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

[Bob kb8tq]

Hi

The mixer sensitivity is based on both ports being saturated. You 
are trying to check the circuit end to end in it’s “as used” configuration. 
There is no real need for an added calibration source, provided you 
pay attention to the basics. 

Bob

> On Jul 5, 2022, at 12:04 PM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> 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 <mailto: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 <mailto: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 <mailto: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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