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

The carrier signal from your device is connected to the VSA through your measurement hardware. You can connect the RF 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 Input > Channels > I + jQ. 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:

Carrier Locking for QAM signals

For QAM 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 Symbol Rate. 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 Result Length also affects the VSA's ability to achieve reliable carrier lock, see the following table for specific result length values. Result Lengths 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)

QPSK

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 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 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:

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 Result Length also affects the VSA's carrier locking. For VSB measurements, the Result Length should be at least 800 symbols. Smaller Result Lengths may cause unreliable carrier locking. The following may also cause loss of carrier lock:

Input Range

The input range must be set correctly to obtain accurate measurements. Input ranges that are too low overload the VSA's ADC. Input ranges 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:

Troubleshooting

The Tips listed above should help successfully set up the digital demodulation measurement. For more information on problems, see Troubleshooting .

See Also

Troubleshooting (Digital Demod)

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