Most of the USB sound cards have a capacitor coupled input. If you jumper
the cap they will go down to DC.
T
----- Original Message -----
From: "Richard (Rick) Karlquist" <rich...@karlquist.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Thursday, March 24, 2016 9:31 AM
Subject: Re: [time-nuts] Spectrum Analyzer Specifications
Thanks for the explanation. I actually built
something like that 30 years ago using an LT-1028
op amp, only it used an HP3582 FFT box instead of
a sound card (which didn't exist at the time).
Does anyone have any experience with the low
frequency cutoff of sound cards? You would
think 20 Hz, but maybe they go lower?? Any
recommended sound cards (or USB things that
work like sound cards)?
Rick
On 3/24/2016 4:42 AM, Bob Camp wrote:
Hi
Pretty simple:
Double balanced mixer, RPD-1 is one option, there are others.
Fairly simple L/C lowpass between the mixer and an op-amp.
20 db positive (non-inverting) op-amp amplifier string after the mixer
Output of the string goes to the sound card. Use a good (dual / quad)
audio op amp
Quadrature amp picks off the output of the first op amp stage, switch and
resistors to set gain, pot to set op point.
====
So what you have is an old style quadrature phase noise amp and “PLL”.
More or less a very junior version
of the 3048 test box. Like any setup of this sort, you check two similar
oscillators. They run in quadrature and
you do a few “measure this with switch in position A” sort of things to
set things up each time.
Nothing exotic.
Bob
On Mar 24, 2016, at 12:25 AM, Richard (Rick) Karlquist
<rich...@karlquist.com> wrote:
On 3/23/2016 3:56 PM, Bob Camp wrote:
and standard covered. If you want to do spurs, spend $40 and build a
phase
noise test set that will drive the sound card on your PC.
Lots of choices ….
Bob
Any documentation on this $40 phase noise test set?
Rick N6RK
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