Datasheet

LTC6957-1/LTC6957-2/
LTC6957-3/LTC6957-4
32
6957f
For more information www.linear.com/LTC6957-1
50Ω load to ground directly, but this creates a DC offset
(the signal is always positive) that the amplifier cannot
take, so a bias tee was included in the DUT signal path.
Only the 122.88MHz sine wave will be in the path without
the DUT, going to the N5500A signal port, until the first
coupler. This coupler allows a spur input to be injected,
while a second coupler allows the size of the spur, relative
to the carrier, to be measured. More on that in a minute.
The three attenuators in this signal path work with the
ZHL-2010+ to manage the dynamic range, while the at-
tenuators on the coupling ports keep these terminals from
degrading the measured noise.
Finally, an ARRA L9428A line stretcher is used to adjust
for quadrature. One last attenuator helps with impedance
matching between the N5500A input and the line stretcher
output port. The E5505A can automatically adjust the signal
source phase/frequency for quadrature when measuring
VCOs or synthesizers, but for additive noise this adjustment
is manual because the adjustment must be made after the
signal is split into the two paths. The line stretcher has a
range
of just 166ps, but with 122.88MHz, up to 20ns of
adjustment
may be needed (1/4 cycle). Not shown is the
various short lengths of SMA cables and barrel couplers
that can also be added or subtracted to adjust the relative
phase of the two signal paths.
applicaTions inForMaTion
To calibrate E5505/N5500 measurements, the gain of the
mixer must be known. The surest way to measure it at
the actual frequencies being used is to inject a calibration
tone. For a 10kHz offset, a 122.89MHz low level (–10dBm)
signal is fed into the first coupler port. The requirements
for this signal are not demanding, so a general purpose
synthesizer that can be frequency locked, such as the
HP8657B, can be used.
The E5505 measures the amplitude of the resulting 10kHz
mixer output, but to put that in context (so that it can later
calculate results in dBc) it needs to know the size of the
injected spur relative to the carrier. Therefore, that relative
difference is measured using a spectrum analyzer con-
nected to the attenuator on the second coupler.
Hopefully the above discussion conveys the meticulous
effort needed to measure additive phase noise of a single
device, at a
single operating
frequency. While the circuitry
in Figure 12 can be used to measure the entire spectrum
of phase noise (all offset frequencies) as well as the phase
noise at other clock frequencies, every clock frequency will
require manual adjusting for quadrature. The input LPFs
will either need to be changed to match the new clock
frequency, or possibly amplitudes at various places will
have to be adjusted to account for the frequency roll-off
therein.