WA8LMF Home Page | Main Ham Page | Updated 15 April 2001

Sound Card Interface with Tone Keyer

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This is an improved version of the audio interface commonly used to connect a PC's soundcard to a transceiver's receive and transmit audio circuits for PSK31, SSTV, . The usual version of this type of interface (including the commercial "RigBlaster") requires the use of a serial port to provide PTT (push-to-talk) control for the radio's transmitter.

This version includes an audio tone detector that keys the transmitter whenever transmit audio is generated by the application running on the PC. No serial port connection is required; a major advantage since PCs in ham shacks seem to never have enough serial ports. This problem becomes particularly acute in APRS setups where packet TNCs, GPS receivers, and modems for Internet access are all competing for a limited number of serial ports and IRQs.  The tone keyer is a modification of a design shown in the help system of K0HEO's WinPix32 soundcard SSTV software. Changes include substituting an NPN transistor for the original PNP (so it can work on a positive power supply), and having the power-MOSFET keying line activate a DPDT relay (instead of keying the radio directly).

Most recent and current Kenwood radios have an 8-pin Mic jack that has fixed low-level receive audio on one of its pins and 8 volts DC (intended to power illuminated touch-tone mics) on another.  Some of the models with this type of connection include the TS-450, TS-690, TM-221, TM-421, TM-731 (when the modular-to-8-pin adapter is used), etc.   I have added the receive audio connection to radios that didn't originally have it such as the TS-50, TM-211, TM-411, TS-711 and 811 VHF/UHF allmodes, etc. (On these radios, the mic jack pin that carries RX audio on newer models was unused). Normally, I pick up the hot end of the receiver volume control pot to this pin [ Fortunately, even on the latest radios, Kenwood seems to continue to use classic analog volume control pots instead of a digitally controlled attenuator ] to provide a fixed-level output that is typically 50-100mV. This level is a perfect match for the usual sound-card line-level input.  

A common mistake in soundcard-interface construction is to use an 8-ohm-to-10K transformer for the receiver-to-soundcard-line-input connection.   The incorrect assumption is that the 8-ohm speaker impedance needs to be "matched" to the sound card Hi-Z input.  You are trying to couple voltage, not maximize power transfer (the condition where impedance matching is required). All that happens is that the roughly 30:1 turns ratio (turns ratio is the square root of the impedance ratio) of such a transformer causes a 30:1  voltage stepup, delivering FAR more signal level (several volts of audio) at the computer than is required.  The soundcard input stage will overload, and make the onscreen level control almost impossible to adjust (the optimum level will wind up somewhere between zero and the first step on the Windows control panel slider!).  The audio levels at a radio's speaker terminal or the rear panel "packet", "data", or Kenwood 13-pin ACC jacks are already at the right level (about .1 to .5 volts ); all that is required is isolation. The classic 1:1 ratio 600-ohm-to-600-ohm audio "line" transformer is exactly what you want.

The interface in the schematic below was designed specifically to be connected to, and powered by, Kenwood transceivers by a single cable connected to the Mic jack. No separate DC connection or wall wart is required.  Obviously, the tone keyer circuit could be used with a separate power source on non-Kenwood radios. I have used this interface on my Yaesu FT-100 after making a RJ-11-to-8-pin adapter for the mic jack,  and after connecting the one unused pin in the mic jack to fixed receive audio as described above. The adapter consists of a surface-mount RJ-11 telephone jack with a standard 8-pin ham mic jack mounted in a 5/8-inch hole punched into the jack's cover. The two extra pins of Kenwood mics ("UP" and "DOWN" buttons) are not connected.
[  Side-note on keying the FT-100:   In order to combine the the mic remote functions that Kenwood uses 8 mic pins to implement, into only 6 conductors of a standard RJ-11 telephone jack, Yaesu uses multiple voltage levels on the PTT line -- not just HI or ground.  Various buttons on the mic pull the same line to ground through different value resistors. Directly grounding the PTT line will cause the radio to start scanning instead of transmitting. You must connect the PTT line to ground through a 27K resistor to actually transmit. ]

I use this interface  on SSTV as well as non-voice modes. On SSTV, voice transmission alternates with sound-card-generated data. I provided automatic switching that disconnects the mic and connects the soundcard output whenever a tone is present by using the second set of poles on the double-pole double-throw PTT relay.

In the interest of maximum isolation and RF immunity, I chose to use a DPDT relay (instead of solid state switches or opto isolators) to switch transmit audio and provide a positive zero-voltage-drop PTT. The maximum current available from the mic jack 8 VDC line is very limited (about 50mA) so a very high sensitivity 5 VDC relay is required. Normal 5 VDC DPDT DIP- package relays have a coil resistance of about 50 ohms. The relay I selected has a coil resistance of over 300 ohms, and will operate from the limited current available.

The SPST toggle switch disables the relay, preventing the interface from keying the transmitter, but the TX led still lights up when a tone is present, allowing the sound card output to be adjusted (using the Windows mixer level controls) for positive keying without putting a signal on the air.

I connected a surplus piezo audio transducer across the TX audio input to the interface to provide a low-level monitor of the transmitted audio (Normally, plugging a cable into the laptop line-out cuts off the internal speakers so the transmit tones can't be heard.) It produces a perfect low-level side tone for the transmitted data and doesn't load the audio line at all. (And if you want to mute it completely, stick a piece of tape over the transducer's hole.)

The schematic was originally created in Corel Draw and laid out for a 11"x17" (US tabloid-size) page. It is presented here as a 600x800 pixel black-and-white GIF image.  The text legends are barely legible in this reduced-size image.  An extremely large black-and-white GIF graphic (3085 x 1964 pixels -- 96K) to provide a high-quality printed image is downloadable below. It will be far too large to fit in your browser without scrolling. I recommend that you right-click on the link and choose 'Save Target As" to save the image as a local file on your hard drive. Then open it in a graphics application that can scale images at print time to "Fit-to-page". The freeware graphics viewer, IrfanView, available on my downloads page is perfect for this purpose.

Schematic as 3085 x 1964 pixel 96K GIF file    TONEKEYR.GIF

As an alternative, I have provided the schematic as a downloadable Acrobat .PDF file. This is a completely scalable vector image -- the Acrobat Reader automatically scales pages at print time to fit the current paper size specified in your print driver (if "Fit to page" is checked in Acrobat's print dialog) and will produce an extremely sharp image at any size. 

pdficon.gif (224 bytes)   Download Tone Keyer Schematic in Acrobat format

Tone Keyer Exterior View
tonekeyo.jpg (47826 bytes)

Tone Keyer Internal View
tonekeyi.jpg (42430 bytes)


Tone Keyer Schematic

tonekeyrs.gif (62856 bytes)