Summary of FTS-8 subtone encoder
Summary: The author needed a Yaesu FTS-8 subtone module for FM satellite access but found originals pricey. They designed a DIY subtone encoder using a PIC microcontroller to interpret radio CPU commands and output PWM subtone signals, followed by a four-pole Chebyshev low-pass active filter (cutoff 280 Hz) and a level potentiometer. The module only generates subtones (no decode), uses simple header pins to fit the radio, and keeps the design inexpensive and easy to build.
Parts used in the FTS-8 subtone encoder project:
- PIC microcontroller
- Op amp (for active low-pass filter)
- Resistors including R2 and R3
- Capacitors (filter and decoupling)
- Four-pole Chebyshev low-pass filter components
- Pulse width modulation output stage (PIC output circuitry)
- Potentiometer for output level adjustment
- Header connectors P1 and P2 (male pins to mate with radio females)
- PCB or protoboard and solder/wire for connector wiring
Many Yaesu transceivers from the 1990s and thereabouts could use an optional FTS-8 module, that provided subtone generation for transmission, and subtone decoding for CTCSS reception. One of the radios that used this module was the FT-736 multiband, multimode full-duplex VHF-UHF rig. I bought one of these radios in 1992, to use it on the many amateur satellites that were operating back then. I included optional band modules in the purchase, but not the FTS-8, because it seemed a bit odd to me to have subtone capability in a satellite radio! After all, satellites don’t use subtones, right?
Well, the years passed, and eventually FM repeater satellites were placed in orbit that did use subtones! I needed an FTS-8. But since these sats were few, and my activity was mostly on the digital sats, which fortunately had no use for subtones, I made do without.
Until recently. The digital satellites are all dead, almost all new satellites are just completely useless little beeping cubes, and if one hopes to have any satellite activity at all, one needs subtones to access the very few FM voice sats that sometimes might operate. So, I was in the market for an FTS-8.
Yaesu stopped making this module long ago. Used modules do pop up on the market, but fetch collector’s prices. I won’t pay 120 dollars for a tiny board with two chips and a couple small components on it! There is also a company that makes a full replacement for the FTS-8, but at 80 dollars it’s still too much, even if at least I would get a new, guaranteed good module. So, in the best ham spirit, I set out to roll my own, and make the design available to all of you.
CTCSS
Is anyone out there who actually uses CTCSS? I mean, who uses it to silence the receiver and only let the squelch open on signals that come with a specific subtone? I think that few people do this, if any at all. CTCSS is more typically used on repeaters, and the normal user stations just need to transmit a subtone, but don’t need capability for receiving any. At least, that is my case, and so I decided to make a module that would only generate subtones, but not decode them. This makes the design much simpler.
I used a PIC microcontroller to read the commands coming from the radio’s CPU, interpret them, and generate the proper subtones as a pulse width modulated digital signal. This is followed by a four pole active low pass filter with a Chebyshev response, calculated for a cutoff frequency of 280Hz, which is just above the highest subtone. The output is a clean sine wave, which is fed to a small potentiometer that allows setting the proper level. And that’s all!
R2 and R3 are there only to keep the op amp circuit biased during RX, when the PIC is in high impedance. While the PIC is running, they don’t do anything.
As you can see, the circuit around the PIC is pretty simple! That’s one of the many nice things about PICs. Use them once, love them forever! The active filter has many more parts, but at least all are commonly available and inexpensive. I still love active filters too, but that’s me! I don’t know about you.
P1 and P2 are header type connectors. Since the females are in the radio, and this module needs the male connectors, I simply soldered pieces of copper wire to my board, in the exact positions to fit the connectors in the radio. It works well, and it would be hard to get anything cheaper than that.
For more detail: FTS-8 subtone encoder
- What does the DIY FTS-8 subtone encoder do?
It generates CTCSS subtones for transmission using PWM from a PIC and an active low-pass filter to produce a clean sine wave. - Does the design decode subtones for reception?
No, the module only generates subtones and does not include decoding capability. - Which microcontroller is used in the project?
A PIC microcontroller is used to read radio CPU commands and generate subtone PWM output. - How is a clean sine subtone produced from the PIC PWM?
The PWM is passed through a four-pole active Chebyshev low-pass filter with a 280 Hz cutoff to produce a clean sine wave. - Why are R2 and R3 included in the circuit?
R2 and R3 keep the op amp circuit biased during RX when the PIC is high impedance; they do nothing while the PIC is running. - How is the module connected to the radio?
Using header connectors P1 and P2; the author soldered wires to the board in the male positions to mate with the radio's female connectors. - What is the filter cutoff frequency and why?
The cutoff is 280 Hz, chosen to be just above the highest subtone frequency. - Is this DIY solution intended to be inexpensive?
Yes, the design uses common, inexpensive components to avoid high costs of original or replacement modules.
