Hello Harold, Thanks for sharing your design, quite interesting! Both the application and that style of data transmission is completely new to me. For some reason, the idea that it is possible to do RF style transmissions over infrared is mind-boggling to me. -Jason White On Sun, Oct 16, 2016 at 8:45 PM, Harold Hallikainen wrote: > >> I'm quite curious about R4, which appears to be increasing the gain of >> the LO transistor. That's odd because (at least in simulations) I have >> had the exact opposite problem, too much gain, which yields a LO >> amplitude of greater than the specified 300mV maximum. Maybe harmonics >> aren't a problem in a direct conversion receiver? > > Yes, R4 is interesting. It's a 22k that's in parallel with the internal > 25k. I don't think it increases the gain since this is an emitter > follower, so the gain is limited to a maximum of 1. I'd be concerned that > this would mess up the DC bias of the circuit. The theory of operation in > the schematic does not explain why they deviate from the chip datasheet. > > So far, my SDR experience is limited to listening to websdr. But, I'm ver= y > impressed with what they've done. > > Somewhat like the NE602, I did a receiver using the SA639. This is an > infrared data receiver ( > http://www.uslinc.com/index.php?option=3Dcom_virtuemart&page=3Dshop.produ= ct_details&flypage=3Dflypage.tpl&category_id=3D23&product_id=3D67&Itemid=3D= 65&vmcchk=3D1&Itemid=3D65 > ). It has a bunch of IR photodiodes in parallel. They are reverse biased > through a tunable inductor becoming resonant at 1.8MHz. The data is FSK a= t > 10kbps at 1.8MHz. The photodiodes drive an op amp-based preamp, then into > the SA639. A crystal based local oscillator converts the signal up to > 10.7MHz where it goes through IF amplifiers with ceramic filters. The > ceramic based quadrature detector from the datasheet is used to demodulat= e > the FSK. The resulting signal drives a comparator in a PIC, then the UART > in the PIC. > > That's my recent "RF" design experience. > > Harold > > > > -- > FCC Rules Updated Daily at http://www.hallikainen.com > Not sent from an iPhone. > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 Jason White --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .