FT-817 Phone Audio interface part 2

Hi all,

I have continued to work on the interface, described in Designing a Phone/Radio interface, and I think things are looking pretty good. I have changed some component values slightly to fit in with easily obtainable parts. One approach is to use thru-hole parts for everything, but this results in quite a big board. Now SMT does scare people. but if we stick to the larger part sizes, it should be easy enough. This does however means steering clear of 0402 and 0201 tiny sized components.

My first try at drawing a PCB used SMD capcitors while keeping everything else thru-hole. Using 1/4 watt thru-hole resistors ends up taking up a lot of space. I decided to change over to SMD resistors as well, but leaving the diodes, transistors and the trimmer potentiometers as thru-hole. This allowed me to get the board size at 2 inches by 1 inch. My resistors are generally 1206 size (this is .12 inches by 0.06 inches, or about 3 by 1.5mm. Most of the caps are 0805 (about 2mm by 1.2mm) size. These are reasonably easy to manage with tweezers.

People have asked what components are used for the transistors and diodes. I am using 2N4401’s for the BJTs and 1N5819 for the schottkies. Both these components should work very well.

In some of my simulations, I have tried quite “out there” scenarios. The circuit actually still works with a 1V power supply, especially with higher audio frequencies on the digital mode being used. It works with 8V, actually very well. I have not bothered higher voltages, perhaps I should give 12V a go, however no phone is going to be supplying that kind of voltage level. I suspect that they (i.e. the various phone manufacturers) are either stepping up the cell voltage output – typically a LiIon cell that will range from 4.2V down to 3.5V or so, and stepping it up to 5V, or just feeding it in unregulated, with a 5 to 10K resistor in series to current limit the supply on the microphone pin. Any of these scenarios will work with this circuit. If anything like a 3V or higher supply is involved, there will be over 50uA to drive the second transistor to sink 450uA of current on the PTT pin. Even if other radios to the FT-817 have different loads on their PTT pins, I can’t imagine it’s orders of magnitude!

On the radio side, there are clear variations. The FT-897 has a lower impedance output on the fixed audio, and a lower Vpp level. I still expect the circuit will work fine. I also had a look at the Elecraft KX3. This does not have a data port, but rather relies on the speaker output and microphone input. The speaker output will be at a higher level than what either the FT-817 or the FT-897 will drive this circuit. The trimmer potentiometer can be turned down to help. Also the radio audio control will change the voltages seen on the output. As for the input, again the potentiometer will have to be turned down, because the circuit is feeding something approaching a line level. Microphone is a good 15dB to 20dB down on that. I would presume that the KX3 would have some forgiveness about the input impedance, as cheap mics are high impedance (10K plus), while high quality mics can be as low as 100 ohs. I’ll need someone to investigate if there is any DC on the microphone input as well, as this could be there for the same reason that there is DC on the phone microphone input. If someone has a Oscilloscope and a KX3, it would be interesting to see the audio output levels, but you would also need to know what load resistor you used.

I have a picture of the circuit for you to enjoy below:

Phone Radio circuit board image

Btw, the diodes are back to front, and this is a consequence of whatever bug is in the schottky files used by GEDA. It is easier to just highlight it as an errata. The circuit will not work if the diodes are not correctly put in place.

I used the geda suite to design the circuit, using gschem to draw schematics, ngspice to do simulations, and PCB to draw the circuit art. These programs can be a little hacky and the help files are not for the uninitiated, but they certainly get the job done. I feel pretty comfortable with these, and I am now also looking to do a design for a bias tee and a preamp for 6m/2m/70cm. More about that later.

Send me an email at vk3wam (at) gmail (dot) com if you are interested in getting one of these interfaces. If people are interested, I could sell the boards at $10 US. I’ll need to look through the cost of the materials if people are interested in kits.

73 de Wayne VK3WAM

EDIT: Here is a slightly updated board design, optimised to remove some of the via holes, improve some spacing and comply with various production house design rules.
Phone to Radio interface board