D4D: A simple QRP transceiver kit for FT8
Kit Assembly Manual
Rev. C
July 8, 2019
CRKITS.COM
Designed and Kitted by BD6CRPCB Layout by BD4RG
Thanks to:
BD4AHS, EA3GHS and WA7DUY for beta testingand valuable feedback
EA2SN (AE2SN) for Spanish manual translation
WB9FLW and K4SWL for wide introduction!
Rev A - May 29, 2019: First release.
Rev B - June 18, 2019: Update modifications and add Raspberry Pi
info.
Rev C - July 8, 2019: Update accessories, modification 5 and FAQ.
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D4D stands for DSB transceiver for Digital mode. It is a Double
Sided Band transceiver kit designed for digital mode, especially for
FT8. If you have chance to try FT8, you will be amazed by the strong
decoding capability due to the communication protocol, digital signal
processing and software. I still remember clearly a YouTube video by
W6LG who communicated with a bulb. I started to think how muchthe transceiver could be simplified if you have a moderate antenna
like a full sized dipole or EFHW (End Fed Half Wave) instead of abulb.
A DSB transceiver is compatible with but much simpler than a SSB
transceiver, however it was never used for FT8 as far as I know. I didsome experiments on my Choc perf board, a specially designed perf
board for RF circuit experiments. I started with a direct conversion
receiver for FT8 and it worked okay. Then I built a DSB transmitterand the transmitted signal can be decoded. By referring to the DSB
designs of AA7EE, VK3YE and ZL2BMI, I combined them using
only one NE602 and a PTT switch and it gave me success to make a
few FT8 QSO’s.
Personally I really enjoyed it because a manual PTT switch will savepower consumption and circuit complexity, but you will need to wellsync with computer, although it was not really a problem for me. Per
request from a few hams, I found a VOX control circuit and modify
the hold time to be compatible with FT8, and I put them together andmade a few improvements on the signal purity and frequency
stability, and it finally became our D4D.
Do we have to worry about the unwanted Lower Side Band (LSB)and Inter-Modulation Distortion (IMD)? Maybe, but for a transmitterof 1-watt, it is not really a big problem for occasional field, SOTA orexperimental purposes. Is it just a toy for a transmitter of 1-watt andonly half of the effective power? Not really, as I can easily make a
few QSO’s as far as 1500 miles range even for 40-meter band when
the propagation opens.
Here are the brief specifications I have measured (subject tochange without notice):
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Summary: Crystal controlled DSB transceiver for 20m(14.074MHz for FT8), 30m (10.130MHz for JS8Call), 40m(7.074MHz for FT8) or 80m (3.573MHz for FT8), other frequencies
could be added per request
Dimensions: 103mm(L) x 66mm(W) x 27mm(H), not including
protruding parts
Weight: about 165 grams
Power supply: 10-14V DC regulated power supply or battery pack,
12V is recommended, center positive, reversed polarity protection
supported
Current consumption in RX: about 15mA (R20* not installed) or
23mA (R20* installed) at 12V
Current consumption in TX: 200-350mA at 12V depending on
audio input levelRF output: 0.5~1.5W at 12V
Antenna connector: BNC connector
Audio in connector: 3.5mm mono, at least 600mV to activate
VOX, connects to headphone connector at PC sound card, no
dedicated PTT connector is required
Audio out connector: 3.5mm mono, connects to microphone
connector at PC sound card
Amber LED: TX status
Green LED: RX status
Frequency accuracy: -400 Hz ~ + 200 Hz
Frequency stability: Okay for FT8 mode per test. If the optional
heater resistor R20* is added, after warm up, long term frequency
stability in 10 min will be improved at the cost of acceptable shortterm frequency stability sacrifice.
Let us briefly go through the circuit.
The schematic will be available for downloading at https://groups.io/
g/crkits/files/D4D%20Kit%20Documentations . The input audio will
activate the VOX circuit of D2 (1N4148), Q5 (2N3906), Q6(2N3904), Q7 (2N3904) and Relay. The relay is a DPDT type andcontrols both antenna and power supply. The LPF consists of L2, L3
and surrounding capacitors, and it is switched to either transmitter
output or receiver input. The power supply is polarity protected by
D1 (1N5817) and switched to either receiver circuit or transmitter
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circuit. The receiver circuit is only for audio amplifier consists of Q1(2N3904) and optional heater resistor R20*, while the transmittercircuit is for RX muter Q8 (2N3904), TX driver Q3 (2N3904) andTX final Q4 (BD139). X1 is a filter in the receiver front end to help
eliminate strong broadcast interference, and X2 is the crystal for thebuilt-in oscillator in U1 (NE602). U2 (78L05) is the 5V regulator for
U1, and Q2 (2N3904) is a buffer amplifier in the TX chain.
Important notes for parts inventory:
A complete part list will be available for downloading at https://groups.io/g/crkits/files/D4D%20Kit%20Documentations . Before
you start soldering, please make sure you have all the parts ready.
Please note C25 (10uF electro) and C26 (10uF electro NP) are very
identical, but you can see NP marking on C26. Please also note that
78L05, 2N3906 and 2N3904 markings are very small and identicalso use magnifying glass if necessary. Report missing parts to yourseller.
Important notes for parts soldering and assembly:• Follow this full manual to solder the parts in several steps. Test at
the end of each step to make sure you are good to proceed. The parts
are all through hole, and the clearance is big enough so you don't
have to worry about the sequence of soldering.
• R20* is the heater for the crystal X2. One pin connects to the 12V
receive power supply and the other pin connects to GND. You willneed to put the resistor on top of the crystal X2. The pin for GND
will be also soldered to the crystal case to help ground the case andbetter heat coupling. Note that you must not short the pin for
12V to the crystal case!
• T1 transformer is not difficult if you have built KN-Q7A kit before.
Use 20cm red and golden enameled wires to twist 5 twists per inch.
Wind 8 turns on the black toroid, scratch the coating of the wire ends,
and pick red wire on one end and golden wire on another end toconnect to make the middle tap by twisting them together. The
middle tap will be soldered in the center bigger pad on the PCB. To
verify you have winded the transformer right, you can measure any
two ends of the total 3 ends and they must be conductive.
• Before you wind L1, L2 and L3 coils, please check part list to cut
correct length of the wire. If you leave the wire too long, you may
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run out of wire in the end. The L1 was originally designed for a fixedinductor, but later changed to a toroidal coil.
• Bend the LED pins to protrude about 2/3 length of the main bodyout of the board edge. The long pin is A and short pin is K.
• Put the Audio IN, Audio OUT, DC IN and ANT connectors in placeand solder. If they are floating, you may have difficulty to put on thepanels.
• Q4 soldering is the key. Plug in Q4 into the board and make sure it
is facing the board center. Put a thermal pad (insulator) in between
Q4 and the rear panel. Use the bigger screw and nut to temporarily
fix them (no more insulator required for screw as TO-126 transistor
has insulation in the hole). Temporarily put on nuts for ANT
connector. Align the DC IN connector hole, and turn over the board
to make sure the board edge is in parallel with the rear panel edge,
then solder. After soldering, fix the nuts tightly.
Important notes for alignment:
You don't need to adjust any part. Use a 12V 1A or higher power
supply and a 5-watt or bigger 50 ohm dummy load or a resonating
antenna to perform current measurements. The overall RX current isaround 23mA if R20* is connected, and TX current is 200-350mA if
you inject audio above 600mV into AUDIO IN connector to trigger
VOX to TX (You can use WSJT-x or JTDX Tune button and pushPwr bar to max and system volume to max as well) . Check FAQ
portion of this full manual to judge if the behavior is as designed,
such as IMD products.

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This is the recommended steps to build your kit.
Step 1: Power supply and LED[ ] Install DC jack.
[ ] Install polarity protection diode D1, check polarity.
[ ] Install relay.
[ ] Install power regulator U2, check marking for 78L05.
[ ] Install C18, check polarity. Short pin on the line side.
[ ] Install LED1, check polarity. Bend the LED pins to protrude
about 2/3 length of the main body out of the board edge. The long
pin is A and short pin is K.
[ ] Install LED2, check polarity. Bend the LED pins to protrude
about 2/3 length of the main body out of the board edge. The long
pin is A and short pin is K.
[ ] Install R12.
[ ] Install IC socket for U1, check orientation.
[ ] Install C1.
Test in this step:
You can use the attached DC jack to make your own power cable.
Note the center pin is the positive. Apply a regulated power supply ora battery pack of 10-14V. You can test power supply voltages after
diode D1 and 5V after regulator U2. LED1 will be on if the circuit
works fine.

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Step 2: VOX circuit[ ] Install AUDIO IN connector. Put it in place and solder.
[ ] Install R13 [For IMD modification, you can refer to last a few
pages].
[ ] Install R14.
[ ] Install R15 and R16.
[ ] Install D2, check polarity.
[ ] Install R17.
[ ] Install Q5, check marking for 2N3906.
[ ] Install Q6 and Q7.
[ ] Install C25, check polarity.
[ ] Install C26, no polarity (NP).
Test in this step:
You can test VOX operating. Power on the board and you will hear
relay click and see amber LED2 on for about 1 sec and off, thengreen LED1 stay on. Connect an audio cable from PC sound card
HEADPHONE connector to AUDIO IN connector. Use Tune
function in WSJT-x or JTDX to inject audio at 1500Hz. Increase thePwr bar to max and system volume to max as well. The relay will be
triggered and the amber LED2 will be on to show it is in TX.

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Step 3: NE602 oscillator[ ] Install C2.
[ ] Install C3.
[ ] Install C4.
[ ] Install C5, C6 and C7 [For IMD modification, you can refer tolast a few pages].
[ ] Install X1. Do not ground the case yet.
[ ] Install X2. Do not ground the case yet.
[ ] Insert U1 into socket, check orientation.
Test in this step:
You can check frequency accuracy or at least confirm it is oscillating.
My way is that I use a short wire to couple to the antenna input of areceiver. Zero beat is the way to confirm frequency. It is better than afrequency counter as the probe might introduce additional
capacitance so the frequency will be impacted.

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Step 4: RX audio amplifier[ ] Install R1.
[ ] Install R2 and R18.
[ ] Install AUDIO OUT connector. Put it in place and solder.
[ ] Install C8, C9 and C10, check polarity.
[ ] Install Q1 and Q8.
[ ] Install R19.
Test in this step:
The normal low impedance headphone should be low sensitivity as
the audio amplifier has high output impedance of a few hundredohms. However, if you touch the input pin (pin 1) of NE602 with ascrewdriver, you should hear some noise from headphone to show it
is working.

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Step 5: LPF[ ] Install C19.
[ ] Do not install C20 for now. It leaves the possibility to change the
LPF to Cauer or Elliptic filter to better suppress 2nd and 3rd
harmonics.
[ ] Install C21 and C22.
[ ] Do not install C23 for now. It leaves the possibility to change the
LPF to Cauer or Elliptic filter to better suppress 2nd and 3rd
harmonics. Note the photo is not correct here.
[ ] Install C24.
[ ] Wind toroids for L2 and L3 as following picture, it shows 16
turns for 40m. Check part list to cut correct length of the wire.
[ ] Install L2.
[ ] Install L3.
[ ] Install BNC connector.
Test in this step:
It completes the RX portion. Connect the antenna and audio cables
and test with software WSJT-x or JTDX. If the propagation opens,
you should receive something. If you are a beginner, you can refer tothe later chapters to know a few basic steps to start decoding.

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Step 6: TX amplifiers[ ] Install R3 and R5.
[ ] Install R6 and R7.
[ ] Install R4 and R8.
[ ] Install R9.
[ ] Install R10 and R11.
[ ] Install C11.
[ ] Install C12, C13, C14, C15, C16 and C17.
[ ] Install Q2 and Q3.
[ ] Wind toroid for L1 as following picture, it shows 8 turns. Checkpart list to cut correct length of the wire.
[ ] Install L1.
[ ] Wind toroid for T1 as following picture. Use 20cm red andgolden enameled wires to twist 5 twists per inch. Wind 8 turns on theblack toroid, scratch the coating of the wire ends, and pick red wireon one end and golden wire on another end to connect them to makethe middle tap by twisting them together. The middle tap will besoldered in the center bigger pad on the PCB. To verify you havewinded the transformer right, you can measure any two ends of thetotal 3 ends and they must be conductive.
[ ] Install T1.
[ ] Install R20* as following picture. R20* is the heater for the
crystal. One pin connects to the 12V receive power supply and theother pin connects to GND. You will need to put the resistor on topof the crystal X2. The pin for GND will be also soldered to the
crystal case to help ground the case and better heat coupling. Notethat you must not short the pin for 12V to the crystal case!
[ ] Install grounding pin for X1 case.
Test in this step:
No test is required in this step.

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Step 7: Assembly[ ] Split the panels and put on the thermal pad as shown.

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[ ] Plug in Q4 as shown. It is facing to the board center. Please do
not solder yet.

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[ ] Put on the screw and nut for Q4, and put on nuts for BNC
connector as well. Make sure the thermal pad is between the rear
panel and Q4 metal plate.

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[ ] Align the DC IN connector hole, and turn over the board to makesure the board edge is in parallel with the rear panel edge, thensolder. After soldering, fix the nuts tightly.

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[ ] Now it is good time to perform test and measurement, before its
final assembly into case.

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[ ] Stick on the rubber feet for case, and use screws to install thepanels to the case, and you will complete the assembly.

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Step 8: Setup your WSJT-x software
Let’s take WSJT-x software as an example. If you have used WSJT-x
for FT8 before, you can just focus on the special settings first:
In main window, push the Pwr bar to max, and check input audio in
30-60 dB range. Also you will need to adjust the system volume to
max as well. When you click Tune button, you should hear loudaudio. Although it can transmit, the distortion is big because of theoverdriving, so you need to slowly reduce Pwr bar to the point tojust trigger the relay stably. Since CAT is not supported, you willneed to manually adjust frequency on this interface. Otherwise, youwill have wrong frequency/band in the log.

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In setting window Radio tab, choose Rig to None and PTT method as
VOX.

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In Audio tab, choose correct audio device you connect your cables to.

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If you are a beginner, at least you will need to enter your call signand grid locator in the setting window General tab. Manually adjust
or sync your system clock within 1 second accuracy. Start withreceiving and decoding. Once you can decode some signals, you canlearn the whole process of a typical FT8 QSO, the signal report like 5(dB SNR) instead of 599 (RST) and other weird things of “that”
digital mode (hihi). If you check Double-click on call sets Tx
enable, the whole QSO becomes easier. That’s why in China FT8 iscalled Senior Citizen’s Toy. If you need some more help, the Internet
is a great source. You can go to YouTube to search for videos for FT8
beginners, or you can read manuals for WSJT-x. Please note that you
need to install WSJT-x version 2.0.0 or later, because previous
versions are no longer supported. If you start with JTDX, you canfind similarity from the above basic settings.

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Step 9: Connect and operate your device
Connect audio cables as follows and enjoy some QSO. Same asnormal QRP operation, it is more likely to achieve successful QSO if
you answer strong signals. If you call CQ, it is recommended thatyou add QRP to get more attention, like CQ QRP BD6CR PM01.
Listen for a minute or two before you transmit, as it is helpful towarm up the radio and reduce thermal drift.
As this radio is tiny and portable, it is strongly recommended thatyou bring it to the field. You can refer to the accessories section
below to get to know about the power supply, sound card andRaspberry Pi suggestions.

FOR LEGACY COMPUTERS WITH SEPARATED MIC
AND HEADPHONE CONNECTORS, JUST CONNECT AUDIO IN TO HEADPHONE CONNECTOR,
AND AUDIO OUT TO MIC CONNECTOR.
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FOR MODERN COMPUTERS WITH SINGLE CONNECTOR FOR HEADSET, USE Y-ADAPTER TO
MAKE CONNECTION. Y-ADAPTER IS AVAILABLE

FOR COMPUTERS LIKE MACBOOK AIR, YADAPTER WON’T WORK, SO USB SOUND CARD
WITH SEPARATED MIC AND HEADPHONE CONNECTS WILL BE USED.

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Modification possibilities
1. Improve IMD by reducing audio input level to NE602. Replace C5
by a 103 monolithic or ceramic capacitor. Replace R13 by a newly
added 22-27 ohm and the original 330 ohm. Remove C6 and picka 1-10uF electro capacitor, and make connections from C6
negative to the interconnection point of the two resistors, and fromC6 positive to C5 and pin 2 of NE602 in the original pad. Add a
103 monolithic or ceramic capacitor at the Audio input (on thephoto, it connects to both 330 ohm and 27 ohm resistors) toeliminate RF interference pickup. This modification will improve
IMD but impact RF output power at the same audio input level,
but you can adjust Pwr bar or system volume to adjust RF power.
This modification is highly recommended to reduce adjacentchannel interference to other stations.

2. Improve carrier leakage by cutting a trace near 78L05 and adding
a 1N4148 diode to reduce power supply voltage of NE602 by
about 0.7V. This is the idea from BH4DCL and my test shows very
effective result without noticeable side effects.

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3. Change LPF to elliptical filter and it will reduce 2nd and 3rd
harmonics significantly. You can follow this web page to try out
https://pa3csg.nl/?page_id=40 .
4. Improve frequency stability by adding a PTC heater to stabilize
the temperature. The frequency drift of a AT cut crystal isminimized when the ambient temperature is about 50 C.
5. For more modification ideas, you can refer to the post by
BH4DCL in Chinese with schematics at https://github.com/cnsit/
radio/blob/D4D/articles/D4D.md . For example, he suggested asimpler IMD modification to just replace C5 by a 105 monolithic
capacitor.

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Accessories

The Y audio adapter is useful when your computer has only one
audio connector for headset. Normally it is a combined 4-pin
connector for microphone and headphone and it is the same as theconnector on your cell phone. The Y adapter will split into two
separated connectors for microphone and headphone, and can beconnected to D4D using supplied audio cables. It is now ready toorder from http://crkits.com, in D4D Frequency and Option drop
down list. Before you order, please test the headphone output level tomake sure it can drive VOX to transmit. In some laptops, the audio
output is too low so you can only consider USB external sound card
like below. Note that this adapter won't work on MacBook Air. We
haven't tested MacBook Pro, but Apple users, you can consider thefollowing Creative SoundBlaster Play!3.

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The Creative SoundBlaster Play!3 is a great companion to D4D.
Although it is the low end in Creative external USB sound card
product line, it offers great SNR up to ~93dB and strong output to beable to drive 300 ohm big headphones. The big output range willoffer flexibility to pursue bigger power (poor IMD) or better IMD(small power). The Creative SoundBlaster Play!3 can be ordered inmany countries. It can replace Y audio adapter.
If you solder the DC plug to the 3x AA size battery holder withpower switch, put in 3x 14500 sized (same as AA size) Li-Ion
batteries of 1200-mAh and it will give you 8 hours of operation. The
power switch is very useful as D4D does not have one. The charging
can be easy if you have a charger of 12.6V 1A. What you need is justan adapter cable. Note that play with Li-Ion battery packs withcare. Protection board is required.
If you go to field, you can consider Raspberry Pi as the replacementof your computer. I shot a YouTube video to introduce the setup of
the tiny portable station. Please refer to:https://youtu.be/la6BSmRS0k0
In the above video, you can see a tiny home made EFHW antenna.
Now it is made available as a PCB kit. It is called Mini Wire Antenna
kit. It is recommended to order along with your D4D kit.
Also, you can find many more accessories in the drop down list when
you place your order online, such as the 6-60m Coil Dipole Antenna
and a low cost USB sound card.

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FAQ:
1. My computer does not have separated connectors for
microphone. What can I do?
Answer:Option 1: Buy a Y-adapter along. It will split the 4-pin headset
connector to two separated 3-pin connectors for microphone andheadphone.
Option 2: Buy a USB sound card with separated microphone andheadphone connectors.
2. VOX seems not triggered. What can I do?
Answer: Check if the audio cables are properly connected. As two
audio cables are the same color and size, you might make mistakes toswap them. Sometimes, you will need to adjust Audio setting tochoose the proper channel Left or Right instead of the default Mono.
Increase headphone volume to the max. You will need to adjust thebars in both system and WSJT-x software. We’ve found in somecomputers, the audio output level is too low to drive VOX. In thiscase, you can find a USB sound card like Creative SoundBlaster
Play!3.
3. It seems my MacBook Air cannot detect this device as
an external microphone. What can I do?
Answer: Buy a USB sound card with separated microphone andheadphone connectors.
4. Is it safe to transmit a short period of time at poweron?
Answer: It is safe as the power level is low.
5. Is it possible to support new FT4 mode?
Answer: Yes, and we need to customize crystals to support.
6. Is it possible to support JS8Call mode?
Answer: Yes, we already supported 10.130MHz. For otherfrequencies like 7.078MHz, we need to work out a way.

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7. Is it normal to see audio overtones in the waterfall
spectrum when I monitor the transmitted signal
nearby?
Answer: It is the inter-modulation distortion products. In order toreduce it, we must reduce power as well, see modification
possibilities chapter. For such a simple design, our recommendationis that we live with it, because the overtones are -10dB or lower. If
you have capability to make modifications, please see the above
modification instructions.
8. How to deal with the frequency oﬀset (accuracy)?
Answer: Frequency offset of -400 ~ +200 Hz is not harmful. Just livewith it. If you cannot view the whole waterfall spectrum range of2700Hz, just change Start 0 Hz setting. For 0~200Hz range, you cankeep unchanged. For -400~-100Hz, you can change 0Hz to400~100Hz.
9. How to deal with the frequency drift?
Answer: Frequency drift of a few hertz won’t impact decoding. If
you really care, you can manually adjust your TX audio frequency
following the station you are communicating with.
10. What should I do if the VOX is mis-triggered?
Answer: Check if it is due to RF interference, especially for end-fed
antennas, although the power output is small. Use coaxial cable toconnect the radio to keep distance from antenna. Add EMI
suppressor toroids at the ends of power cable and audio cables. If it isdue to hum, check your power supply quality or ground the case.
11. Is it legal to operate FT8 mode with a DSB transceiver?
Answer: Check with your local regulatory agency. This is something
new so it will take quite a discussion. It is recommended that you
strictly follow the instructions to operate when it is clarified.

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