[time-nuts] Phase noise measurement (was - no subject)
bruce.griffiths at xtra.co.nz
Mon Aug 23 05:17:44 UTC 2010
Is a better link, in that one can actually view the circuit schematics.
There are a few simple refinements that will dramatically improve the
low frequency PSRR of the single ended JFET circuits in the HPS5.1:
1) split the 3k3 resistor feeding the green LEDs into into 2 series 1k6
resistors and bypass the common node of these 2 resistors to ground.
This low pass filters the noise current flowing in the LEDs due to power
2) It would probably be even more effective if the base of the cascode
transistor were driven by a voltage equal to the JFET source voltage
plus about 3.7V.
It should, for example, be possible to use a selected JFET to do this.
3) The output servo should drive the noninverting input of the opamp via
a CBCS cascode (or equivalent) with a load resistor connected to the
input stage positive supply rail.
This should improve the PSRR dramatically. I use something similar in
one of my low noise preamps albeit with a few LEDs in series with the
resistor to provide most of the voltage drop as in my case the required
voltage drop is reasonably predictable. This reduces the noise
contribution from the servo integrator.
Bruce Griffiths wrote:
> The link isnt particularly useful as guests cant view the attachments
> and registration is disabled!!!!
> dk4xp at arcor.de wrote:
>>> Wenzel Audio Amp referred to in this email. Perfect! I drive with it a
>>> 3561A and a 7L5! Works for me. The only problem is getting any more
>>> Any recommendations?
>> NXP BF862, available from digi-key.
>> I have used it in a similar hookup with good success. Its virtue is the
>> low noise voltage AND low input capacitance at the same time.
>> You could deploy MANY of them in parallel until you get
>> into the capacitance range of a single Interfet device.
>> One heroic effort for audio is here:
>> http://www.diy-audio-engineering.org/index.php?board=2.0 HPS5.1
>> I currently use 3 pairs of SSM2210 in front of a AD797.
>> regards, Gerhard
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