HP 8340B Manual page 79

4-16.
FM ACCURACY AND FLATNESS (Cont'd)
FM Flatness Description
The FM Flatness test uses
a
delay line
discriminator. The dischminator is formed by
driving
a
mixer
with
a
modulated carrier into
the LO port, and
a
delayed carrier into
the RF port.
The
delay is produced
with
a
3 foot length of cable (the length does not have to be precise). When connected
like this,
the
mixer responds to phase differences between its
two inputs. The cable exhibits
a
constant time delay.
As the carrier
frequency changes,
the cable
causes
a
phase
shift at the mixer inputs. This phase shift
varies linearly with frequency.
The mixer
only
operates
as a
phase detector when its
inputs are 90° out
of phase relative to one another (quadrature).
A
carrier frequency must chosen such that the cable
delay causes
a
phase
shift of 90° (this
will be explained in the
procedure).
The output of the discriminator is
fed to the 50 Ohm
input of
an HP 3585A spectrum analyzer. By using
the tracking
generator output to drive the
FM
input
of the DUT,
a
swept response of FM Flatness
will
be seen on the spectrum analyzer CRT. The response
relative to 100
kHz is verified against the
specification.
This
test also checks control circuitry on the
A23 FM Driver assembly. (That
is
why FM
is
checked in
bands other than the fundamental band of the
YIG
oscillator).
Equipment
Spectrum Analyzer
.............................................
HP 3585A
Mixer
.................................................
HP P/N 0955-0307
Power Amplifier
...............................................
HP 8447F
Power
Splitter
................................................
HP 11667B
Step Attenuator
................................................
HP 355D
Delay Line
...........................................
HP
P/N 08503-20038
(>
1
meter
in
length)
Adapters:
APC 3.5
(m)
to
APC 3.5 (m)
.............................
HP P/N 1250-1748
SMA
(m) to
BNC
(f)
...................................
HP P/N 1250-1200
Cables:
Semi-Rigid,
SMA
(m) to
SMA
(m)
......................
HP P/N 08340-20124
BNC (m)
to
BNC
(m)
...................................
HP P/N 8120-8140
(4 required)
4-78
Performance Tests
HP 8340B/41B