HP 8340B Manual page 75

4-16. FM ACCURACY AND FLATNESS (Cont'd)
FM Flatness Description
The FM Flatness test uses a delay line discriminator. The discriminator 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
of phase relative to one another (quadrature). A carrier frequency must chosen such that the cable
delay causes a phase shift of
The output of the discriminator is fed to the SO 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
Mixer
Power Amplifier
Power Splitter
Step Attenuator
Delay Line
(> 1 meter in length)
Adapters:
APC 3.5
SMA(m)toBNC(f)
Cables:
Semi-Rigid, SMA
BNC(m)toBNC(m)
(4 required)
4-78 Performance Tests
(this will be explained in the procedure).
900
to APC 3.5
(in)
(in)
to SMA
(in) (in)
out
900
HP3S8SA
HP P/N 095S-0307
HP8447F
HP11667B
HP3SSD
HP P/N 08S03-20038
HP P/N 12S0-1748
HP P/N 1250-1200
HP P/N 08340-20124
HP P/N 8120-8140
HP 8340B/41B