Setting up a Digital Demod Measurement
When setting up a digital demodulation measurement, there are several things that should be considered. This topic contains the information needed to connect your signal to an 89600 VSA and successfully demodulate digital signals.
- Connecting Your Signal to the VSA
- Carrier locking (QAM)
- Carrier locking and pilot search (VSB)
- Input Range
- Tips When Using Digital Demodulation
Connecting Your Signal to the VSA
The carrier signal from your device is connected to the VSA through your measurement hardware. You can connect the RF Radio Frequency: A generic term for radio-based technologies, operating between the Low Frequency range (30k Hz) and the Extra High Frequency range (300 GHz). signal to any input port of the measurement hardware and use the Input Channel Configuration dialog to map the hardware channel to the desired measurement channel. Digital Demod can analyze one or two channels of data.
If the input measurement hardware has I+jQ baseband input channels, you can connect the raw I (In-phase) and Q (Quadrature phase) signals (refer to the Measurement Platforms book for information on configuring the instrument). To do this, select . With this configuration, the VSA skips the quadrature mixer from the demodulator's block diagram, which allows the I and Q components of the signal to be connected separately.
Notes for I+jQ input:
- Selecting an configuration enables DC coupling.
- Use User Correction to correct for any offset or gain imbalance between the I and Q signals.
- Range is set independently for the I and Q inputs, but can be coupled by selecting the Couple Paired Channels parameter.
Carrier Locking for QAM signals
For QAM Quadrature Amplitude Modulation signals, the VSA's center frequency must be close to the transmitted carrier frequency to achieve carrier lock. The required proximity of the center frequency to the carrier frequency varies depending on the signal type, symbol rate, and system noise. If symbol locking appears poor, better carrier locking may be achieved by observing the Freq Err value in the symbol table of any successfully locked measurement and adding that amount to the center frequency.
Frequency Offset limits for Reliable Frequency Lock
In general, to obtain reliable VSA carrier frequency lock, the frequency difference between the VSA's center frequency and carrier frequency should be within 10% of the . For QAM signals the frequency difference tolerance is dependent on the QAM format; see the following table for specific values.
Result Length Limits for Reliable Frequency Lock
For QAM signals, the also affects the VSA's ability to achieve reliable carrier lock, see the following table for specific result length values. less than the specified values may result in unreliable carrier lock. Frequency offsets greater than the specified value may also result in unreliable carrier lock.
|
Digital Modulation Format |
Minimum Result Length (0 Hz frequency offset) |
Maximum frequency offset (Result Length = 1000 symbols) |
|---|---|---|
|
50 symbols |
9.6% symbol rate |
|
|
16 QAM |
50 symbols |
4.8% symbol rate |
|
32 QAM |
75 symbols |
3.15% symbol rate |
|
64 QAM |
150 symbols |
4.65% symbol rate |
|
128 QAM |
250 symbols |
0.3% symbol rate |
|
256 QAM |
400 symbols |
0.3% symbol rate |
|
512 QAM |
1000 symbols |
0.15% symbol rate |
|
1024 QAM |
1000 symbols |
0.15% symbol rate |
|
2048-QAM |
4000 symbols |
0.1% symbol rate* |
|
4096-QAM |
4000 symbols |
0.1% symbol rate* |
|
|
|
|
|
16 DVB QAM |
50 symbols |
4.65% symbol rate |
|
32 DVB QAM |
75 symbols |
2.85% symbol rate |
|
64 DVB QAM |
150 symbols |
4.65% symbol rate |
|
128 DVB QAM |
250 symbols |
0.45% symbol rate |
|
256 DVB QAM |
400 symbols |
0.15% symbol rate |
|
|
|
|
|
Star16QAM |
50 symbols |
4.95% symbol rate |
|
Star32QAM |
75 symbols |
4.95% symbol rate |
*Using Low SNR Signal-to-Noise Ratio Enhancement is required to make frequency locking robust for this QAM format.
QAM formats with 512 or more states are very sensitive to noise. Even an EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. of approximately 2% can cause symbol detection errors in the algorithm. This can cause a positive bias on EVM (an EVM appears better than it really would be for that signal). Analysis of these higher-order QAM formats is not intended with a Signal to Noise Ratio of less than 40dB.
The following may also affect carrier locking:
- A frequency span that is too narrow.
- An incorrect range setting.
- Using the wrong measured or reference filter.
Carrier locking and pilot search (VSB)
The same parameters that affect carrier locking for QAM signals also affect carrier locking for VSB signals. For VSB signals, the VSA must also find the pilot signal to achieve carrier lock.
If your VSB signal uses a high-side pilot, configure the VSA to demodulate a high-side pilot by selecting mirror frequency spectrum. If this is not done, the VSA cannot find the pilot signal and cannot lock to your signal.
If the VSA cannot lock to your signal, it displays CARRIER LOCK?. There are several conditions that may cause loss of carrier lock, one of which is an unsuccessful pilot search. If carrier lock is not obtained after several measurements, the VSA assumes the pilot search failed and repeats the pilot search.
CARRIER LOCK? may appear even when the data appears to be locked. If this occurs, the results may be inaccurate. Normally, this condition clears in a few measurements as the instrument obtains better estimates of the carrier frequency.
The following paragraphs describe other conditions that may cause loss of carrier lock. The VSA always assumes pilot search failed if there are carrier lock problems, even if the cause is one of the conditions below.
Like QAM signals, the VSA's center frequency must be close to the transmitted carrier frequency to achieve carrier lock for VSB signals. The following formulas show how to calculate the ideal center frequency. Use the formula for low-side pilot if the pilot is on the left (low) side of the spectrum; use high-side pilot if the pilot is on the right (high) side of the spectrum.
Center Frequency (High Side Pilot) = Pilot Frequency - (Symbol Rate / 4)
Center Frequency (Low Side Pilot) = (Symbol Rate / 4) + Pilot Frequency
The also affects the VSA's carrier locking. For VSB measurements, the should be at least 800 symbols. Smaller may cause unreliable carrier locking. The following may also cause loss of carrier lock:
- A frequency span that is too narrow.
- An incorrect range setting.
- Using the wrong measured or reference filter.
- Not using mirror frequency spectrum.
- A pilot that is not in phase with the transmitted data. Pilot-phase problems may be compensated for by using clock adjust to change when the VSA samples the I/Q trajectory.
Input Range
The input range must be set correctly to obtain accurate measurements. that are too low overload the VSA's ADC Analog-to-Digital Converter. that are too high increase noise, which increases errors reported in error summary data, such as EVM.
Tips When Using Digital Demodulation
The following summarizes the major points from the previous paragraphs and contains additional information for using Digital Demodulation:
- Modulation Reference: The VSA does not need an ideal signal to generate the reference signal. The VSA uses your signal to internally generate the ideal, reference signal.
- Carrier Lock and Detection: The VSA does not need a sync word to perform carrier locking or detection. See also the next paragraph on "Center Frequency."
- Center Frequency: The VSA's center frequency must be close to the transmitted carrier frequency to achieve carrier lock. The required proximity of the center frequency to the carrier frequency varies depending on the signal type, symbol rate, and system noise. If symbol locking appears poor, better carrier locking may be achieved by observing the Freq Err value in the symbol table (for QAM, PSK Phase Shift Keying: A broad classification of modulation techniques where the information to be transmitted is contained in the phase of the carrier wave., and MSK Minimum Shift Keying formats) of any successfully locked measurement and add that amount to the center frequency. As a general rule, the carrier-frequency-to-center-frequency error should be within 3% of the symbol rate.
- Symbol Clock: The VSA does not need an external symbol clock; the VSA internally generates the symbol clock.
- Maximum Symbol Rate: The maximum symbol rate depends on the alpha of the system and on the span (information bandwidth). See symbol rate for details.
- Span: The minimum span (information bandwidth) depends on the alpha of the system and on the symbol rate. The maximum span is limited by the symbol rate. See span for details.
- Time Gating: Time gating is not available for digital demodulation.
- Averaging: Averaging is not available for demodulated trace data in digital demodulation but may be applied to numeric error data in the symbol table. For details, see Available Features.
- Accuracy: To meet specifications, your signal and its modulation must be fully contained in the selected measurement span.
Troubleshooting
The Tips listed above should help successfully set up the digital demodulation measurement. For more information on problems, see Troubleshooting .
See Also
