Post
by Albert H » Wed Jan 31, 2024 12:14 pm
Hi Zozo.
The SAA1057 is a fine choice for an exciter, as long as the resistor on pin 1 is increased from the datasheet value of 180Ω to 1kΩ. As we know, the IC was designed for the Philips digitally tuned portable receivers (I used to have one - it was excellent).
The IC was made in huge quantities, because Philips believed that it would be a big seller. Unfortunately, Philips failed entirely to make their controller IC available as they considered its code "proprietary", and the group at Eindhoven that designed it was disbanded. It was an ASIC with keyboard matrix reading, LCD drive for frequency display, I²C bus, and memories for each band for frequently used frequencies, and was a work of art! If they had made that IC available along with the '1057, it would have been the "go-to" tuning option for many other portable radio manufacturers.
The SAA1057s ended up being sold off for pennies, because the radio manufacturers developed their own solutions (or bought the Sharp chipset). This made it fair game for the pirates.
One of the nicer features of the '1057 is the split supplies - the Vcc3 input for example will work up to 33V, giving a big voltage swing for the varicaps. I've made use of this by developing 33V regulated by a zener on the board, using a simple voltage multiplier circuit, and giving the Vcc3 input that raised voltage. This allows the sensitivity of the varicap in the VCO to be made smaller, so that the modulation sensitivity is also reduced, making the modulator less prone to hum and noise issues, whilst allowing the VCO to tune right across the band without any adjustment.
The code that Jan Kolar made available on his "Pira" site works well enough. The only thing it lacks (because of the way the SAA1057 works) is a reliable "locked" detection output. The simplest and most practical solution for that is simply to delay the switch-on of the PA supply for several seconds after power-on. This is viable because of the reliability of the '1057 PLL - it tunes and locks wherever you've set the frequency to within a few seconds - every time!
I've used the IC to tune Band IV and Band V exciters and receivers by adding a prescaler IC and using the SW tuning range. This made for a very cheap solution, especially since I used the 12F609 PIC (62p in quantity!) to load data into the SAA1057. On the pirate gear I sold, I sanded the tops of the PIC, SAA1057 and prescaler to frustrate the copying solder-jockeys. The prescaler I used was the SAB6456, which Philips developed for TV tunerheads, and was another IC that was made in huge quantities and therefore cost next to nothing!
"Why is my rig humming?"
"Because it doesn't know the words!" 
