Hi Oleg,
I like the simplicity here.
Would not GNUradio be a good platform to encode the calibration stuff a
little more gift-wrapped?
What spectrum-analyzer software do you use? (Just curious)
This simple setup would be useful for many purposes.
Depending on the oscillator, the EFC can need inversion, so I sketched
for a similar design such that I could jumper it for either polarity. In
that design I put the inverter after the integrating op-amp.
Cheers,
Magnus
On 03/28/2016 10:04 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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