On Wed, 10 Dec 2014 15:33:54 -0500 (EST), bitrex <bitrex@de.lete.earthlink.net> wrote:> >> During the 1990's, most of the RF, IF, demod, and audio circuitry was >> crammed into a single IC, surrounded by a few discrete components, and >> run by a micro. However, the receiver was still fundamentally an >> analog design full of LC and ceramic filters. >>=20 > >Thanks for the reply. I'm still curious how in these later > designs the input filter was tuned to be offset from the LO by > the IF frequency; I don't think they're using variable caps or > diodes. I think I'm missing something - when you have have a PLL > synthesized LO is there a way that the need to tune the front end > filter can be eliminatedIf you use a much higher than usual IF you can get image rejection quite = a bit easier. Thus (little or) no RF side tuning at all. ?-) =20
Semi OT: History of consumer radio designs
Started by ●December 9, 2014
Reply by ●December 11, 20142014-12-11
Reply by ●December 12, 20142014-12-12
On Wed, 10 Dec 2014 15:33:54 -0500 (EST), bitrex <bitrex@de.lete.earthlink.net> wrote:> I'm still curious how in these later > designs the input filter was tuned to be offset from the LO by > the IF frequency; I don't think they're using variable caps or > diodes. I think I'm missing something - when you have have a PLL > synthesized LO is there a way that the need to tune the front end > filter can be eliminatedWhich later designs? I guess you mean PLL design. The front end was always broadbanded from 88 to 108 MHz. With a 10.7 MHz IF frequency, the 20 Mhz band pass is just a little smaller than 2 * 10.7 MHz, which would be the frequency range most like to produce receiver image reception. A tracking filter front end would be nice in high signal strength areas, where some additional off frequency rejection would be useful to reduce intermod. However, with FM capture effect, it doesn't buy much for consumer FM listeners, who rarely listens to weak signals, where it would most beneficial. On the other foot, it's not that difficult to build a tracking filter RF amplifier, that tracks the LO frequency. All that's needed is a lookup table that relates the RF tuning frequency with the varactor tuning voltage. If I had surplus CPU cycles, I can do it by sweeping across the approximate receive frequency, stopping at the varactor voltage with the best SNR, and storing the voltage in RAM or FLASH. Extrapolated to its logical extreme, I helped with the design of a fairly conventional VHF/UHF NBFM receiver and PLL LO, that had no tuning adjustments. However, it never went into production because the savings in production tuning labor and lower component costs were negated by the increased cost and complexity of the design. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●December 12, 20142014-12-12
On Wed, 10 Dec 2014 09:47:56 GMT, Jan Panteltje <panteltje@yahoo.com> wrote:>One or 2 years ago I bought a Tecsun radio: > http://www.ebay.com/itm/111288607108It's a very good radio (due to the SiLabs chipset) for a different reason. The IF bandwidth is sufficiently narrow, and DSP demodulator sufficiently noise immune, to remove the overlapping HD Radio sidebands from the receive signal. Most other non-DSP receivers will pickup the HD Radio junk, and all you hear is distorted garbage. With these receivers, it's clean. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Reply by ●December 12, 20142014-12-12
On a sunny day (Fri, 12 Dec 2014 13:07:27 +1100) it happened Clifford Heath <no.spam@please.net> wrote in <25siw.309932$ZT5.40650@fx07.iad>:>If I can get the patch I'm sure you can too. >It's a encrypted data file to program the internal >DSP, so knowing the registers is not enough; you >need to be able to produce the DSP code and encrypt >it. > >We got the SSB patch because for DF use, you tune to >maybe 1KHz off the fundamental CW signal to get a >heterodyne whistle.I did receive SSB with it, used my Raspberry Pi as signal generator http://panteltje.com/panteltje/newsflex/download.html#freq_pi to beat against the station, so say if station is at 14 MHz, set raspi signal to 14 MHz, and keep it close to the radio. That other Tecsun I have with SSB has no USB LSB selector, took a bit getting used to it.
Reply by ●December 12, 20142014-12-12
