MBeez.Houston@XEROX.COM (01/16/90)
MARC I tried to get through to you but had no luck and the reply may be of interest to everyone. The article was written by a friend of mine, Hugh Wells. I sent your question to him and thius is his reply. __mike ---------------------------------------------------------------- Sender: Hugh E. Wells:El Segundo:Xerox Date: 10 Jan 90 17:02:10 CST (Wednesday) Subject: Re: tv tuner question From: Hugh E. Wells To: MBeez:Houston In-Reply-to: MBeez:Houston:Xerox's message of 10-January-90 (Wednesday) 12:42:11 PST It was nice of Mark to identify my article. on 800-900mhz the Easy Way. He indicated the article is a little short. For what it was intended, the article did its job. For a longer disertation on TV tuners he can refer to Ham Radio, January 1988, pg 25. Relative to the tuner that Mark received from Fair, the catalog voltages are still correct, e.g., B+ 15V, AGC 3-10V, and Tuning voltage 0-30V. These are typical voltages for all varactor tuners. Judging from Mark's comment about not hearing a signal, I believe that he has too many variables working at the same time. It will be necessary to work from a known position. Example: Set the AGC voltage to about 3 volts, B+ to +15-20 volts and the scanner to a fixed frequency between 46-47mhz. Then adjust the tuning voltage slowly from zero up. Stop on the first TV signal received, and adjust the AGC for maximum strength. The scanner will saturate as the signal increases -- it may be necessary to short the tuner antenna terminals temporarily to reduce the strength of the incoming signal. Again, adjust the AGC for maximum signal strength. The bottom line is to inject a known signal into the tuner in order to adjust the AGC to the correct value of maximum sensitivity. Once the AGC is adjusted, then adjusting the tuning voltage will allow tuning through the entire range of the tuner. It will be noted that Bearcat scanners have a bandwidth of about 25khz. When adjusting the tuning voltage, very slow movement is required as a signal will be passed over without even realizing it was there. I've found that a 10-turn pot is a necessity for adjusting the tuning voltage as it provides a little mechanical bandspread control. Hopefully, those Fair Radio tuners will tune into the 900mhz region. Some of the new tuners have limited tuning which stops at about channel 80. The tuner change occurred shortly after channels 80-83 were given up for commercial use. Relative to L11 and L13, no adjustment need be made. In fact, a reduction in sensitivity will most likely occur if any internal circuit adjustments are made. Yes, the tuner is broadband, but the varactors tune the front-end, and the I.F. output is tuned to approx. 47mhz. The Q of the I.F. coil is low enough to allow a 5mhz bandwidth. Adjusting the coil will not change the Q or sensitivity. For cellular phone reception, I suggest a good outside antenna up reasonably high, perhaps 20-30 feet, depending upon terrain. I've had excellent results with the Radio Shack discone and have received signals from about 20-30 miles. If Mark has any further questions, he may contact me via either telephone or snail-mail. I'll be happy to work with him. 73 Hugh 1411 18th St Manhattan Beach, CA 90266 (213) 546-2137 ----------------------------------------------------------------