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Old 13th Jan 2018, 10:19 pm   #15
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Ultra VLF signals?

Very much so.

Also it needs a good clean local oscillator with low sideband energy 72 kHz out from its carrier, because that will leak through the mixer straight into the IF.

The Racal RA1772 went so far as having a crystal notch filter in its LO feed which switched in when low frequencies were tuned, to avoid the noise floor lifting at LF. One HP instrument also did this.

Oddly, the Racal RA1792, which is very much a cost-cut box, is considered useful at these low frequencies, but it has very basic RF filtering and no notch in the LO feed.

A lot of the usual Japanese amateur radio receivers limit their tuning to 30kHz and higher, which could be interpreted an an indication of oscillator phase noise, but it's more likely imposed by transformer bandwidth and the mixer architecture.

However, radios with 'simple' single loop fractional-N synthesisers can have noise pedestals extending 1MHz out.

David
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