[time-nuts] homebrew 13 dBm distribution amplifier based on NIST design 5 to 100 MHz
jmiles at pop.net
Wed Sep 24 22:39:06 EDT 2008
> It (LTSpice) has some severe limitations for most of the simulations I
You might bring those up with Mike Engelhardt (the author). He doesn't miss
> These need to be supplemented with on board filtering as they aren't
> quite as quiet as you need.
> Either the NIST style darlington buffered RC low pass filter (one per
> amplifier) and/or a modified (stabilises the shunt transistor re by
> making its collector current approximately PTAT) version of Wenzel's
> active power supply noise filter can be used.
The Jung article at Gerhard's link claims 3 nv/root-Hz at 1 kHz. Wenzel's
page claims 20 nv/root-Hz at 1 kHz. What figures would be expected from the
modified version you're talking about?
> You can easily measure the phase noise for low offset frequencies using
> a low noise mixer with appropriate (not 50 ohm) IF termination followed
> by a low noise (audio frequency) preamp driving a sound card. A 24 bit
> sound card is ideal, however 16 bit sound cards just need a little more
> preamp gain. No need for a PLL just split the output of a low noise OCXO
> or similar source drive the mixer LO port with one output and the
> isolation amplifier with the other whilst the isolation amplifier output
> drives the mixer RF port. You will need to adjust the phasing between
> the LO and Rf signals so that they are approximately in quadrature by
> using a suitable length of coax or other means. You can even take
> advantage of the 2 channel (stereo) sound card inputs to do get well
> below the mixer noise and/or sound card noise floor by using cross
> correlation techniques.
What's the current thinking re: FFT window functions for noise measurement?
Does it matter what you use, as long as the window's equivalent noise
bandwidth is factored in?
-- john, KE5FX
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