HP 70427A User Manual page 468

Table 13-10
NOTE
600 MHz Bandpass Filter
The 600 MHz power amplifier filter reduces the broadband reference noise
down to the thermal noise floor at carrier offsets greater than 20 MHz. This
is necessary to prevent 30 MHz noise sidebands from being induced on each
of the microwave comb line frequencies by the SRD comb generator. The
step-recovery-diode (SRD) comb generator has a bias network resonance at
30 MHz which will cause parametric amplification of any 30 MHz sideband
energy present on the 600 MHz reference.
The filter is a four-section bandpass filter, with a center frequency of 600
MHz, and less than 1 dB of insertion loss. The stop-band loss relative to loss
at the center frequency is 25 dB minimum from DC to 560 MHz and from
640 to 3000 MHz. The VSWR in the passband is 1.35:1 or better, input and
output, at 600 MHz.
Step Recovery Diode
The SRD comb generator is a modified HP 33004A 500 MHz Comb
Generator re-designed to operate at a 600 MHz input frequency, with an
input level of +27 dBm to +30 dBm. The comb generator generates voltage
impulses with rise times on the order of 65 ps. This fast rise time is rich in
harmonic energy. The individual comb frequency power is distributed as a
sin x
function, where x is proportional to the pulse rise time. The usable
----------
x
output frequency range is 600 MHz to 15.0 GHz. The usable output
frequency range is defined as output levels greater than –20 dBm.
SRD's Typical Residual Phase Noise Performance
Carrier Offset (Hz)
Residual Phase Noise
The noise is referred to the input frequency of a single SRD.
GaAs Edge-sharpening Transmission Line
The edge-sharpener is a high-impedance coplanar-waveguide transmission
line with varactor diodes spaced periodically along its length. The
edge-sharpening nonlinear transmission line changes its characteristic
impedance as a function of input voltage, which also changes its velocity
factor.
The edge-sharpener sharpens the leading edge of SRD pulse sources by as
much as 4:1 depending on the input pulse characteristics. When the input
pulse voltage is low, the velocity factor is low. As the pulse voltage increases
the velocity factor increases. This action causes the leading part of the
leading edge to slow down relative to the later part of the leading edge. The
result is a faster pulse rise time.
HP 70428A Microwave Deck Assembly
(SRD) Comb Generator
1 10 100
–130 –140 –150
HP 70427A/HP 70428A User's Guide 13-33
Block Diagram
1 k 10 k >100 k
–160 –168 –176