[BC] Freq agile AM transmitters

[Lamar Owen]

On Tuesday 24 April 2007, Jim Tonne wrote:
> Willie:  How are you going to make a "frequency-agile"
> output harmonic filter?   The kind where you "just set
> the cutoff with a thumbwheel switch?"
>
> Making an oscillator move to a particular frequency is
> a piece of cake.  Assuming the transmitter itself has
> some harmonic content then an output lowpass or
> bandpass will also be needed.  I am not sure that
> would be a piece of cake.

The conelrad people thought it would be easy.  And it never did work right.

Most frequency agile transmitters that I have seen that were 'no-tune' types 
used switched coils and caps for the output and harmonic suppression 
networks.  This is very easy to do at 100 Watts; much less easy at 5 or 10KW.

I have a Hull model 924 agile marine transmitter; it has a whole board of 
relays, coils, and caps to do this network switching.  Unit's like SGC's do 
the same.

As I don't have a newer model amateur HF rig, not sure how the bandswitching 
is done these days, but the output networks on all the rigs I've seen have 
had similar.

As the AM band spans more than an octave and a half in frequency range, 
wideband tuning is very very difficult, and some sort of bandswitching (with 
RF contactors) would have to be done, more than likely.

HOWEVER, having said that, I HAVE seen widebanded RF amplifiers that used 
non-tuned wideband transformers good for an octave or so of frequency range; 
in particular, I have a pair of old Coast Guard beacon transmitters that have 
ferrite core out put transformers, and can transmit from around 100KHz to 
over 700KHz with no tuning. 

 The tradeoff is terrible efficiency; these are basically class AB push-pull 
audio amps, just running in the hundreds of KHz range.  These transmitters, 
made by Monitor, use two 4CX1000's in the final for about 3.5KW output.  They 
are basically driven by multiple crystals; to get a beacon on 330KHz and 
another on 250KHz, for instance, two crystal oscillators, one at 300 and one 
at 250, were used, and the broadband linear output just amplified the sum.  
There is of course no harmonic suppression at all, so everything has to be 
linear in operation, and the push-pull has to be very well balanced.

The other alternative would be a 'Power DAC' like the Harris DX transmitters, 
running highly oversampled and a good output filter.  Harmonic suppression 
could be very good with a nice broadband design (the DAC output couldn't have 
as low of an output impedance as the DX design, which requires an impedance 
transformer at the DAC summing node); with a 50 ohm output impedance obtained 
without a transformer the efficiency would be rather poor.

Designing a transmitter with no tuned circuits in the RF output that still 
meets 73.44 would be a challenge.
-- 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772
(828)862-5554
www.pari.edu