That explains the "low pass filter" needed at the end...ditch the = extremely high ones and keep the lower ones. Odd behavior but I think I = have it down now!! Thanks!!! >>> firmware@TDS.NET 11/12/03 11:51AM >>> When you mix the freqs you get a very dirty signal that contains the sum freq (extremely high) and the difference freq (low, usually audio) Considering the enormous difference between these two components they are very easy to separate with lowpass filters. (do I here a "click" yet?) :-) -Denny ----- Original Message ----- From: "Tim Hart" To: Sent: Wednesday, November 12, 2003 11:21 AM Subject: Re: [OT]: Simple questions... The heterodyne just sounds counterintuitive.... Ultra High + Local High =3D Low? AM is a good example....I've not actually built an AM radio...but I guess it does the same thing. Turns 600 AM into something I can hear. One person out on the web converted an AM radio into a heterodyne.... I'll keep reading....it will "click" sooner or later.... Once again Thanks for the help!!! Tim >>> rixy@VVALLEY.COM 11/11/03 04:22PM >>> Just looked at the bat detector schematic. It's just lie I explained. The LM567 is the oscillator. Q1 and Q2 make up a preamp and high pass filter. Both feed the fet mixer Q3 via R7/C2 and R11/C5. The resultat "tone" or ticking is amplified by U1. Simple. Rick Tim Hart wrote: > I found some "Bat Detector" circuits that seem to be exactly what I'm wanting to do. I'm still lost as to why a local signal mixed with an ultrahigh signal makes something you can hear? > > They are used an LMC567 "Phase Locked Loop IC" to make the local oscillations. Which is Greek to me... I've used the LM386 before so I'm cool with that...but the rest still doesn't make much sense. > > Maybe you can make sense of it... > http://www.njsas.org/projects/bat_detector/populel_sch.html=20 > > I appreciate the help!!! > Tim -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu=20 -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu