PSK31 interface for Kenwood TS-680s / TS-140 HF Transceiver
This past week I’ve been struggling with building a good interface between my Apple Powerbook and my HF Transceiver for PSK31 (and other modes). The interface had to handle audio (both going from the radio to the computer, and vice-versa), as well as keying the transmitter. Not wanting to spend money on a USB to RS-232 converter, I opted to build a simple vox circuit to transmit whenever audio is detected from the Powerbook’s headphone output.
I used NIHFX’s vox circuit from his web site with a few modifications. For one, I used radioshack and my personal parts bin for all the parts. That meant using two separate LM741 op-amps instead of the single op-amp that N1HFX had in his schematic. Furthermore, the given schematic was for a microphone input. I lowered the gain of the first op-amp by changing the feedback resistor (R3) to 2k rather than 100k. I think that gives the firs op-amp a voltage gain of 2:1, or 3dB. I also added shunt capacitors to the input, PTT output, and even the power supply inputs, to block RF. You can calculate the value of a shunt capacitor as follows:
C = 1 / (2 * pi * Z * F)
Where C is in farads, Z is the impedance of the circuit (in ohms), and F is in Hz (frequency). I recommend you put in around 10kHz. That way, by the time you consider RF going through the circuit, the signal will be almost totally anahilated (say, down by 40dB). This will keep RF out of your transmitted audio, as well as keeping the RF from keying the vox circuit. This is especially critical with the audio from the computer to the transmitter.
I determined that my Powerbook’s only audio output (described as “Headphone/Line out”) is about 47 ohms. You can determine the output impedance of most devices by outputting a steady tone and viewing the voltage on a scope or digital multimeter. Now add shunt (parallel) resistors until the voltage drops in half (or down by 6 dB). The resistor value required for this is equal to the impedance of the source. Just remember by Kirchoff’s rules, the voltage drop across the shunt resistor equals the imaginary source resistor. So then the resistors must be the same!
Based on the Kenwood’s schematic (and I found the service manual with the schematics online, link to service manual, link to user’s manual), the input impedance to the kenwood’s PKD (packet data input, pin 11 on the back ACC2 13-pin plug), is 10k ohm. That’s rather high, and invites lots of problems such as RF.
Connecting a 47-ohm output into a 10k-ohm input implies a loss of about 20 dB or maybe more, if a transformer isn’t used. Keep in mind though, the input sensitivity of the Kenwood’s packet data input is just over that of a microphone’s level. So loosing some level is actually good in this case.
But there’s a stupid problem with this setup. Kenwood, for whatever reason, did not intend for the PKD Ground (pin 12) to be connected to the chassis, PSU, or PTT ground. In fact, you must use only the PKD ground for the audio input. If you use the wrong ground, your ALC meter will peg or wave back and forth. You do not want this, because for whatever reason, there’s a serious oscillation saturating the audio circuitry that is beyond the transmitter’s passband.
In a vox circuit, all three grounds are needed — PTT, Audio, and PSU. According the the schematic of the kenwood TS-680s, all these grounds are internally connected. But they aren’t, it’s a lie. I found that connecting the PTT (PKS, Packet Send) line’s ground to PSU ground was just fine. To isolate the audio line, connect the PKD and PKD Ground lines from the kenwood to a transformer. I used a 2k-ohm to 10k-ohm transformer. The key is to connect PKD and PKD Ground to one side, and connect the vox circuit’s audio input to the other side, and to make no connection from PKD Ground to other grounds. Keep it separate!
I also added a shunt capacitor across the high-impedance side of the transformer, because high impedance lines are very susceptible to interference problems. Use the above formula. To lower the audio level even more, I put in a 10 dB pad on the low-impedance side of the transformer, and I lowered the Kenwood’s data input level. Here’s how I lowered it:
Remove the radio’s top lid, and set aside. Remove the internal screws that allow the top circuit to hinge open. With the VFO knob facing you, the top circuit hinges to the left. Now locate VR1. There are several VR1’s. You want to adjust the one on the vertical circuit board attached to the face plate. The knob faces up, so it’s easy to turn. It’s located just behind the M Ch. knob on the front. Mine says “103″ on it, and has an orange screw. The default setting is 50%. You can “read” the position of the knob by hooking a DC volt meter up from chassis ground to the center pin of the pot. Low values (rotating CCW) indicate lower level of audio. I turned mine down to 2.5k-ohm.
Lastly, you can get a nice constant-level data output from pin 3 (use pin 4 for ground, or I found the chassis ground worked fine). This output is independent of the volume setting on the front panel, and also doesn’t contain any interface beeps. You should also use PKS (pin 9) as PTT. This has the added advantage of muting the microphone on the rig, so you won’t have to disconnect the mic. Remember to disengage the speech processor, or else you won’t get any audio through.
And now it works great! With very low power (say, 20 watts or so), you really can work the world from PSK31. My favorite program to use is W7AY’s cocoaModem, which does not only PSK31, but also RTTY and several other neat digital modes.
Remember, to operate PSK31, you must put your radio in Upper Side Band (USB) mode, even on the low-bands.
Here are a list of frequencies you’ll fine PSK31 in use, from VK6YSF’s web site:
On these frequencies, you may find as many as 20 simultanious QSOs. In cocoaModem, just select the QSO you want to monitor or participate in. The wide bandwidth of USB (3kHz or so) allows for LOTS of 31Hz-bandwidth PSK31 QSOs in very close space.
Enjoy!