[time-nuts] Re: DIY Low offset Phase Noise Analyzer

[Bob kb8tq]

Hi

If you are running a high gain op-amp to buffer things into a
sound card *and* using the same op-amp output to drive the
EFC, then you will have problems. 

Simple answer is to use a couple of op amps.

Buffer the mixer with something low noise. Get the output of the
mixer up to the point it almost saturates the op amp. Just how 
much gain that is depends a lot on your parts and power supplies.
Running +/- 18V supplies into this op amp is no at all unusual.

Since this output is linear, you have the full range of the beat note
present. Nothing has been lost (yet). 

One path off this device goes to the high gain stage to the sound
card. If the beat note is present, you will have clipping there. 

The other path goes to whatever you do to run the EFC. There are
*many* approaches that could be used. One of many is a variable
gain / variable roll off amp to “set” the PLL corner. That is followed
by a simple summing amp to tune out the DC offset on the EFC.

One thing to note is that once you get past the fancy amp in the 
first stage, the noise properties of the other amps are much less
important ( assuming the supplies are fairly high ). This makes the
rest of the circuit pretty cheap to build.

Bob

> On Jul 4, 2022, at 6:02 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi David,
> Let me explain the DIY "measurement instrument"
> DOCXO into LO port of a mixer, DUT into RF, IF port with low pass filter to both steer the Vtune of the DOCXO and into an opamp to amplify 1-100kHz into PC audio input. On PC audio spectrum analyzer.
> So yes, the limiting factor is the measurement instrument, probably the opamp
> Further improvements of the setup has reduced the noise level at 10kHz offset to -145dBc/Hz so the phase noise of the DOCXO plus the AR60 is at least -145dBc/Hz at 10kHz offset. If the DOXCO is, as the spec states, -150dBc/Hz at 10kHz offset that the AR60 can not be above -145dBc/Hz at 10kHz.
> 
> On 4-7-2022 12:01, David C. Partridge via time-nuts wrote:
>> Are you sure the PN floor of the measurement instrument isn't the limiting factor?
>> 
>> David
>> 
>> -----Original Message-----
>> From: Erik Kaashoek via time-nuts <time-nuts at lists.febo.com>
>> Sent: 04 July 2022 10:14
>> To: time nuts <time-nuts at lists.febo.com>; Erik Kaashoek <erik at kaashoek.com>
>> Cc: Erik Kaashoek <erik at kaashoek.com>
>> Subject: [time-nuts] DIY Low offset Phase Noise Analyzer
>> 
>> :
>> 
>> At 10kHz offset the Phase Noise of the DOCXO should be -150dBm but
>> unfortunately either the noise of the ultra low noise opamps or the
>> Phase Noise of the AR60 is almost 20dB higher.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com
>> To unsubscribe send an email to time-nuts-leave at lists.febo.com
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com