RCD originally offered active delay lines in two versions… standard (operating temp range of 0º to +70ºC) and military (Opt. ER, -55º to +125ºC). Most of the +125ºC IC’s (utilized inside the Opt ER delay lines) were discontinued resulting in product supply problems and exorbitant price increases. In discussions with customers, it was determined that nearly all had no real need of such temperature extremes. Delay lines that would operate at a low range of -30º to -40ºC, and a high range of +80º to +85ºC would be perfectly suitable. In response, RCD developed an “industrial grade” version rated for operation at -40º to +85ºC (designated as option 39). Essentially all of our Opt. ER customers have switched to opt.39. Some of the special +125ºC IC’s are still available but generally very costly so we don’t recommend it them unless absolutely necessary.