Author: James

Solution: This document provides step-by-step instructions on using the Rigol DSA-800 series of Spectrum Analysers to measure the characteristics of an RF Amplifier.

In addition to the DSA-800 Spectrum Analyser, you will need an RF Source, cabling, and adapters.

Measure the amplifier

1. Connect the RF generator output to the RF input of the instrument using the appropriate cabling and connectors.

NOTE: If your instrument is equipped with a Preamplifier, you can enable it to lower the displayed noise floor by pressing the following sequence:

Press AMP button > Down Arrow > RF Preamp On

2. You can use the Auto button to center and zoom on the waveform. You can also use the Freq and Span buttons to manually manipulate the displayed data.

Another option to center the waveform is to press Freq button > Peak → CF. This will automatically align the center of the display with the peak of the trace.

3. Freeze the unamplified trace by pressing Trace > Trace Type > Freeze. You can use the Marker button to create a marker. This can be used to find the peak frequency and amplitude of the displayed waveform.

4. Disable the RF Generator output.

5. Disconnect RF generator from the instrument RF Input and connect it to the Amplifer input.

6. Connect the Amplifier output to the instrument RF Input.

7. Enable the RF generator.

8. Enable a second trace to visualize the amplified signal by pressing the Trace button > Select Trace 2.
9. Set the trace type to Clear/Write by pressing Trace > Type > Clear/Write. 10.You can use the Auto button or manually center the trace using the Freq,
Span, and Amp buttons.

11. Readjust the amplitude scale by pressing Amp > Auto

12. You can enable an additional marker for the new trace by pressing Marker > Select the marker you would like to use

13. Now, select the trace you want to mark by pressing Marker > Marker Trace
> select trace of interest

• Be sure Normal is selected to enable the marker

14. You can also enable a marker table by pressing Marker > Down Arrow > Mkr Table ON. This allows a convenient way to compare markers and values between traces.

15. Alternately, you can use the Trace Math function to create a Trace difference on the screen.

16. Enable Trace Math by pressing Trace > Trace Math

17. Set Function to A-B

18. Set A = T1

19. Set B = T2

20. Set Operate > On

21. Set Amplitude by pressing AMPL > Auto

NOTE: New trace appears. This represents Trace 1 – Trace 2.

22. Set Marker to Math Trace by pressing Marker > select Marker 3 23.Set Marker Trace to Math by pressing Marker > Marker Trace > Math

23.Set Marker Trace to Math by pressing Marker > Marker Trace > Math

• You can then move the marker to the smoothest portion of Trace 3

• You can then move the marker to the smoothest portion of Trace 3.

This document provides step-by-step instructions on using the Rigol
DSA-800 series of Spectrum Analysers to measure the characteristics of an RF
Bandpass filter.
NOTE: The DSA must have a Tracking Generator to effectively perform the
following test.
Normalize the trace (Optional)
Many elements in an RF signal path can have nonlinear characteristics. In many
cases, these nonlinear effects on your base measurements can be minimised by
normalising the instrument.
1. Connect tracking generator output to RF input using the same cabling that
you will be using to test your device. Any element, like an adapter, used
during normalization should also be used during device measurement as any
changes to the RF signal path could effect the accuracy of the measurement.
2. Enable the tracking generator by pressing the TG button > TG On
3. Store the reference trace by pressing the TG button > Normalize > Stor Ref
4. Enable normalization by pressing the TG button > Normalize > Normalize
On
Measure the filter
1. Connect the tracking generator output to the filter input using the appropriate
cabling and connectors.
2. Connect the filter output to the instrument RF input.

3. Set the tracking generator amplitude by pressing the TG button and the TG
Amplitude. You can use the keypad or wheel to enter the correct value.

NOTE: If your instrument is equipped with a Preamplifier,
you can enable it to lower the displayed noise floor
by pressing the following sequence:
Amplitude button > Down Arrow > RF Preamp On
4. Enable the Tracking generator by pressing the TG button > RF Source ON
You can see the small bump in the figure below.

Figure 1: Before Auto.
5. You can use the Auto button to center and zoom on the waveform. You can
also use the Freq and Span buttons to manually manipulate the displayed
data.

Figure 2: After Auto.
6. You can now enable the Marker function to measure the bandwidth and
attenuation or passband characteristics of the filter.
7. Press Marker Fctn > N dB BW and set the function to the amplitude of
interest. In this example, we are measuring the 3dB Bandwidth of our filter.

Solution: Almost any electronic design slated for commercial use is subject to EMC (Electromagnetic Compatibility) testing. Any company intending to sell  these products into a country must ensure that the product is tested versus specifications set forth by the regulatory body of that country. Here in the US, the FCC specifies rules on EMC testing. CISPR and IEC are also used throughout the world.

To be sold legally, a sample of the electronic product must pass a series of tests. In many cases, companies can self-certify, but they must have detailed reports of the test conditions and data. Many companies choose to have these tests performed by accredited compliance company. This full compliance testing can be very expensive with many labs charging thousands of dollars for a single day of testing. Testing a product for full compliance can also require specialised testing environments and no changes can be made during the testing. Any failures in compliance testing require that the design heads back to Engineering for analysis and possible redesign. This can cause delays in product release and an obvious increase in design costs.

One method to lower the additional costs associated with EMC compliance is to perform EMC testing throughout the design process well before sending the product off for full compliance testing. This pre-compliance testing can be very cost effective and can be tailored to closely match the conditions used for compliance testing. This will increase your confidence in passing compliance the first time through, lower your test costs, and speed your time to market.

Measuring Radiated EMI
The most simple form of pre-compliance measurements for radiated emissions can be performed using a spectrum analyser, like the Rigol DSA-815 (9kHz to 1.5GHz), and near field electric (E) and magnetic (H) probes.

Figure 1 and 2: Near field E and H probes.

The most simple test is to configure the DSA to use the peak detector and set the RBW and Span for the area of interest per the regulatory requirements for your device. Then select the proper E or H probe for your design and scan over the surface of the design.

Probe orientation (rotation, distance) is also important to consider. The probes act as an antenna, picking up radiated emissions from seams, openings, traces, and other elements that could be emitting RF. A through scan of all of the circuit elements, connectors, knobs, openings in the case, and seams is crucial.

Figure 3: Using a near field H probe to test a power supply.

For the first pass, configure the spectrum analyser to use the peak detector. This will provide you with a “worst case” reading on the radiated RF and it is the quickest path to determining the problem areas. Larger probes will give you a faster scanning rate, albeit with less special resolution.

Once you have a good idea of your problem areas, you can get more detail by implementing a few common techniques. If you can, select a spectrum analyser that has the standard configuration used in full compliance testing. This includes a Quasi-Peak detector mode, EMI filter, and Resolution Bandwidth (RBW) settings that match the full test requirements specified for your product.

This type of setup will increase testing time but should be used on the problem areas. A full compliance test configuration will mean your pre-compliance testing will provide a greater degree of visibility into the EMI profile of your design.

On many instruments, you can also store cable and antenna correction factors that will allow you to see the true signal, without the added errors from the setup.

The next step in radiated testing includes using antennas in place of the near field probes, a rotating platform for the equipment under test (EUT), and can include a special room that minimises environmental factors (semi-anechoic). These setups are beyond the scope of this document, but there are references at the end that provide good references for the details of the setup.

Figure 4: Compliance Setup for Radiated Emissions Testing

Measuring Conducted EMI
Conducted EMI testing requires analysing the RF energy that is coupled from the instrument or test circuit to the main power line it is connected to.

Like Radiated EMI, Conducted EMI is also measured using a spectrum analyser, but it also requires a transient limiter and a Linear Impedance Stabilisation Network (LISN). A LISN isolates the power mains from the equipment under test, isolates any noise generated by the EUT,  and couples the signals generated by the EUT to the spectrum analyser.

Figure 3: Standard Conducted Emissions pre-compliance setup via a transient limiter. Note: be sure to choose a limiter suited to your own system and testing regime and do your own checks first: transients can easily exceed the input rating of your analyser (e.g. at switch-on – it is good practice to avoid switching on the LISN with the analyser connected) A modern DSA’s internal level indicator is not intended to warn against fast transients and internal attenuators do not provide greater input circuit protection – if in doubt do your own measurements first to ensure you always operate within the stated input limit envelope.

As with emissions testing, the best start is a scan over the frequency range of interest using the peak detector on the spectrum analyser. Then, performing a quasi-Peak scan using the EMI filter for the problem areas. This will minimise test time while maintaining a high degree of confidence in your test.

Summary
EMC Compliance testing is mandatory for the majority of electronic products that are slated for sale throughout the world. For the cost of 1 day of compliance testing, you can have a pre-compliance setup that you can use to continually monitor and improve your design. This will help speed product development and  save the company money.

Advanced Measurement Functions
Option DSA1000-AMK provides plenty of advanced measurement functions including T-Power (Time domain Power), ACP (Adjacent Channel Power), Chan Pwr (Channel Power), OBW (Occupied Bandwidth), EBW (Emission Bandwidth), C/N Ratio, Harmo Dist (Harmonic Distortion), TOI (Third Order Intermodulation) and Pass/Fail. The measure mode can be Single or Continuous and you can control the measurement status: Restart, Pause or Resume.

Pressing the front panel key Meas, the corresponding menu will appear on the right of the screen. Press Meas Fctn and choose a measurement function. The screen will be divided into two windows. The upper one is for basic measure, displaying sweep trace, and the lower one shows the measurement results.
1. T-Power (Time domain Power)
Enables the Zero Span Mode and calculates the power within time domain. The measurable power types are Peak, Average and RMS.

Yes, but you should still take care never to exceed the DSA815’s max input power or voltage (+20dBm or 100mW and 50VDC abs. max – pulsed waveforms can easily exceed this e.g. direct from a LISN – see below). The DSA internal attenuator function is variable to give relative 0-30dB and whilst it does not increase the permissible power, it is designed to allow the instrument to optimise its dynamic range, enabling it to measure louder signals without distortion and maintaining good accuracy.

Note: A LISN isolates the power mains from the equipment under test, isolates any noise generated by the EUT, and couples the signals generated by the EUT to the spectrum analyser.

Figure 3: Standard Conducted Emissions pre-compliance setup via a transient limiter. IMPORTANT: it is up to you to choose a limiter suited to your own system and testing regime and do your own checks first: transients can easily exceed the input rating of your analyser (e.g. at switch-on – it is generally regarded as good LISN practice to avoid switching on the LISN with any analyser connected!) A modern DSA’s internal level indicator is not intended to warn against fast transients and internal attenuators do not provide greater input circuit protection.
CONCLUSION: if in doubt do your own measurements in any test environment on a resilient instrument (such as a suitably voltage-rated fast scope) first to ensure you always operate your more sensitive instruments, such as spectrum analysers, within the stated input limit envelope.

First follow the procedure for Factory Reset. Then check the noise performance is as it should be. You will need a carefully sequence of settings for this… use the quick video tip on our RIGOL pages.