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PostHeaderIcon Digital VOX Decoder

Written by Don Brown AC0TS

Digital VOX Decoder Instructions

Transmit audio from the computer is connected to J1, isolated from the transceiver and doubled in value by T1,and then peak rectified by the voltage doublers consisting of D1, D2, C1, and C2, which is used to bias Q1 on, causing it to saturate and pull down the transceiver push-to-talk pin, putting the transceiver into transmit. R5 and VR1 form an attenuator, and VR1 is used to adjust the transmit audio to the right level for a clean transmitted signal. Q1 gets its collector voltage from the transceiver push-to-talk circuit, which must be the common “open-collector" push-to-talk circuit. If this voltage does not exist, the interface will not work. This interface is intended to be used with all the popular digital modes like PSK31, RTTY, Olivia, etc., which modulate a continuous carrier. For modes that require fast attack and recovery times, such as keyboard CW, Hellschreiber, Packet, or Winmor, a serial port switched interface like the Classic Sound Card Interface in the July 2010 QST should be used instead.

Circuit Board Assembly

Mount and solder all parts to the circuit board as indicated by the legend on the circuit board. Pay close attention to where the "+" ends of the electrolytic capacitors and the black bands of the diodes are placed before soldering. Cut and trim each part after soldering. Mount the resistors and diodes first, then the rest of the parts, and lastly, the tie points. If a hole is accidentally filled with solder, heat the hole to melt the solder, and push a sharp round wooden toothpick into the hole. On the opposite side of the circuit board, melt and draw the solder away from the hole, and when the solder has cooled, withdraw the toothpick, leaving the hole ready to accept another part. Check that there are no solder bridges between adjacent holes, such as those for the transistor or electrolytic capacitors, and melt away any solder bridges with the soldering iron.

Mount and solder resistor R1, 4.7K (yellow, violet, red, gold)

Mount and solder resistor R2, 2.2K (red, red, red, gold).

Mount and solder resistor R3, 680 ohms (blue, gray , brown, gold).

Mount and solder resistor R4, 10K, (brown, black, orange, gold).

Mount and solder resistor R5, 4.7K, (yellow, violet, red, gold).

Mount and solder D1, 1N4148, with band positioned as shown. D1 is next to R1.

Mount and solder D2, 1N4148, with band positioned as shown. D2 is next to D1.

Mount and solder all pins of J1.

Mount and solder all pins of J2.

Solder all pins of pre-mounted T1.

Solder all pins of pre-mounted T2.

Mount and solder electrolytic capacitor C1, 10 uF/50v, with plus end as shown. C1 is next to R1.

Mount and solder electrolytic capacitor C2, 47 uF/50v, with plus end as shown. C2 is next to Q1.

Mount and solder transistor Q1, 2N4401, with flat positioned as shown. Q1 is between the ends of D2 and R2.

Mount and solder 500 ohm potentiometer, VR1.

 

Preparing the Cable to the Transceiver

Decide how long a cable you want to use and cut off the end with the female connector, leaving about 6" of length on the female connector in case you are using an accessory connector, or a microphone connector.

In this case, plug the male end of the cable into the female end and strip and tin the wires for connecting to the connector you are going to use.

On the cut end of the cable, strip about 3/8" of insulation off, and then strip each wire about 3/16" and tin the wires.

 

Identifying the Wires

If you are using a microphone plug or an accessory plug that has a DC voltage on it, refer to the transceiver manual for the correct pin connections. Do not connect anything to a pin that has a DC voltage on it, unless it is a microphone input pin.

If you are using the MiniDin data jack on the transceiver, identify each wire by first plugging the cable into the transceiver and turning it on. Touch each of the wires, in turn, to the electrical ground of the transceiver. The wire that makes the transceiver go into transmit is the push-to-talk wire.

Solder this wire to the tie point marked "PTT" on the circuit board.

Touch each of the other wires to the now-identified push-to-talk wire. Solder the one that makes the transceiver go into transmit to the "Ground" tie point.

Run the digital software (like DigiPan - www.digipan.net) and place it in transmit. Verify that you can hear transmit audio coming from the computer speaker.

Connect a stereo audio cable from J1 to the computer speaker or earphone jack an   place the transceiver in SSB mode (or FM for FM-only transceivers).

Raise the Windows Volume or Speaker level control slider (and also WAVE slider, if present), until the transceiver switches into transmit.

Using a small screwdriver, turn VR1 fully clockwise.

Touch each of the remaining wires to the tie point marked "Data In" and solder the one that makes the transceiver generate RF output to the "Data In" tie point.

If using an FM-only transceiver, monitor with a second receiver, or scanner, and solder the wire to the tie point that produces a transmit audio sound.

With the transceiver still in transmit, adjust VR1 until the transceiver output power is one-third to one-half of maximum rated power.

Place the software into receive mode.

Connect a stereo audio cable from J2 to the computer microphone jack and raise the Windows microphone level control to maximum.

Touch each of the remaining wires to the tie point marked "Data Out" and solder the one that creates visible noise on the waterfall to the "Data Out" tie point.

Cut off the remaining wires and, if present, the cable shield wire.

 

Modifying the Enclosure

Place the wired circuit board into one half of the enclosure.

With a pencil, make a mark where each jack and the cable touches the edge of the enclosure.

Remove the circuit board, snap on the other half of the enclosure, and drill a small pilot hole where the edges of the enclosure top and bottom meet, at the place where you made the pencil marks. Leaving the enclosure snapped together, enlarge the pilot holes to 1/4" diameter.

Open the case by pressing the sides where the enclosure halves meet

Place the circuit board back into the enclosure. Note which jack is J2, and snap the two halves together.

If not still connected, connect a stereo audio cable from J2 to the computer microphone jack.

If not still connected, connect a stereo audio cable from J1 (the other interface jack) to the computer earphone or speaker jack.

If not still connected, plug the transceiver cable into the transceiver data port, accessory port, or microphone port, depending on which one you are using.

Adjust the Windows Volume Control levels until the transceiver goes into transmit when the software is in transmit. The level should be correct from the previous adjustment of VR1. If not, open the case and adjust VR1 until the transceiver is one-third to one-half power, or for an FM-only transceiver, the audio level that is about the same as that for phone operation as heard on a second receiver.

Adjust the Windows Microphone Control level until there is a faint haze of noise on the waterfall. If using DigiPan, it will be yellow on a blue background.

To prevent any possible feedback, it is a good idea to mute both Microphone and Line inputs on the Windows Volume Control panel (not the Recording Control panel, as that is used to control the waterfall signal level).

This completes the assembly and adjustment of the interface. Review each step above if necessary for proper operation of the interface.

 

Mouser Part Numbers (Quantity needed, part, number): 

(1) 2N4401 transistor, 512-2N4401BU 

(2) 1N4148 diodes, 512-1N4148

(1) Bourns trimmer potentiometer, 652-3362P-1-501LF

(2) Isolation transformers, 42XL016-RC

(2) 4.7k resistors, 291-4.7K-RC

(1) 680 ohm resistor, 291-680-RC

(1) 10K resistor, 291-10K-RC

(1) 2.2K resistor, 291-2.2K-RC

(1) Electrolytic capacitor, 10 uF, 647-UVR1H100MDD

(1) Electrolytic capacitor, 47 uF, 647-UVR-1H470MED

 

schematic


For questions contact Don Brown at This e-mail address is being protected from spambots. You need JavaScript enabled to view it