The QRPGuys LA3ZA Zero Beat Indicator gives you a visual indication when you are tuned to the callers frequency, based on the tone of the received signal you hear on your transceiver. The theory is simple. The device takes into account the amount of built-in offset your transceiver has, usually around 300-800Hz, and illuminates an led when it detects that specific tone you hear when you are tuned to transmit on the callers specific frequency.
It incorporates a CMOS LMC567 Tone Decoder IC that compares the audio tone you are receiving to a calibrated tone you determine during the setup, then illuminates an LED when that frequency is met. The board can be used outboard with the 3.5mm audio jack with it’s self-contained battery supply or mounted inside in a smaller configuration, and run from any 2.0V – 9.0V max. source from inside the transceiver using the auxiliary audio input. It is a low power device and uses less than 1mA to power the circuitry. The device will detect and respond to a tone of ~300-1000Hz.
A web search will show these devices have been around for a number of years and Sverre, LA3ZA, upgraded an older TTL design to CMOS technology and has given us permission to provide it as a kit.
My original circuit is in the post QRP Zero-beat indicator for the Elecraft K2 (2002), and a revised one in the post K2 Fixed Level Audio Output (2009). The main difference from the kit version is that I have 220 ohms in series with 470 uF from the output pin 8 to ground. It adds a little hang to the LED light making it less agile. This is something I found more pleasing to look at, but which QRPGuys decided to skip. My zero-beat indicator in action can be seen in this YouTube video.
I think they made a really nice kit out of this and am very happy with the result. A full description is on the QRPGuys website. And no, I will not make any profit from this, but please go ahead and buy the kit from them anyway!
This blog post first appeared on the LA3ZA Radio & Electronics Blog.