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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