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Explanation: I'd keep 'of the order of', since here we're not talking about anything like the approxmiate frequency, but reather, the order of magnitude of the frequency — e.g. it's not thousands or millions of hertz.
-------------------------------------------------- Note added at 17 hrs (2010-07-27 09:19:43 GMT) --------------------------------------------------
Or the most natural-sounding way around it would be to say 'of the order of a hertz or so'.
It is clear that the writer wants to exemplify that his technology can handle a very wide range of frequencies starting with sismic waves (very low frequencies) all the way up to radio waves (high to ultra high frequencies). When he says "de l'ordre du Herz" he clearly means of about a herz or so. So for him the frequency range is from about 1Hz to about 10 Giga. An order of magnitude is an approximation whereas here he's being highly specific. He's spelling it out for us.
...saying "in the Hz range" is that if you fial to specify some kind of number, it tends to naturally lead to an interpretation of 'hertz, as against some other unit of measurement...' — what is actually crucial here is the number of hertz involved, and not the actual unit of measurement per se. Consider that the writer might well have said "From 1 (Hz) to 10 giga Hz" — the first (Hz) is really needed to avoid its being misread as '1–10 GHz'.
That's the whole problem — although it works in FR, because it uses the def. article and so 'du' Hz is OK, in EN it doesn't work, as we would naturally use the indef. article — of the order of a Hz; however, we don't usually say 'a hertz' in EN, since it isn't an everyday unit like a foot or a metre; so it feels much more natural to say '1 Hz' instead of 'a Hz' (and even more so if you use the abbreviation!) I don't think you should be slavishly worried about the fact that a figure is not used in the original, I feel sure the writer would have done so if it had been the most natural way of expressing it in FR; and after all, they didn't shy from using 10 Gig — let's not forget that when you get up into the gigahertz region, one can legitimately talk about a band of frequencies, whereas is sounds a little odd to talk about the '1 Hz frequency band' (though of course it is techncially perfectly valid to do so!)
We must not lose sight of the fact that we are simply talking about 'orders of magnitude' here...
The most natural-sounding way around it might be to say 'of the order of a hertz or so'
The whole point is, 'hertz' is the unit of measurement (exactly the same in both cases!), while G is simply the multiplier, giga, = 1,000 million.
So the contrast is perfectly valid between 1 and 10,000 million Hz — all they're doing is expressing what a vast range of values their wonderful invention can cover!
I agree it's best to avoid using a figure, but we are only talking 'orders of magnitude' here, so it would be OK — but mediamatrix has come up with a suggestion in his peer comment to my answer which avoids quoting a specific figure (and thereby being obliged to use the abbreviation).
I'm not sure about putting 1Hz as there is no 1 in the source text, I understood that the waves processed were like Wendy Leech said measured in Hz rather than GHz
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which can be measured in Hertz
Explanation: because these waves are smaller, they can be measured in hertz, rather than a larger unit of measurement.
Wendy Leech United Kingdom Local time: 19:35 Works in field Native speaker of: English
The translation workplace 
peer agreement (net): -1
