overview - 50 mhz and up50mhzandup.org/advanced rf tips and tricks.pdf · 2016/3/2 5 rigol...
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2016/3/2
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Overview
Available and capable test equipment is always
important to RF design and debug whether at the
office or as a hobby.
RIGOL brings a portfolio of capable tools, but are
there ways we can do more than expected with
affordable RF test equipment?
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Advanced RF Measurements
1) Filter and Amplifier tests with a Tracking Generator
2) Using mixers as paired converters
3) Testing filters and amplifiers at higher frequencies
4) VSWR and reflectivity measurements
• Antenna characterization
5) Noise Figure Measurements• Methodology for basic NF measurements without an NF meter
• Passive devices, active devices, and systems
6) System modulation tests• Testing IF modulation through your system with emulation
7) Phase measurements• Determining phase changes in your system
8) Conclusions
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Testing devices with a tracking generator
- A Tracking Generator is a synchronized swept source always
outputting power at the same frequency the detector is at as they
scan together
- This allows a clear view of the response of a device across frequency
Response of a 40 MHz Band
Pass Filter at 950 MHz
Filter and Amplifier tests
Key points and limitations
- All RIGOL tracking generators
extend to the top frequency of the
Analyzer (not all other units do)
- TGs are settable from -20 to 0 dBm
- N dB BW markers can be used to
simplify measurements
- Use traces and trace math to
compare IN and OUT of an
amplifier
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Normalization improves accuracy by providing an offset calibration
adjustment for the integrated source at each frequency.
This can be used for simple devices or for more complex systems
Tracking Generator Performance Normalized TG Output
Normalization
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Mixers as paired converters
Mixers can be used to extend the frequency range for many tests using
a paired upconverter and downconverter section
Example of setup for making measurements at 10 GHz
RF Signal
Source
@ 500 MHz-
1.5 GHz
LO @ 9.5
GHz
Test signal from 10-
11 GHz
RF IN
10 GHz
DUT
RF OUTSpectrum
Analyzer
@ 500 MHz-
1.5 GHz
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Testing passives with mixers
Applying the converter methodology to our passive tests
- Filter after down conversion not needed since analyzer is scanning
synchronously with source. RBW filter is handling it.
LO @ 9.5
GHz
Test signal from 10-
11 GHz
RF IN
10 GHz
DUT
RF OUT
1.5 GHz
Spectrum Analyzer
Tracking Generator
Output
Analyzer input
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Testing passives with mixers
Tracking generator Normalization
- Essentially an offset calibration for power
- Normalize with a Thru connection before testing DUT
Adjust test range with a higher bandwidth Analyzer. This requires a
lower frequency LO which may be easier to achieve and increases the
range of the scan
LO @ 6
GHz
Test signal from 6-
13.5 GHz
RF IN
10 GHz
DUT
RF OUT
Tracking Generator
Output
7.5 GHz
Spectrum Analyzer
Analyzer input
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VSWR and reflectivity measurements
VSWR measurements require:
- Directional coupler
- Analyzer with tracking generator
- Broader power range can be tested with a signal source
- Calibration done with a “OPEN” cal test (No antenna connected)
Calculations: The VSWR can be calculated by the following
a = Return Loss (dB)
r = Reflection coefficient of the DUT
s = VSWR
r = 10^(-0.05 * a)
s = (1 + |r|) / (1- |r|)
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VSWR and reflectivity measurements
For our example of an ~800MHz antenna, we have the following:
a = -26.22 dB
r = 10^(-0.05 * 26.22) = 10^(-1.31) = 0.05
s = (1+0.05) / (1-0.05) = 1.11
Math is done in the instrument and ratio is calculated at peak power
transmission frequency
Reference: http://beyondmeasure.rigoltech.com/acton/attachment/1579/f-0684/1/-/-/-/-
/VSWR%20with%20a%20DSA800%20%28includes%20VSWR-DSA800%20and%20standard%20measurement%20technique%29.pdf
Test VSWR from
DC – 7.5 GHz
RF IN
Antenna
Tracking Generator
Output
7.5 GHz
Spectrum Analyzer
Analyzer input
Directional Coupler
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VSWR and reflectivity measurements
Using mixers to test at higher frequencies is possible, but you need a
directional coupler capable of the actual test range. The coupler should
be connected directly to the device to measure the directionality at the
device.
LO @ 6
GHz
Test VSWR from 6-
13.5 GHzRF IN
Antenna
Tracking Generator
Output
7.5 GHz
Spectrum Analyzer
Analyzer input
Directional Coupler
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Noise Figure Measurements
Noise Figure measurements using a Spectrum Analyzer can be made
using the gain method. Summarized from maxim reference document
https://www.maximintegrated.com/en/app-notes/index.mvp/id/2875
Noise Factor (F) = Total Output Noise Power / Output noise from input src
One factor is the noise power density from Brownian motion.
In spectral power density and at room temperature this simplifies to:
NF = PNOUTD + 174dBm/Hz – Gain
First, we measure the Gain of the device or system at the frequency of
interest. We can do this with the tracking generator as we did earlier
testing passives.
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Noise Figure Measurements
NF = PNOUTD + 174dBm/Hz – Gain
Second, we test the Spectral Noise Power of the system with no input
and with the input terminated with a 50 Ohm load.
50 Ohm
Termination
LO src
RF Signal
Source
Power Supply
RF SYSTEM IN LO (if applicable)
RF SYSTEM
Vcc RF SYSTEM OUT
Spectrum
Analyzer
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Noise Figure Measurements
NF = PNOUTD + 174dBm/Hz – Gain
Accuracy:
The DSA815 has inherent noise of about -155 dBc/Hz. Measurements
should only be attempted above that. Therefore, the Noise Figure + the
system Gain should be at least 20-25 dB to make accurate
measurements. 50 Ohm
Termination
LO src
RF Signal
Source
Power Supply
RF SYSTEM IN LO (if applicable)
RF SYSTEM
Vcc RF SYSTEM OUT
Spectrum
Analyzer
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Noise Figure Measurements
NF = PNOUTD + 174dBm/Hz – Gain
Frequency Converting Noise Figure measurements
- Once you measure the gain and adjust for the offset of the mixers you
can then measure the noise. View the mixers as part of the system under
test. The noise figure of the mixers and filter is equal to their attenuation.
Reference: University of Kansas – Noise figure of Passive Deviceshttp://www.ittc.ku.edu/~jstiles/622/handouts/Noise%20Figure%20of%20Passive%20Devices.pdf
50 Ohm
Termination
RF Signal
Source
Spectrum
Analyzer
LO @ 6
GHz
RF IN
10 GHz
DUT
RF OUT
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System modulation tests
How to verify signal modulation through the system
- Use a RF signal source to emulate the 144 MHz radio output with
known modulation data
- Monitor modulated signals in zero span mode
- Measure deviation using max hold traces ->
Reference: http://beyondmeasure.rigoltech.com/acton/attachment/1579/f-059e/1/-/-/-
/-/FM%20Deviation%20with%20a%20DSA.pdf
RF IN
RF OUT
Modulated Signal
Output
Zero Span Analyzer
Time Domain Signal
TX System Under test
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System modulation tests
View the modulation on carrier with a spectrum analyzer
- Time domain measurements help verify signal to noise ratio and optimal
levels for modulated source
- Important for adjusting components in the TX or RX paths
sinusoidal FM modulation slower FM with visible noise floor
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Phase measurements with mixers
Phase measurements of RF signals
- Use down converters with phase linked LO signals to preserve phase
offset
- Option 1 is to convert to a frequency easily measured with an
oscilloscope
- Be careful that you have enough voltage from the mixer to view easily on
the scope
Reference: http://www.av.it.pt/nbcarvalho/docs/ci47.pdf
RF signal
Source LO
@ RF – 50
MHz
RF Signal
Source
RF IN
DUT
RF OUT
CH 1
Oscilloscope
CH 2
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Phase measurements with mixers
Phase measurements of RF signals
- Option 2 is to configure the system to output DC and measure with an
RMS Voltmeter
- This method requires 3 measurements (Vrms between signals and
each signal to ground)
- Result is a DCV value related to phase. To get a absolute phase value
you would need to know the DCV at 180°offset
Reference: http://etc.unitbv.ro/~olteanu/Tehnici%20de%20masurare%20in%20tc/Phase%20Measurement.pdf
RF signal
Source LO
@ RF
(difference
= DC)
RF Signal
Source
RF IN
DUT
RF OUT
HI
DMM
LO
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Summary
Combining power, phase, and noise measurements across frequency to
completely characterize RF systems and components
Utilize mixers to convert to easier to measure frequencies
Use tracking generators for synchronization across frequency or add a
RF signal source for power range, modulation options, or increased
flexibility
Add directional couplers to measure reflection, determine impedance
matching, and conduct VSWR measurements
Make noise figure measurements on an RF system using the gain
method
Select an option for phase measurements based on mixer and
instrumentation requirements
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Digital Oscilloscopes
- Phase and time domain
measurements
- 50 MHz – 1 GHz
DSG3000 RF Signal Sources
- 3 and 6 GHz
- AM, FM, and Phase modulations
- Pulse, pulse train, and IQ options
RIGOL Instruments
DSA800 Spectrum Analyzers
- High performance and value
- 1.5, 3.2, or 7.5 GHz
- Tracking generator to top freq
DSG800 RF Signal Sources
- 1.5 and 3 GHz
- FM, AM, and Phase modulations
- Pulse and Pulse train mod options
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References and Resources
1) http://www.microwaves101.com/encyclopedias/mixer-noise-figure
2) http://www.reeve.com/Documents/Noise/Reeve_Noise_6_NFMeas
SpecAnalyz.pdf
3) https://www.maximintegrated.com/en/app-notes/index.mvp/id/2875
4) http://www.av.it.pt/nbcarvalho/docs/ci47.pdf
5) http://www.ittc.ku.edu/~jstiles/622/handouts/Noise%20Figure%20of
%20Passive%20Devices.pdf
6) http://etc.unitbv.ro/~olteanu/Tehnici%20de%20masurare%20in%20t
c/Phase%20Measurement.pdf