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

[Erik Kaashoek]

Why would using a calibrated noise source into a 1Hz per bin FFT not be a
good level check? Just subtract 3 dB due to both widebands folding and you
have a reference.
A noise source should also be great for checking flatness I think.
A noise source at -90dBm/Hz around 10 MHz is cheap on eBay. Calibration can
be done with any SA having noise markers.

On Tue, Jul 5, 2022, 21:34 Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> One of the “gotcha’s” with this is the need for dual supplies. That’s what
> lets you
> get to an 30 something volt peak to peak output on the initial amp. It
> also keeps
> all the signals ground referenced through the system.
>
> The downside ( as noted in posts from a number of years ago) is that
> sequencing
> of the supplies can matter. Having just one supply running and the other
> missing in
> action can put a bit of current where you might not want it to go. The
> common
> example is the mixer output port. Depending on how things are done, there
> are
> other possible paths. There are a lot of ways to deal with this.
>
> One quick check of “how am I doing?”: Look at the fundamental amplitude of
> the
> undistorted beat note. Then look at the 1 Hz normalized noise with both
> ports driven
> from the same device. Your noise should be more than 145 db down. The same
> should also be true of the preamp output with the input terminated in your
> magic
> 50 or 500 ohm resistance. Since the preamp likely does not pass the beat
> note
> without saturating, there is a bit of math here or there to get this all
> done.
>
> Is this gizmo going to have a flat floor right into 1 Hz? Nope. All these
> magic floor
> numbers are talking about >= 1 KHz off carrier. 1/F noise is a very real
> thing in
> your mixer, and in the op amp used for the preamp. Fortunately it also
> impacts
> signal sources. You are not likely to need -155 dbc / Hz data at 1Hz off
> carrier.
>
> Bob
>
> > On Jul 5, 2022, at 10:51 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> >
> > Bob, See below
> > Thanks.
> > Erik.
> >
> > On 5-7-2022 19:37, Bob kb8tq wrote:
> >> Hi
> >>
> >> One “cute trick” that can be done on the lowpass filter:
> >>
> >> Ideally you would like to terminate the mixer properly at the DUT and
> DUT x 2
> >> frequencies. Open circuit (or short circuit) is ok at audio. The quick
> and easy way
> >> to do this is to put an appropriate resistor ( 50 ohm, 500 ohm,
> whatever ) in series
> >> with C2 on your schematic. It is common to put an RF choke across it to
> get it
> >> “out of the way” as you head to audio.
> > That is exactly what I did, forgot to show in schematic.
> >>
> >> Spice is your friend in this case. It’s way easier to pump it into
> LTSpice or something
> >> similar and tinker then trying to come up with some sort of math
> solution.
> >>
> >> Stuffing in a resistor at the output of the lowpass is also a common
> thing. Again the
> >> value is a “that depends” sort of thing. 500 ohms or 5K are the likely
> candidates. The
> >> main value is to take the port to zero when the mixer is not being
> driven.
> > I used 5k
> >>
> >> No, none of this is a big deal.
> >>
> >> Since the ref out of the OCXO is likely a pretty noisy item, I would
> not get it anywhere
> >> near the low noise audio part of the circuit. The mixer output is
> typically grounded and
> >> the ref out is ignored.
> > There is an RC filter between the REF out and the buffer opamp, but I
> can move the mixer to ground and feed the bottom of the tune potentiometer
> (R3/R4), will test, but U2 still need 5V as reference for operation.
> > An easier method may be to connect the auto bias capacitor C1 not to
> ground but to the buffered Vref, this puts the opamp into balanced input
> mode thus eliminating any remaining noise from Vref.
> >> The EFC is driven off of the lock section of the circuit to keep
> >> things in quadrature.
> >>
> >> If things are “locking up” without a lock circuit then indeed the
> earlier post about injection
> >> locking applies.
> > Will test for locking with mixer output shorted.
> >>
> >> Bob
> >>
> >>> On Jul 5, 2022, at 3:15 AM, Erik Kaashoek via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >>>
> >>> Here a small schematic of the simple Phase Noise Analyzer
> >>> Hope this answers some of the questions.
> >>> The +5 V reference from the OCXO is buffered by U1.
> >>> The 3 ports of the used ADE-1 mixer are galvanically  isolated greatly
> helping to reduce ground loops.
> >>> The output of the mixer is low passed using C2,L1 and C3 and used as
> input to the tuning of the OCXO
> >>> R4/R4 are actually two potmeters linked with summing resistors for
> coarse/fine frequency adjustment.
> >>> Inside the OCXO is a R/C low pass filter with a corner frequency of
> about 0.5 Hz.
> >>> The potmeter setting do influence the loop gain but in practice this
> is not a problem.
> >>> The mixer output is also amplified by U2 using automatic bias done
> with a large C1 and send into the audio input of a PC running the FFT
> program.
> >>> The 10MHz output from the DUT goes into RF_GND and RF_IN
> >>> For simplicity the supply decoupling capacitors and the output DC
> blocking capacitor are not drawn.
> >>> The opamps use single +12 V, just like the OCXO.
> >>> The OCXO must have better (or just equal) phase noise performance
> compared to the oscillator being measured (the DUT)
> >>> The +12 V supply comes from a bench supply with floating ground
> >>>
> >>> By shorting C3 one can check for unwanted behavior like injection
> locking and measure the  internally generated noise.
> >>> R5 is added to measure noise levels when no DUT is connected.
> >>>
> >>> The performance is surprisingly good, although one has to use a
> frequency counter to bring the DUT and OCXO close enough for lock.
> >>> To check for 90 degrees lock the R3/R4 potmeters are tuned to maximum
> noise level while still having lock.
> >>> Listening to the audio out is like listening to a DSB receiver. One
> can hear any disturbance or stray 10MHz. like the 10MHz house clock
> distribution cable being too close (not connected) to the Phase Noise
> Analyzer.
> >>> Shielding is important to keep the noise down.
> >>> By ensuring the FFT has a bandwidth of 1Hz a calibrated noise source
> can be use to establish a power level reference, much needed because of the
> undefined gains in the PC audio path.
> >>>
> >>> I know this design is far away from what many people on this list are
> used to, but it was good enough for me to quickly check the performance of
> some oscillators.
> >>> Erik.<PNA.pdf>_______________________________________________
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> >
>
>