Page 95 - Complete Wireless Design

Modulation

94 Chapter Two

drop approximately 3 dB from each input signal, will send the remainder into
the step attenuator. The step attenuator permits the operator to vary both of
the two signal generators’ amplitudes by exactly the same amount at the same
time, speeding up testing. From the attenuator the two-tone test signal is
placed into the input of the receiver at the desired amplitude, while the level
of the third-order IMD products are measured as dB below the carrier, or dBc.
This IMD level will then be checked to confirm that it meets the receiver’s
design specifications.

P1dB compression point test. To reduce spectral splatter and the BER, ampli-
fiers must be backed off a set amount from their P1dB point, the amount
depending on the modulation in use. We can find the P1dB by the following
method: Set up test equipment as shown in Fig. 2.47. Set the signal generator
frequency to the center of the bandpass of the DUT (the amplifier). Employ
attenuator pads if the amplifier’s output will overdrive or damage the spectrum
analyzer. Increase the signal generator’s power output 1 dB at a time until the
spectrum analyzer’s measurement does not track the input dB for dB. The
amplifier’s P1dB will be the signal generator’s output level, while the sig-
INPUT
nal amplitude as read on the spectrum analyzer, plus any attenuation dialed
into the attenuator, is the P1dB OUT in dBm. The amplifier’s third-order intercept
point (TOIP) can be approximated by adding 10 dB to the P1dB value.
OUT
Phase noise tests. Now, more than ever, decreasing phase noise to decrease
the BER is not an option: The lower the LO-generated phase noise, the better
for the digital radio system. Highly accurate phase noise testing is possible,
but takes special, and very costly, equipment and test setups. To perform lim-
ited-accuracy phase noise tests on any oscillator, follow the procedure below,
which is useful only if the spectrum analyzer employed in the test has a low-
er phase noise than the DUT:

1. Attach the LO output to the spectrum analyzer’s input.
2. Set the spectrum analyzer to the same frequency as the LO.
3. Switch on video averaging on the spectrum analyzer.
4. The difference between the amplitude of the carrier and the noise ampli-
tude, minus 10 log RBW, is approximately equal to the phase noise in
dBc/Hz. (RBW is the resolution bandwidth as set on the spectrum analyzer.)

Reference spur measurement. To check for high reference spurs in the output of
a phase-locked loop (PLL), which will damage the BER of the wireless device:

1. Attach the PLL’s VCO output to the spectrum analyzer’s input.
2. Set the PLL and the spectrum analyzer to the same center frequency. Open
the spectrum analyzer’s SPAN to allow viewing of all reference spurs (refer-
ence spurs are located at f above and below the PLL’s output frequency,
comp
as well as at their harmonics).

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