rearrange the equation terms, with FOUT = 7050000, and CLKIN = 125000000, so TW =

(7050000 × 4294967296)/125000000

TW = 242236155.4944, which truncates to an integer.

It follows that your measured frequency using that tuning word (TW) and the other constants produces the exact desired frequency. You can rearrange the equation to

242236155.4944 = FOUT ×34.359738368 where the right side number is the tuning constant.

Our frequency output was a little off the mark. When we plugged in the actual output from the VFO for FOUT, and changed our tuning constant to 34.35910479585483, our output frequency was dead on. You will need to make a similar adjustment, using either a frequency generator or an accurate receiver to determine your offset multiplier.

Near the top of the program code, you will see a line that says

#define MYTUNINGCONSTANT

34.35910479585483 // Your calculated

TUNING CONSTANT

Once you determine the tuning constant for your VFO, you can replace that constant with yours in the program code line above.

Now recompile, upload the new version of the code to the Nano board, and you’re done!

Using the Transceiver

Power your rig with either a 9 V (for 1.8 W output) or 12 V (for 3 W output) supply.

You may want to put a heatsink on the power transistor if it gets too hot. I use a

13.8 V power supply when at home and a small 12 V SLA battery in the field.

The AD9850 draws its power from the

Nano board. The Nano can accept voltages between 6 V and 18 V but we regulate it to

5 V. We added a Mini 360 voltage regulator

U1 (see Figure 7) to the Nano/VFO board to reduce the stress on the Nano regulator.

When you turn the rig on, there is a short

“splash” screen, then the display looks like that shown in Figures 1 and 9. The top number displays frequency. The number on the second line is the frequency increment. That is, as you rotate the encoder, each detent changes the frequency by the amount of the frequency increment value

(100 Hz shown). The text word towards the right side of the second line is a reminder of the frequency limits that apply to US hams on the 40 meter band. For example, if you tuned down to 7,024.990 kHz, the text word changes to EXTRA since you must hold an Amateur Extra class license to operate on that frequency.

The frequency and increment values are read from EEPROM when the rig powers up. If you tune to a new frequency and stay on that frequency for more than 60 seconds, that frequency and increment value are written to EEPROM automatically. If another minute passes and you have not changed frequency, the frequency and increment values are not updated. The reason for not updating is because EEPROM has a finite write life of about 100,000 cycles before it loses reliability. If you had left your rig on, and we didn’t check for a frequency change, after about 70 days of continuous operation the EEPROM could become unreliable. Using our approach reduces this possibility.

The next time you apply power to the rig, the frequency and increment values are read from EEPROM and sent to the display. This and the band edge markers are done in software, so you can modify this feature if you wish. The code is well commented.

Conclusion

Driving forces behind this project are to show that a viable transceiver need not be expensive, and to encourage more hams to use microcontrollers in our hobby. This modular transceiver performs surprisingly well. With an effective antenna and favorable conditions, 3 W is enough to work the world. We think you’ll find the transceiver easy to build and fun to use, and you’ll get a sense of pride sending RIG HR IS HB.

You need not be an expert programmer to use microcontrollers.

5 The Arduino family of microcontrollers is open source, so there is much free plug-and-play software available. Once you start programming, chances are you will be hooked, and will want to extend the feature set of the transceiver.

For example, you may wish to add an electronic keyer, “canned” contest messages, battery voltage reading, clock, and S meter to the transceiver.

6

1

Notes

The Arduino programming software from arduino.

cc/en/Main/Software.

2 www.arrl.org/qst-in-depth

3 Our PCB uses plated through holes and is silkscreened. See the assembly instruction manual for details and availability.

4 Jack Purdum, Dennis Kidder, Arduino Projects for

Amateur Radio, pp 439 – 477, Item No. 5007,

ARRL Item no. 0161, available from your ARRL dealer, or from the ARRL Store.Telephone tollfree in the US 888-277-5289, or 860-594-0355, fax 860-594-0303; www.arrl.org/shop/; pub-

5

[email protected].

If you have no programming experience, see Jack

Purdum’s Beginning C for Arduino, 2nd Edition,

2015.

6 Glen Popiel, Arduino for Ham Radio, Item No.

0161, ARRL Item no. 0161, available from your

ARRL dealer, or from the ARRL Store. Telephone toll-free in the US 888-277-5289, or 860-

594-0355, fax 860-594-0303; www.arrl.org/

shop/; [email protected].

All photos courtesy of the authors.

Dr Jack Purdum, W8TEE, retired from Purdue

University College of Technology in 2009. He authored 18 programming texts, including Arduino

Projects for Amateur Radio, and continues writing about various programming and ham radio topics. Jack is a Life Member of the ARRL and has been licensed continuously since 1954. You can reach him at [email protected].

6 March 2016 ARRL, the national association for Amateur Radio ® www.arrl.org

For updates to this article, see the QST Feedback page at www.arrl.org/feedback .