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 <[email protected]> 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 <[email protected]> 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 < > [email protected]> 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>_______________________________________________ > >>> time-nuts mailing list -- [email protected] > >>> To unsubscribe send an email to [email protected] > > > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe send an email to [email protected]
