Overview
Ten-Tec's Model 1254 is a general coverage AM and SSB super heterodyne receiver that comes in kit form. Controls are simple, and the display is numeric only, using six seven segment, and three fixed LEDs. While it is relatively easy to assemble, looks great, and performs well for its price, its design is strictly based on the technology and available parts from many years ago. Everything on its two circuit boards are hand solderable through hole parts, not a single part is surface mount. Almost no upgraded parts have been incorporated into the circuitry over the years.
The radio has a few limitations in its design that are important to address. One major problem is it only tunes in coarse 2.5 KHz steps when in SSB mode, and in 5 KHz steps in AM mode. These are not fine enough steps for practical usage, especially when trying to receive transmissions on single side band. In an effort to get around the large tuning steps the radio is equipped with a "clarifier" control. Its function is to slightly pull the second IF frequency mixer's oscillator off its 455 KHz center in an effort to shift the tuning frequency. Still its range of off tuning isn't broad enough to receive far off step frequency stations clearly.
The radio has the ability to store frequencies and recall them by a channel number, but the total amount of storage space available in the radio is limited. There is no computer interface available to control the radio externally, to program its memorized frequency channels, or to read digitized sound data through. Neither is there is an analog line level output. There is no DSP processing available for the radios baseband analog signals. The processor used in the radio, a PIC, uses one time programmable memory for its firmware, and once it has been programmed at the factory there is no way to update it.
A separate problem is a poor wall plug external power supply. The audio power amplifier is unregulated and unfiltered in the radio and allows a good deal of that power supply's 120 Hz hum into the audio. This is especially noticeable using headphones.
To solve these problems an upgrade kit has been developed for this radio. This upgrade kit resolves these shortcomings with minimal physical impact or design changes. No part changes, or circuit board cut and patches are required. Only a couple of wire ties, and removing and replacing of the main board's socketed controlling PIC processor with a new carrier daughter board is required. The new daughter board, only slightly larger than the processor it replaces, contains a modern low power high speed processor with DSP functionality, large internal and external program and storage FLASH, interface electronics, analog signal conditioning, I/O connectors, and a 3.3 Volt linear power supply.
Totally new firmware has been developled to control the displays, control a newly added LED on the existing display board, read from the existing keys and optical encoder, and implement a new control algorithm for the radio's existing PLL used in the tuner's VCOs. The new firmware allows the radio to be tuned in either the original 2.5 KHz steps, or new 1.25 KHz steps, 1 KHz steps, or even all the way down to 100 Hz steps, and for the tuned frequencies to be displayed directly.
A line level audio output has been added. A digital data and control interface is available and can be programmed to be compatiable with hardware and control software for either Yaesu or ICOM radios. A USB interface is also available. The existing keys now also set new modes of radio operation, including DSP noise reduction functions. There are large banks of storage available for frequency channel memories.
Technical Description
The upgrade is primarily the result of replacing the radio's PIC16C5x processor with another processor of new design from SiLabs. Since the PIC variant that is currently used great advances have been made in processor speed, program and data RAM size, reprogramablity, and on chip peripherals.
The Silicon Labs F360 chip used in this upgrade is a tiny 5mm square leadless chip. It has many advanced features including:
- 100 MIPS processor core with pipelined instruction execution
- 1280 bytes interal data RAM
- 32 kB Flash in-system programable memory
- A 10 bit 200 ksps ADC, single-ended or differential inputs
- A 10 bit DAC
- 2-cycle 16 x 16 MAC engine for DSP processing
Supporting the processor on the upgrade board are a couple of transistors for level conversion, an op-amp for audio signal gain into the processor's ADC, and a serial 1 megabyte Flash.
The upgrade PCB is two layer and measures 1.15" by 1.575" by .062", and it plugs in the socket formally for the PIC processor.
There is a 4 wire connector used for the external computer control and data interface. There is a 2 wire connection for audio signal in, and a 2 wire connection for audio out to the audio power amplifier. There is another 2 wire audio output that is at line level. There is a standard 10 pin JTAG port for processor programing and in system development.
- Firmware is in ANSI standard C with assembler where required.
- Programming is written according to structured and variable naming standard guidelines.
- System design is modularized.
Conclusion
The Ten-Tec 1254 was already a worthwhile investment, but with this low cost upgrade its features are greatly enhanced, making it the equivalent of more expensive receivers.