A Simple PC to Radio Interface
Jose I. Calderon, DU1ANV
Makiling Amateur Radio Society (MARS)
The first article written by this author on a homebrew project, “A multi-mode PC to Radio Interface for PSK, CW, PACKET, etc”, created a clamor by readers and fellow Hams for a simpler project to introduce new Hams who are beginners in home brewing. This second construction project will perform the same function, less the amenities and convenience of the more advanced interface project described earlier. The first project was intended for the seasoned home brewer who wants a good looking add-on to the station setup plus finger tip operation for easy mode switching, adjustment and calibration.
The simple interface project
presented here is the circuit that I designed during the early days of my trying
the new mode of communication via packet Radio through the use of software based
packet radio programs via the computer’s soundcard, rather than using an external
terminal node controller (TNC). After my completion of this simple project, my
old reliable but very expensive PK-232 MBX external TNC went to the display
shelf in the shack.
This circuit can be used to mate your personal computer digital mode programs to the radio. With this simple but functional circuit, you can work PACKET, PSK, SSTV, RTTY and other software based digital mode programs, including CW if the home brewer will be willing to take some minutes to analyze and re-wire the TTL circuit. This circuit can make the PC and Radio to handshake and let you similarly enjoy the digital highway in Ham Radio with all its simplicity.
The interface uses minimum number of essential parts. There are no built-in controls in order to simplify this construction project. The circuit is a straight forward input/output contraption to be interfaced between PC Soundcard to radio via the use of the common audio shielded cable, with of course the use of an extra microphone cable taken from the junk box. However, input/output levels coming to and from the radio must be adjusted separately by adjusting the radio’s audio volume control and the PC’s soundcard mixer controls (more on this later).
The principle of the interface is the same. The modulation and mode that is generated by the computer software program must be transferred via the associated soundcard output to the radio mic input. On the other hand, the received audio information must be transferred from the radio’s speaker output to the input of the computer’s soundcard, all at the required signal levels. These two operations must be transferred at the right time and sequence by a switch (TX/RX) automatically.
The Modulation and receive interface: The circuit of this interface is shown in Figure 1. (A). The design was optimized by using a resistor, R2 (10Ω ), to act as the load resistor for the speaker (normally 4-8Ω impedance) output. Caution! ….This load resistor must be at least 2 watts power rating. Since the Line input impedance of most soundcards are in the range of 600 to 2000 ohms, a resistor, R1 (470Ω), was added in series to form an impedance match to the required input impedance of the soundcard. This pad configuration will approach proper balance of input/output impedance matching without excessive loading, whichever circuit is looking at the receive interface.
The same design concept of impedance matching is incorporated in the modulation interface. However, since the microphone input of the radio requires a relatively low input voltage (about 300 mV to 500 mV at 600 input impedance), a voltage divider pad is required to reduce the soundcard output to the level required by the microphone input and keeping the correct impedance match also in mind. The voltage divider I used in this circuit is the combination of R3 (10KΩ ) and R4 (100Ω ). Hence, the Line OUT is fooled by looking at a high impedance load and at the same time, the mic input is fooled by looking at a low impedance as if it were a microphone load connected to it. Therefore the output/input circuit is matched with minimal signal degradation.
The TTL Switch: The Transistor Transistor Logic switch is the electronic circuit that is used to switch the PTT to go to RX mode and TX mode automatically as it receives the commands from the computer, see Fig.1 (B). The input of this circuit is fed by the command from the DB9 RS232 serial output terminal of the computer (usually called COM port). Only two terminals are used for this purpose. The GND terminal, and either the RTS or DTR terminal of the RS232 is used. Please refer to the article “A multi-Mode PC to Radio Interface” in this website for the detailed terminal designations and schematic diagram of the DB9/RS232 configuration.
Figure 1. The Interface Circuit
(A). The receive and modulation Interface
(B). The TTL Switch
You need the following parts to assemble before starting to wire and solder the components:
R1 – 470Ω , ¼ w (Color code: Yellow – Violet – Brown)
R2 – 10Ω , 2 watts (color code: Brown – Black – Black)
R3 – 10KΩ , ¼ w (Color code: Brown – Black - Orange)
R4 – 100Ω , ¼ w (color code: Brown – Black – Brown)
D1 – 1N4148 (Band is cathode [K] – Signal switching diode)
D2 – 1N4001 (Band is cathode [K] - Rectifier diode)
3. Transistor :
Q1 – 2N2222 metal case (TO18). General purpose signal and switching transistor
4. Audio mono-plugs : Three (3) pieces of 3.5 mm diameter
5. Audio shielded cable (Mono type) : About 2 meters long
6. One (1) piece of DB9/RS232 female cable plug
7. One (1) piece of Hobby copper clad circuit board
8. One (1) PTT/Microphone cable with matching microphone plug
Tools required: A low power (25 watts)
soldering iron, a supply of soldering lead, Long nose pliers and diagonal wire
cutter and other hobby station hardware.
Part Items 1-5 can be obtained from any local radio and electronic store in your neighborhood. The copper clad circuit board (Item 7) is also available in some bigger stores. It comes pre-etched in square copper clad square blocks (which I used) or you can use another type, with PC traces pre-etched in parallel. Or, make your own! The required circuit board area is not large. About 3 to 4 square inches of circuit area is enough.
The PTT/microphone cable (Item 8) can
also be purchased from big radio stores but I am very sure that you already have
one in the junk box. Those old and kaput stock microphones thrown in the box
will serve as a perfect source of this cable.
Lastly, the DB9 female computer cable plug is cheap and available in any computer store. If you are a computer geek, you may even find it in your junk box.
Assembly and Wiring
First, prepare the circuit board and clean to remove any burs. In order to save space and you want to have a relatively compact interface, you should mount and solder the components in any of the techniques as shown in Fig. 2. If you have opted to buy the square copper clad blocks, the orientation of the components as they will be soldered will look like the layout shown in Fig 3. Use this expanded view drawing of the PC board layout and wiring setup as a guide in positioning and soldering the components.
Fig. 2. Mounting and Soldering Techniques
Fig. 3. The Circuit Board and Wiring setup (expanded view)
This “dead bug” technique of soldering the components is quick and easy to follow. At the same time, trouble-shooting is a cinch including replacement of parts in case the renegade, uncle Murphy’s Law, begins to set in. The orientation of the diodes and the switch transistor is a little bit critical for the new home-brewer. The signal diode (D1) is a miniature type and the color band (K) may not be very obvious to the eye. It is better to use your ohmmeter as the continuity and polarity checker to identify the cathode (K) terminal before installation. If reversed, the electronic switch will not work. The same is true to the protection diode (D2). In general, most volt-ohmmeters (VOM) have test prods, one colored Black and the other is colored Red. The polarity of the black prod is actually positive (+) and the Red prod is negative (-) in continuity or resistance test mode (R x scale). In this case, connecting one lead of the diode to one test prod and the other lead to the other test prod, one can check which one is the cathode side lead of the diode. The test position of the Red prod that makes the VOM needle to move forward (full scale) is the cathode side. Reversing the position of the test prods in the diode leads will reverse bias the diode and the needle will not move forward.
Caution!....if by reversing the test prod position but still the VOM needle will move forward in the same extent as before, this condition indicates a shorted diode and should not be used. The other critical terminal positioning is the switch transistor. Invert and point it’s bottom towards your eye and position the ear guide to the left side. The nearest terminal to this ear is the emitter (E), tracing clockwise to the middle is the Base (B) and the last is the collector terminal (C). To mount this transistor to the pc board copper square soldering terminals, you have to carefully bend the Base lead at the bottom of the transistor to it’s opposite side to match the connection and soldering point when mounted upright (See Fig. 3). Once the components are installed and solder bridges are soldered, Re-check your assembly by using Fig. 3 as a guide before proceeding to the final wiring setup.
The last step is connecting the various wiring harness between the computer and the radio transceiver. The wiring setup shown in Fig. 3 is self explanatory and should be easy for the home-brewer rookie to follow. The length of these harnesses depends on preference but the shortest length possible is best desired to prevent RF pickup during operation. The ends of these harnesses must be terminated by the appropriate plugs that will mate with the radio in use and the computer. In general, the soundcard Line input/Line output jacks require 3.5 mm phone plugs and the same to the speaker extension plug. The RS232 cable that connects from the TTL input to the DB9 female plug can be the same as the audio shielded cable used in the modulation and receive interface circuit. Finally, connect the microphone /PTT cable from the interface board to the matching microphone connector plug.
You may leave the circuit board as is, without cover, but it is highly recommended that the whole should be enclosed by a small aluminum metal box to prevent RF pickup. This is critical if you work mostly in the HF bands via PSK, RTTY and SSTV mode. In the VHF range, I have worked in PACKET radio mode without any problems of RF pickup with the circuit board left dangling and without cover between my computer and VHF transceiver. But, for the sake of aesthetics, a decent box cover is in order.
Setup and calibration
Using Fig. 3 as a guide, connect the appropriate plugs to the Computer’s soundcard Line IN and Line OUT then, the DB9 plug to the computer’s COM port. The receive audio input of the Interface must likewise be plugged into the extension speaker output jack of the radio transceiver, and finally, connect the matching PTT/mic plug to the transceiver’s mic/PTT input after removing the standard mic connected to it.
Load and start your digital program (PSK, SSTV, Packet…etc….) and turn ON your radio transceiver. If you are using the PSK31 program mode, tune your radio to the upper portion of the CW band. You should be able to see the narrow band PSK trace in the program window’s waterfall display if there are stations working in this mode. Please refer to the tuning and calibration guide that was fully discussed in the first article “A multi-Mode PC to
Radio Interface “
This simple Interface does not have built-in level controls. Hence, the adjustments of inputs and outputs to and from the soundcard and radio transceiver must be adjusted by the mixer controls of the soundcard and the received audio volume control of the transceiver.
Once the program is running and the radio transceiver is tuned to the working frequency,
calibrate the setup as follows:
1. Right click the speaker icon (normally in the task bar) then left click “Adjust audio properties” to bring the audio soundcard properties window.
2. Click the audio tab to bring the audio playback and recording configuration tab.
3. Under Sound playback, click “Volume” to bring the playback mixer slider controls.
• Slide the volume control to about 75% of the bar scale.
• Slide the wave control to 75% (make sure that it is not muted)
(Note: These slide bars will control the audio modulation level going to the microphone input of the radio via the soundcard’s line OUTPUT)
4. Under Sound Recording, click “volume” to bring the recording mixer slider controls.
• Slide the stereo balance (if any) control all the way up the bar scale
• Slide the Line IN control to 75% (make sure it is not muted)
• Select and “MUTE” the microphone control.
(Note: These slide bars and setting will control the receive audio coming from the audio extension speaker of the transceiver for processing via the soundcard’s line INPUT)
This completes the initial configuration of the setup when using an onboard “Realtek" AC97 compliant soundcard. Other soundcards may have different controls but the principle is the same. During actual operation however, you may need to readjust these controls including the adjustment of the radio’s audio volume control to achieve a perfect receive and drive levels to suit the digital program and radio transceiver requirements. After these calibration and adjustments, you will need later to tweak only the computer’s speaker (click the speaker icon in the task bar) volume control and the radio’s volume control to adjust proper levels during operation. Remember! .... DO NOT OVER-DRIVE YOUR TRANSCEIVER! Over modulation will result to a high percentage of Inter-Modulation Distortion (IMD, the enemy of digital modes). Use the computer’s speaker volume control in the task bar (actually Line OUT master control – the speaker icon) to control modulation drive. Lower power levels are adequate even for DX contacts compared to SSB (J3E). Likewise, too much receive audio level will overload the input and will squash all receive signals drowned in noise, leaving your poor computer confused as to which one is to be processed. You have enough latitude in the radio’s volume control to reduce receive audio level going to the soundcard. In PSK31, you will know if you are overdriving the soundcard’s line IN. Watch the spectrogram window (The waterfall). If there is too much audio level, the spectrogram will be bathed with too many yellow speckles. You cannot see the signal trace! In this case, you should reduce the signal going to the line IN by reducing received audio through the Radio’s volume control.
Gud Luck! ….. And will be seeing you down the digital band…….DU1ANV/Joe
“If anything can go wrong, it will”
…….. Murphy’s Law
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