A Multi-mode PC to Radio Interface For PSK, CW, PACKET, etc

A Multi-mode PC to Radio Interface For PSK, CW, PACKET, etc Jose I. Calderon (DU1ANV) Makiling Amateur Radio Society In Recent years, the proliferations of soundcard based multi mode communication softwares have invaded the Radio amateur Ham Shack. It all started with keyboard CW. A computer based generation of the Morse characters that allowed the ham operator to use his PC to key his CW transceiver with ease. Speeds up to 60 words or more per minute was made possible by these keyboard CW programs that made perfect morse characters the way it should be. The appearance of these PC-keyboard CW keyers placed the old reliable manual keyer to be set aside to become only the alternate - Just in case status inside the shack. Today, other computer-based digital modes of modulating the radio transceiver has emerged to the point that Amateur Radio operating not only to become even more enjoyable but extended the exciting world of Ham Radio into a Super Hobby. PSK mode of modulation has recently become a craze among ham operators due to its reliability and speed of transmissions compared to other digital modes. Unlike packet that requires a mandatory retransmission of acknowledgement burst of packets. PSK on the other hand uses varicodes to confirm the integrity of characters as they are being sent by the computer to the radio, ensuring the characters being sent is correct. Hence, sending PSK signals and receiving takes shorter time. Result a more enjoyable keyboard to keyboard QSO between stations. PSK is the acronym for Phase Shift Keying at 31 bauds (actually 31.25 bauds). It was introduced by the Ham Operator, SP9VRC. This relatively new mode went into leaps and bounds in recent years as more new hams adopted it. Operators with no monitoring PC program can hear the PSK31 signals in the receiver similar to a two tone ssb test signal converted to square waves. The familiar warbling tones are a PSK signal ID. You can hear these signals in the upper portion of each CW segment of the amateur HF-bands. If you are a CW buff, I am pretty sure that you are hearing these warbling tones as you tune across the CW band. To transmit and receive PSK31, you need a software program to encode and decode PSK. This is all done by your computer when the PSK program is properly loaded. Once the program is loaded to your computer, open the program to let the PSK program window appear on the screen. You can begin meantime to receive/decode PSK by feeding received audio (from radio speaker) to the computer microphone (position your computer mic in front of your radio receiver speaker). Tune to a PSK signal by left clicking your mouse pointer to a PSK trail displayed by the waterfall spectrogram window and you will be amazed to see the information being received and printed in your PSK program s Rx window. PSK signals appear as two parallel yellowish green trails resembling railroad tracks. These tracks are the PSK tones inverted 180 degrees apart at very narrow bandwidth (about 31 Hz.). By analogy, imagine or draw a two wire transmission line. At one instance the tone is sent via the left wire and the next instance, the tone is sent via the right wire, hence the warbling tone and railroad track display in the waterfall spectrogram window. The decoded characters are printed automatically in the receive window which can be read with ease. By watching the waterfall, PSK signals can be identified by their thin narrowband trails while PACTOR transmissions have much wider bandwidth trails and appear like being over modulated with lots of splatter-like whiskers. 1/9

Of course, the crude interface between your radio and the computer as described above is not the ideal. It merely demonstrates how you will eagerly want to decode PSK and eavesdrop what the amateurs are talking about. You cannot transmit with the above mentioned setup! All PC soundcard-based digital programs for amateur radio require a mating Interface between PC to Radio. To become a 100 percent digital such as a PSK31 capable station including other modes, you need a dedicated computer-to-radio interface to transmit and receive the signals. I have been using a crude outboard interface for quite sometime now. A connect - disconnect style. I finally decided to make a more dedicated one for use in my shack. My personal requirements included: One that does not require a messy plug and unplug contraption. It must be semi-permanent that removal from the station set up is possible when there is a need to test other rigs or for easy removal and transport during field days. Being a CW buff, I must add also CW capability. Including a choice of keyboard, outboard electronic and manual keying as desired. Going from CW to PSK mode must be done by merely flipping a switch and everything else is done by the computer. A circuit must be added within the interface to tweak and adjust audio drive levels of receiver audio and soundcard audio and vice versa. In particular, PSK audio must not overload the radio input to prevent excessive overdrive to cause severe Inter-Modulation Distortion during transmit (IMD -- the enemy of PSK and other digital modes). Too much drive will cause the narrowband PSK signal to cause splatter to neighboring psk trails causing QRM. Finally, the setup must be switched quickly from digital/cw mode to voice without physically removing the Interface. In short, I need to build a multimode PC to Radio Interface. The Circuit design The Interface circuit that I designed is shown in Fig 1. (Circuit A and circuit B). It is simple but functional to fit my requirements. Building the unit can be completed over a two-day weekend break. Most of the parts can be obtained from your already existing, and now burgeoning junk parts from previous projects thrown in the junk-box. The isolation transformers (T1 and T2) are something that you must seriously consider to prevent all sorts of problems when mating your computer to the radio transceiver due to possible ground loops. Notice that the circuits going to the soundcard are floating via transformers T1 and T2. The Interface circuit is divided into two general sections. They are: Circuit A, the receive and modulation interface - During receive mode, speaker audio is fed to J1. The receive audio is sampled through the signal divider (1000 resistor), to prevent loading the speaker audio line into the attenuator (Pot 1). The signal is transferred via the isolation transformer, the output of which, is made available at J3 where the received audio is sent via a shielded cable to the soundcard Line Input for processing by the soundcard and the PSK program. The signal is finally displayed in the waterfall spectrogram (called trace or trail) while simultaneously displaying the decoded characters in the receive window. During transmit mode, the characters that are typed in the transmit window is processed by the program and the PSK modulation signal is sent via the soundcard Line Out. This signal is sent through a shielded cable to the input jack (J4) of the interface with the associated attenuator circuit. The modulation signal is transferred through the isolation transformer (T2) and finally fed to the audio mic input via a shielded audio cable to modulate the transmitter. 2/9

Fig 1. The Multi-mode PC Soundcard to Radio Interface circuit. A. The Receive and Modulation Interface: J3 - To soundcard LINE IN Z= 1000 NC - CT T 1 Pot 1 Z= 8 1000 J1 - To radio ext spkr jack 50 K pot J2 - To external spkr J4 - Stereo jack from soundcard LINE OUT J5 - Stereo jack to external stereo amplified spkrs Ring Tip 1000 Pot 2 T 2 50 K pot Z= 600 See text Z= 600 P 1 To radio Mic input P2 To mic B. The TTL and Mode Switch Interface: RTS D1, D2 1N4148 s 2.7 K Q 1-2N2222 P 3 (+) PTT Spare D3 1N4148 D4 1N4001 P 4 PTT DTR D5, D6 1N4148 s 2.7 K Q 2-2N2222 SW1 PSK CW J6 To Radio Key jack Spare D7 1N4148 D8 1N4001 J7 Manual keyed Ring / shield Tip 3.5 mm Stereo plugs (Mono plugs have no ring contacts) Signal / Shield 3.5 mm Jacks : J1 - J7 3/9

Circuit B, the Transistor-Transistor Logic (TTL) circuit. The two separate circuits are electronic switches and are identical in design. The upper diagram serves as the electronic switch for the PTT (labeled in many radio models as standby or tx terminal pin in the mic wiring diagram) of the transceiver. The second circuit (lower diagram) serves as the electronic keying circuit for keyboard CW work when you use a computer keying program such as CWType. The command signals for each function is taken from the serial COM port (such as COM 1 port) of the computer. This is normally available at the back of all PC s in DB9 RS232 configuration. Only three (3) terminals are used to accomplish the switching function. The DTR (Data Terminal Ready) for CW-Key function and the RTS (Request To Send) for PSK function, or vice versa, and the terminal of the COM port. How the circuit B works - When the program/computer is idle, the DTR and the RTS terminals of the COM 1 port are -12/-5 vdc with respect to, cutting the bias to the base terminals of Q1 and Q2 resulting to non-conduction of current from each collector to, via the emitter junctions (OFF state). When the transmit command is evoked by the computer, the DTR and RTS terminals will swing to a positive voltage of +12/+5 vdc with respect to. This positive bias triggers the two transistors to conduct current (ON state) from each collector to via the emitters. This happens because the PTT and Key terminal (when connected to the interface) of the radio have a normally open terminal voltage of +5 to +7 Vdc. During this conduction state, the resistance between collectors and emitters appear to be very low (almost zero) as seen from the PTT side and or the CW key side of the radio. Hence these terminals become shorted to, actuating the PTT and or the CW key (as if they were manually shorted to ). When the base of the transistors are relieved of the positive bias (computer is idle), the collectors will stop conducting current (OFF state) and the radio will see an open connection of the PTT and CW Key to, thereby stopping the transmission and the radio will go back to receive mode ----- (QSK). The diodes (D4 and D8) were installed in the circuit to prevent any negative spikes from damaging the transistors when the PTT and or the CW key terminals of the radio will open and close. Diodes D3 and D7 in the base circuit of the transistors serves as additional security for the same purpose which are also aided by the blocking diodes (D1, D2, D5, D6). Any negative voltage appearing at the DTR and RTS terminals are blocked by these diodes (like one way valves) from entering the base circuit. There are also two extra diode inputs to serve as spare just in case the active diodes will fail. These measures will make the TTL circuits highly stable and reliable without all sorts of problems. To avoid using two different COM ports for each mode, the RTS terminal is connected to the PTT switching circuit via the first diode (D1) and the DTR terminal is connected to the CW switching circuit via fifth diode (D5). This configuration forces the Com port to grab both circuits. A manual switch (SW1, spst) was added to select from CW to PSK to resolve the problem. This is also an added security to avoid loop back when I use the accessory jack at the back panel (Acc2) of my TS 450S for PTT and audio IN, and the VOX operated Key Jack of the radio, simultaneously. This setup also allows me to leave the standard microphone permanently connected to the mic jack of the radio for immediate voice operation as desired. Construction Notes - The audio isolation transformers (T1 and T2) are small enough to fit together with the TTL circuit components in a 3 x 2 inch pre-perforated hobby Pc-board. For easy servicing, the components were assembled by soldering the components into the pc board copper (copper side up). Your imagination and caliber as a home-brewer is required to find the right audio isolation transformers. I was lucky to find that my old junkbox still contains the Radio Shack transformers. Although rusty now, but still working perfectly. Transformer T1 has a winding primary impedance of 1000 and a secondary impedance of 8 (Radio Shack cat. No. 273-1380). The output of this transformer goes to J3 (a MONO jack). Transformer T2 has a 1:1 transformer ratio with a mean impedance of 600 in each winding (Radio Shack cat. No. 273-1374). The input of this transformer comes from the TIP 4/9

of J4 (a STEREO jack) and the attenuator Pot 2. These transformers however are not critical. You can salvage the small old driver transformers from antique transistor radios (circa 1960) or dismantle old telephone modems and extract the transformers. You can also buy these or beg from fellow Hams. Any transformer having two separate windings with nominal impedance of 600 each winding will do. This is because a potentiometer was added in the circuit to adjust proper audio levels to prevent over-drive. During the assembly, please take note if your microphone/audio input requires a separate GROUND POINT! If so, you must remove the ground connection of the modulator circuit interface to prevent ground loops. Otherwise, solder the mic terminal of T2 to circuit ground. Different models of transceivers require different ground points of PTT and Mic. A final caution - Make sure that the PTT terminal and CW key terminal of your radio model has an open circuit voltage of between +5 to +7 volts Dc or thereabout (Pull to ground to activate). This TTL circuit is not suitable for keying electron tube rigs (Plate and grid block keying) that require more than 20 up to 100 volts open circuit. Doing so will ZZZaaappp the living daylights of your TTL transistors! Assembly and hookup The completed wiring and assembly on the pc board was nice and cute. Now I will have to find a way to enclose the whole in an appropriate metal box for good presentation and assume some permanency with console-like appearance to match the shack. I later found an old data switch box lying in the shack. I used this in the early days of my computing before USB ports arrive to connect two printers to my computer (A-B type). The switch box has one input port with DB25 pins and two 36-pin Centronic output ports (A or B switching). I thought that by using this box will allow me later to use two different radios (Radio A and Radio B). There was enough space for the pc-board to be installed inside this box. The two potentiometers and two keying jacks were installed in the front panel of the switch box including the mode switch. I de-soldered all the wires coming from the data switch to the Centronic terminal output ports for later terminal assignments during final hookup. The Centronic port and its companion male connector have some pins that are bridged together and all must be removed by clipping the bridges but leaving the pins intact. Below is the procedure I followed in the final hookup between the COM port of the computer and the Interface to the radio, using the switchbox: 1. Buy or get from the junk box an RS232 cable with a DB9 (female) connector at one end and a DB25 (male) at the other end. This cheap cable is available from computer stores. 2. Identify the DTR, RTS and wires/terminals by using an Ohmmeter/continuity tester. The DB9 to DB25 connector should have the standard Serial port RS232 pin configurations that is shown below: RS232 Pin configuration Terminal designation DB9 DB25 RTS Pin 7 Pin 4 DTR Pin 4 Pin 20 Pin 5 Pin 7 5/9

The wires must agree to the wiring diagram shown in Fig 2 below. If not, you must open the case of the DB25 and rewire it yourself. Only three wires are needed (DTR, RTS and ) and ignore the rest but leave them connected to their original terminals. Fig 2. RS232 DB9 to DB25 wiring pin configuration 6o 7o 8o 9o 1o 2o 3o 4o 5o RTS DTR Extrension cable RTS 14o 20o 1o 4o 7o Protective ground DTR Extrension cable DB9 (female) RS232 connector Protective ground 25o 13o DB25 (male) RS232 connector 3. Once completed, plug the DB25 end to the input port of the Data switch and proceed to identify which switch terminal is associated to DTR, RTS and. In my case, the switch goes to A port and or B port (the Centronic terminals). Therefore, the switch must be a bank of multiple single pole double throw (spdt) type. Selecting only the three connection points is easy by using your ohmmeter as continuity tester. The final wiring hookup is shown below: 4 o 7 o 20 o DB25 I/O input port socket of Data switch box RTS DTR NC SW 2 Gang switch Interface OFF Interface ON B Cw - PSK - Etc Interface INPUT from COM port of computer via RS232 cable Switch of switch Box (A or B) A B A B A B A Other switch contacts (Not used) To D1or D2 To TTL To D5 or D6 To P4 To P3 To P2 To P1 NC Centronic output B port socket of Data switch box Centronic male connector Mic +PPT PPT Gnd To Radio transceiver. Salvaged mic/ptt cable Interface OUTPUT to Radio via Salvaged mic/ptt cable 6/9

4. Final check : As an additional insurance, you must check your final wiring to review the specific connection points of inputs and outputs of circuit A and B and the data switch connections. Make sure that the DTR, RTS and are coming from the correct terminals of the computer s serial COM port and your RS232 cable is correctly wired. As a final guide, the common serial port of today s computers is a DB9 (9-pins) and has the following standard pin configuration: Those who are using outboard TNC s (Terminal Node controllers) for Packet operation use the same RS232 cable configuration. Final Hookup and Setup To setup the interface, you need the following extension harness cables; the length of each depends on your preference (I used 1 meter of each cable). Use well shielded audio cables! Use 3.5 mm mini plugs at each end except the harness from Interface output port to radio. See the table below for wiring and connecting instructions and type of harness: Cable Harness Type From Harness Points Cable 1 Mono audio cable Radio ext spkr jack J 1 of Interface Cable 2 Mono audio cable J 3 of interface Cable 3 Stereo audio cable J 4 of interface Cable 4 Cable 5 Shielded cable (RG 11 or RG 58 or a good audio cable) Microphone/PTT cable CW key jack of Radio Centronic output port of switch box To Soundcard LINE IN Soundcard LINE OUT J 6 of interface Radio mic/ptt Socket OR To accessory jack in back panel of radio Remarks May or may not be shielded Shielded Shielded Shielded Shielded 7/9

The above cables complete the interface setup. And the Interface should be ready to fire once connected to the computer s COM port and to the radio. Those radios without accessory jacks for PTT and audio input must use the mic/ PTT input in the front panel of the radio by disconnecting the standard microphone and replace it with an appropriate plug similar to its standard microphone connector and wire the output of the Interface to this. Plug your extension speaker to J2 of the interface and finally, if you want to maintain the computer s sound card outboard amplified speakers, plug the input of this amplifier to J5 of the interface. That s it folks! You are ready to enjoy PSK and other computer soundcard based digital modes. Calibration and Use Beg, barrow, or download the PSK31 software and load it to your computer. I use Digipan7 because it is a very popular program and it is completely free! Others like MixW, WinPSK etc are sharewares and are limited unless you buy. The Digipan program is powerful and complete and yet very good for beginners. Once loaded to your computer, Open Digipan and a window appears in the screen. The Digipan window is divided into 3 sections: Rx window (top), Tx window (middle) and the beautiful waterfall/ spectrogram window (bottom). Click Configure and fill the blanks (enter the your callsign etc ). Configure also the COM port to be used to connect to the interface. Ensure that the interface is properly connected between the COM port and the transceiver. Receiving PSK31 (RX or monitoring signals): Turn ON your radio and tune to the upper portion of the CW band. If there are stations transmitting in PSK, you should see the familiar PSK trail in the spectrogram (like falling debris in a waterfall) and hear the warbling tones. Adjust your radio speaker to a comfortable level. To tune to a PSK signal, bring your mouse pointer to the trail and left click it. If you see a blue background with too many yellowish speckles (too much noise), reduce the Rx audio by tweaking the Potentiometer, Pot1, until the spectrogram is clean blue with some little yellowish green speckles. But you may need to tweak again later when receiving real time PSK QSO. This completes the calibration for the receive mode. If the waterfall color does not respond, check the following: 1. Check your wiring! 2. Check your Soundcard (It must be configured as Full duplex ) 3. Ensure that the recording level of the soundcard mixer Line IN is active Transmiting PSK31 (Transmit): Left-click the T/R button located at the program window, your radio should go to transmit mode and hear the PSK tone. Now, while in this mode, watch the ALC of your radio. Adjust potentiometer, Pot2 (your Tx drive) so that the ALC is just about to disappear (almost NO ALC). The PSK tone should be driving your transceiver to about 20-30 watts. If so, the calibration is now complete. Hit T/R again to stop transmitting. Type some words in the keyboard like CQ CQ CQ DE DU1ANV (or your call) and hit T/R again. Once you are in transmit mode, you should see the characters you typed in the transmit window now being printed in the Rx window. While transmitting, watch the ALC so that there is no ALC reading or just about to Kick the ALC meter. If otherwise, you must reduce the ALC by reducing TX drive by adjusting the Potentiometer (Pot2). If your radio does not respond, check also the first two items mentioned above. In addition, check the wave out of your 8/9

soundcard and adjust the level control. If your radio does not go to transmit mode, check the COM port configuration of the PSK program you are using. Be careful! --- Most radios are not 100% duty cycle compliant. Radio transceivers like my TS 450S was designed for continues FSK (Frequency Shift Keying) duty. So, if yours is not, DO NOT OVERDRIVE YOUR RADIO. Otherwise, PSK and or other digital modes will cook your radio and the rig will leave you and go to the fields of ambrosia forever. Drive your transceiver down to 20 to 30 percent of your PEP power rating if the radio is Not rated for continues duty. Once the ALC is properly adjusted, but if the power is still more than that recommendation, you have to reduce the power by adjusting the power knob on the front panel of your radio. Please read the specifications of your radio manual! And, do not overmodulate your transmitter! Always watch the ALC meter while transmitting. Over modulation will easily result to high Intermodulation distortion that causes splatter and hard copying and QRM to adjacent PSK transmissions. PSK has a very narrow bandwidth (31 Hz). High IMD is bad for PSK and other digital modes. After Calibration : Go back to the Help menu of the PSK software program and continue configuring your personal PSK program window. Configure also your soundcard settings as these level controls will interact during your initial calibrations and during actual transmissions. Once these controls are adjusted to optimum, the settings will remain permanent unless you use the same soundcard for other programs. You will be mostly using the Potentiometer level controls of the interface (Pot1 and pot2) to adjust the receive and modulation levels. Read all the Help topics and documentation of the PSK program to familiarize its operation and use of the various Macro commands and dial scales of the waterfall/spectrogram window. In the Digipan program, the operating frequency of your transceiver will be automatically tuned by the program to within the boundaries of it s IF bandwidth filter. If you set your radio at dial 7.035 Mhz (LSB) and using your IF filter at 2.5 khz for example, Digipan will tune from 7035 KHz to 7032.5 KHz (7035 KHz as Zero Beat) in increments of 30 Hz bandwidth signals within the pass-band of the IF filter. This is done via the dial scales of the waterfall window. Of course, transmitting at 7035 KHz is not recommended due to the zero beat signal. To transmit at 7035 (radio dial), set digipan scale about 500Hz above zero beat and transmit digipan s signal at 7035 KHz by using the waterfall spectrogram scale. If you want to send CW via the keyboard, you must download the program, CWType from the Internet site and load it to your computer. I prefer to copy Morse by ear and transmit Morse via the keyboard, Hybrid bio-electronic style hi hi.. You can also connect a manual keyer or the output of your electronic keyer to J7 of the interface. You must however, check the compatibility of the electronic keyer you intend to plug into J7 to ensure that no conflicts with the TTL circuit will arise!! That s all Folks!. I look forward to having PSK31 or CW QSO with all of you down the band. It is hoped that this multi-mode interface will bring satisfaction and Joy to all Hams who care to tread the radio amateur digital highway..73 de.du1anv/joe 9/9