User`s guide

4
Digital
Phase
Comparator
HP 71500A
with IBASIC
CH1
CH2
RF carrier
Modulator
AM or FM
Receiver
Internal
LO
Modulation
source
(AM shown)
Digital
Demod
HP-IB
Figure 1: Measurement block diagram.
The direct phase technique
requires that both the test and
reference signals be at the same
frequency in order for phase to
be measured. For measuring
frequency translating devices, a
reference DUT or external
mixer is needed to ensure this
condition.
Modulation Delay
The second technique measures
modulation delay. A modulated
carrier is passed through the
DUT, and then demodulated.
The phase of the demodulated
signal is compared to the
original modulating signal to
determine the delta phase at
that particular frequency. The
carrier must be stepped across
the frequency band of interest
for the complete group delay
measurement. The modulation
type can be AM, FM or PM.
Group delay is calculated as:
t
d
= –––––––
Ø
e
360xf
m
–
(t
d
in seconds, Ø
e
{envelope phase shift} in
degrees, f
m
{modulation
frequency} in hertz)
The HP 71500A group delay
personality uses this technique.
The aperture of this
measurement is equal to two
times the modulation frequency.
Key Features and
Benefits
The key features and benefits
for the HP 71500A solution are
described below. A more
detailed comparison to other
group delay techniques is
provided in Appendix A.
Accuracy to better than 1 ns.
Using normalization and zoom
FFT processing, accuracy down
to +/- 0.2 ns can be achieved.
This is adequate for most
applications.
Access to internal LOs not
required. Since it is the
modulation on the carrier that
is of concern and not the carrier
itself, access to internal LOs or
frequency references is
unnecessary. The vector network
analyzer method measures
phase of the RF signals directly,
and therefore requires LO or
reference access.
No need for external
reference mixers. The vector
network analyzer approach
needs an extra external mixer
or DUT to provide a reference
signal that is the same frequency
as the test signal. While this
method provides excellent
relative measurements
(referenced to a golden standard),
absolute delay measurements
are difficult due to the need to
calibrate the mixers. The HP
71500A solution measures
absolute group delay directly,
providing a simpler and more
traceable measurement.
Frequency coverage extends to
40 GHz. No extra upconverters
or downconverters are needed to
cover most applications, such
as satellite transponder
measurements. This lowers the
measurement system cost and
complexity.
FM group delay. The FM
demodulation routine internal
to the HP 71500A eliminates
the need for an external FM
discriminator. Receivers that
have internal limiters or
saturated amplifiers require the
use of FM, since any AM is
stripped away. An example of a
receiver with a saturated
amplifier is a typical Ku band
satellite transponder.
Amplitude and phase. The
program also provides the
amplitude and phase response
of the DUT with no additional
measurement time.