FM wireless broadcasters Surplus & auction update A homopolar traction motor New power measurement chip Understanding the Faraday Disk
et’s start off this month with a hot helpline topic that has both a useful new solution and a rather long history of caller interest. In particular, let’s talk about developments in… L
Lots of folks do want to broadcast over the radio. Perhaps for their own "underground" or "pirate" community radio station; to deliver a commercial "please buy my nice house" realtor message; to free a stage or studio performer from trailing wires; for use in wireless modems or in short haul telemetry; for survelliance, alarms, or baby monitors; or simply to couple a portable CD player into any nearby home or car radio.
It only takes a few milliwatts to go a few feet. A hundred feet will need a hundred milliwatts. Your local town coverage may take ten watts or so. As you might guess, the FCC places very stringent limits on what you are legally allowed to either transmit or broadcast. Anything more than 30 milliwatts or so is probably illegal.
The FM band resides from 88.1 to 107.9 Megahertz. All the FM station channels are precisely spaced every 200 kHz. Thus the channels always end in an "odd" number such as 93.3 MHz or 99.9 MHz. Long ago and far away, the original "FM broadcasters" were a simple oscillator, often based on the superb 2N918 transistor. A varactor or even plain old collector voltage modulation was used to get its mono frequency modulation.
Alas, in those days, most "analog" FM receivers all used powerful AFC automatic frequency control circuits. These receivers could easily follow any in-band FM carrier. No matter how far off frequency it was or how much it drifted.
But nearly all of today’s better FM receivers are digitally synthesized and work only with signals that are both stable and precisely on-channel. Thus, most of the older and simpler low cost FM broadcaster circuits will no longer work!
The first step in the right direction was a potent chip known as the Rohm BA1404. This provided high quality by way of its 38 kHz crystal derived stereo modulation. But the internal RF oscillator was still flakey enough that it drifted unacceptably.
How can you build your own FM oscillator that is both stable and on channel? There are two main routes that you can use.
Any stable FM system is really a contradiction in terms. For you’ll want to change your frequency for your musical or voice content. Yet you want its center frequency to be rock stable.
The "correct" and complex way to deal with this is with a variation on a PLL called the frequency lock loop. You’ll first build an oscillator that is linear frequency modulatable. Then you build a crystal oscillator of the same center frequency. The average
frequency of the two get compared and integrated, or low pass filtered.
Your resulting error voltage from this low pass filtering is a near dc waveform that’s used to continuously correct the FM oscillator. Thus long term forcing it to the proper channel frequency.
This frequency lock loop method is how "real" FM broadcasters do it. The big problem is that a lot of parts are needed in a fancy circuit.
Instead, can an ordinary crystal get "pulled" enough in frequency by a capacitive loading? The usual rule of thumb is that you can pull a crystal up to one tenth of a percent. Which does translate to 100 kHz up at 100 MHz. But that tenth of a percent is usually an outside limit. Sadly, the pulling process is usually nonlinear.
Both Sony and Pioneer found that they could pull special crystals in hard-to-design but simple low cost
50K "L-R balance"
500 "mod depth"
0.0012 10 F
150K nc nc nc
18 17 16 15 14 13 12 11 10 RIN BAL BAL VCCXOUTCAR MOD REF OSC
LIN AFB GND MOB XIN XOUTAMP GND OFB 123456789
varactor bias network
stereo audio input
38 kHz 10 pf
mono - stereo
to RF modulator
Fig. 1 – THE FM MULTIPLEX PORTION of the new Radio Shack 12-2051 FM Stereo Transmitter. The BA1404 generates a stereo subcarrier.
Copyright c 1998 by Don Lancaster and Synergetics (520) 428-4073 www.tinaja.com All commercial rights and all electronic media rights fully reserved. Reposting is expressly forbidden.