[BC] FM Exciters (Pic's)

[Phil Alexander]

On 25 Feb 2006 at 11:19, Harold Hallikainen wrote:

> 
> We need a definition of "crystal controlled." You cannot swing a crystal
> far enough to directly FM a crystal oscillator (you can for narrow band
> FM, but not for wide band broadcast). The various methods of generating FM
> that come to mind are:
> 
> 1. Serosoid (spelling is bad, I'm sure), which involved phase modulating
> the output of a crystal oscillator (not the oscillator itself), then
> frequency multiplying like mad to get to the output frequency.

I believe it is "Serrasoid" and I know the Gates M-5534? and M-6095 used
crystals just above 100 kHz in large JBK ovens in a phase to frequency
modulation scheme.

> 2. Phasitron, similar to above, but with a special Phasitron tube.

The Phasitron used a crystal double the frequency of the Serrasoid
because the unique beam tube could swing the phase much further. This
eliminated a multiplier stage while giving cleaner sounding modulation.
> 
> 3. FM oscillator (either on carrier frequency or a lower frequency that is
> hetrodyned up to reduce noise) that is AFC locked to a crystal oscillator.
> This is a Frequency Lock instead of a phase lock.
> 
> 4. FM oscillator that is phase locked to a divided down crystal
> oscillator. Both the FMO and the reference oscillator are divided down to
> a pretty low frequency (in the kHz, as I recall) so the FMO can be FMd
> without losing lock.

I may be remembering it incorrectly, it has been 40 years since I've 
looked at a manual, but as I recall, both the tube type RCA exciter 
as used in the BTF-xxD transmitters and the Gates/Harris used in the 
FM-xxH/H3 transmitters directly modulated an oscillator that was 
frequency locked to crystal control by dividing the carrier frequency 
down to a little above 100 kHz, comparing it with a crystal at that 
frequency and generating a DC bias superimposed on the modulating audio 
supplied to a variactor in a ?? Colpitts ?? oscillator circuit. The RCA 
had a cute little 1" CRT for reading Lissajous patterns to make sure 
the divider was correctly tuned from stage to stage to get the correct 
total division otherwise the output could be entirely out of the band.

> 5. Direct Digital Synthesis. A crystal oscillator is used as a clock to
> advance a "phase accumulator." On each clock, a number is added to the
> existing accumulated phase, advancing the phase in a circle. The
> accumulated phase goes through a sine lookup table, then a D/A converter.
> FM is accomplished by varying the amount of phase that is added on each
> clock. A higher number results in a higher frequency.
> 
> 6. DSP. I'm not sure if there are any special DSP alorightms for
> generating FM other than just doing the same as DDS.

There is, of course, use of GPS 10 MHz for phase locking DDS in the
case of synchronized exciters where transmitters are operated
synchronously.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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