[time-nuts] Oleg' s PN test Re: A new member & PN test set

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 28 19:23:17 EDT 2016


That's probably a good example of how not to do it well.

1) The chosen mixer isnt as low noise as the various Minicircuits phase 
detectors.

2) The 50 ohm load after the filter merely serves to halve the phase detector 
gain. The IF port is terminated by a 15nF capacitor at RF and LO frequencies 
and their harmonics. This produces a frequency dependent gain, however it will 
likely be relatively flat over the sound card bandwidth.

3) Saturating both mixer ports increases the phase detector gain substantially 
and has the lowest noise 

4) Cascading the PLL circuitry with the preamp causes interaction between the 
Preamp gain settings and the PLL bandwidth. Driving the PLL circuit in 
parallel with the preamp input directly from the low pass filtered mixer output 
avoids this issue as well as your 0.1x amplifier in the PLL section.

5) Another method of calibration is desirable in order to account for 
potentially  non flat frequency response.

There are a large number of NIST papers on PN measurement including a few on 
the effect of mixer IF port terminations. A recent one 
(http://tf.boulder.nist.gov/general/pdf/2556.pdf) compares the PN performance 
of various mixers used as phase detectors.

An OCXO like the 10811A has an EFC gain of around 0.1Hz/volt.
The PLL bandwidth should ideally be less than 1/10 of the lowest offset 
frequency for which the PN is to be measured.
If the system frequency response is measured then the PLL bandwidth can be a 
little higher albeit with a reduction is sensitivity and an increase in system 
PN at the low offset frequency end of the range.

AS is the PN noise of this test set is far too high to measure the PN of state 
of the art OCXOs or indeed most modern OXCOs.

Bruce

On Monday, March 28, 2016 11:04:45 PM Oleg Skydan wrote:
> Hi, everybody!
> 
> OK. Let's start. Here is the schematics of the "test set"
> http://skydan.in.ua/PNTestSet/PN%20Test%20set.pdf . It consists of three
> small
> boards:
> 1. Mixer board - a simple mixer (500MHz ADE-1+) with 200kHz pi-LPF at the
> mixer output.
> 2. LNA board - a non-inverting low noise AF amplifier based on AD797 with
> switchable 20/40dB gain.
> 3. PLL board - contains two TL071 OP amps. One is inverting amplifier, the
> other is PLL integrator. The R4,R8,R2,R7,C8 sets the PLL parameters - gain,
> passband and damping factor. Loop parameters are also dependent of signal
> levels and VCO tuning sensitivity. So you may need to correct them if your
> setup differs from mine - VCXO's I use have tuning sensitivity approx
> 100Hz/V and I set RF level at mixer near 0dBm with LO level near +7dBm. If
> you want to build universal test set you will need to use some switchs to
> allow setting different loop parameters (I just use my soldering iron and
> change parts if needed :) ).
> 
> The power supply is a simple design based on 7812/7912 regulators.
> 
> I use the E-MU 0202 USB external sound card and laptop PC as the AF spectrum
> analyzer.
> 
> You will also need some cables. Different fixed attenuators or switchable
> one will be also helpful.
> 
> I also have several homemade low noise VCXOs for some frequencies (7MHz,
> 10MHz, 14.318MHz, 60MHz) which I use as the reference signal sources to make
> measurements at these frequencies.
> 
> Another option is to test two identical oscillators (or other signal
> sources). Assuming that both signals will have identical phase noise
> characteristics we can correct the results by 3dB (or just add 3dB
> correction during calibration).
> 
> The calibration and use is simple.
> 1. Set LNA gain to 20dB.
> 2. Set the FFT parameters - flattop window, small (2048..4096) points number
> and short averaging in SA software.
> 3. Connect reference signal to LO mixer port and signal you are going to
> test to RF mixer port through the attenuator. Do not close the PLL yet.
> 4. Set the beat level a bit less then the sound card full scale using the
> attenuator. Check the beat harmonics levels - they should be at least 30dB
> lower then the beat level (add more attenuation if harmonics are higher).
> 5. Now set the spectrum analyzer calibration so that beat level is at -27dB
> if you measure against low noise reference VXCO, or -30dB if you use two
> identical oscillators.
> 6. Switch the LNA to 40dB gain.
> 7. Set SA software to Blackman window, 131072points/96kHz SR/necessary
> averaging, close PLL, wait for the lock, measure the phase noise.
> 
> Why I am calibrating to -27/-30dB:
> 20dB because the LNA gain is 20dB less during the calibration (compared to
> measurement time)
> 1dB because of FFT parameters 96k/131072 = 0.73Hz * 1.73 (Blackman window) =
> 1.267Hz, 10 log10(1.267) = 1.03dB
> 6dB is the correction inherent to used calibration method
> additional 3dB needed in case of testing identical oscillators.
> 
> Now some words about results. The noise floor of this test set depends of
> the signals levels, and with the optimal levels it is in -160..-170dBc/Hz
> range (depending of the offset from the carrier). It completely satisfies my
> needs, better results can be achieved with the higher level mixer and/or
> better LNA. I just used parts that I had :).
> 
> Here http://skydan.in.ua/PNTestSet/Screen%20(420)-e.png is an example of the
> phase noise measurements results of my homemade low noise 60MHz VCXO (two
> identical units were measured). The results at the offsets greater then
> 1kHz should be corrected cause the oscillators noise is too close to test
> set noise (the real oscillator noise is a bit lower then the displayed
> one). The test set noise floor and calibration spectrum is also there.
> 
> The boards also have other use.
> 
> For example I was able to measure my home made 60MHz VCXO harmonic content
> http://skydan.in.ua/PNTestSet/Screen%20(414)-e.png using the mixer, LNA
> boards and signal generator. I have no spectrum analyzer so it is a big help
> to me :).
> 
> Power supply noise can be investigated with the LNA board and sound card.
> Look at this screen http://skydan.in.ua/PNTestSet/Screen%20(431)-e.png to
> see how bad the LDO regulator noise can be and a great difference in noise
> with the simple transistor filter (sorry there are a lot of power line noise
> pickup - I needed just to quickly check the power supply noise, so did not
> pay a lot attention to minimize them).
> 
> The low noise VCXOs with the combiner and attenuator can be used to measure
> IMD3 of the receiver. If you add the mixer, LNA and signal generator you can
> measure the IMD3 of the separate units (mixers, filters, amplifiers and
> etc.).
> 
> The low noise VCXO can also be used to test reciprocal mixing DR of the
> receiver.
> 
> Other useful combinations are possible.
> 
> If you like I can post the photos of the boards. They a bit ugly :). Every
> time I use them I think about mounting them in personal metal boxes, but I
> always find something more important to do...
> 
> Best wishes,
> Oleg
> P.S. I am not aware of the attachments rules of this list, so put the links
> in the message instead of attaching files. Is it OK? Should I attach files
> next time?
> 
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