[sdiy] Equation for converting V PP to dBm?

Justin Owen juzowen at gmail.com
Sat Dec 31 15:42:14 CET 2011

-----Original Message-----
From: Tom Wiltshire [tom at electricdruid.net]

On 31 Dec 2011, at 13:04, Justin Owen wrote:

>> ...and this: http://www.sdiy.org/juz/2164_comp_CV_DB_01.jpg - which shows CV against DB.
>> Interestingly, as Andre mentioned - my measurements start to fall off below approx. -40dB / 0.05 V PP - but I'm pretty sure I can work on that.

> I had a discussion with George Hearn about this at SDIY UK 2010. He'd been studying the SSM2164, and noted that the VCA drops off sharply to zero at the far end. He didn't consider it a problem since it closes the VCA completely, which is probably what you want. Although I don't remember him saying that he'd see the effect from -40dB onwards, more like -80dB.

> But he had spotted the same pattern, so you're seeing a chip characteristic, not a measurement effect, I reckon.

That's interesting. When I said "I can work on that" - I was referring to being able to get better quality measurements at below -40dB. For the 'actual' 2164 measurements I used the multimeter on my scope rather than a dedicated AC multimeter because it's convenient and it gives V PP - but below about 0.5 V PP - it's not hugely accurate.

Using a multimeter will mean I'm working in RMS - but I think I can deal with that now.

Still - using the 2164 as a benchmark and knowing that it potentially has a sharp drop-off below -80dB makes me happier about using a CV of 0 to  2.5V which should get me down to about -75dB, which, for now - is fine (and very convenient)... I think.

Ta - J

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