HP 70427A User Manual page 455

Block Diagram
600 MHz Reference Loop (A7A2)
driven into collector current cutoff. Its output level is about +11 to +15 dBm
into 50Ω. This 600 MHz signal is the phase detector reference for the
600 MHz Reference Clean-up loop.
Phase Detector/2.3 MHz LPF/20 dB Amplifier
The phase detector/amplifier block combines the 600 MHz reference output
with the output of the 600 MHz STWO, through the 80 dB isolation
amplifier, to produce a phase error voltage. This phase error voltage will be
integrated, and then fed back to the STWO to phase-lock the oscillator.
The phase detector is a doubly balanced mixer. One input is the 600 MHz
Multiplier output. The other input is a +11 to +16 dBm signal from the
80 dB isolation amplifier. The two signals are multiplied together which
produces a sum and difference product. The 1200 MHz sum product, and the
600 MHz feedthrough is terminated in 50Ω by the 2.3 MHz low pass filter's
diplexed input. The difference product is proportional to the phase error
between the two inputs, where a 90 degree phase difference between the two
inputs will produce 0 V output. The phase detector sensitivity is 0.5 to 0.7
V/rad.
The 2.3 MHz lowpass filter (LPF) performs two functions. First, it provides
the termination of the undesired 600 and 1200 MHz signals produced in the
phase detector. Second, it rejects the undesired harmonics of 100 MHz,
produced in the 100 to 600 MHz Multiplier, by 60 dB. These 100 MHz
harmonics will produce 100 MHz sidebands on the PLL output if allowed to
reach the VCXO tuning port. The filter was designed in two sections. The
input is a 3 pole, 0.1 dB ripple, 3 MHz Chebyshev LPF, with a 50Ω resistor
in series with the input capacitor. The 50Ω resistor terminates frequencies
above the filter cutoff frequency. The majority of the stop band performance
is from the 5 pole, 2 zero, 3 MHz elliptic filter section. Phase delay in the
LPF must be minimized to prevent PLL peaking due to excess phase shift.
The output of the filter is terminated in 100Ω. The 100Ω termination is a
compromise between greater phase detector sensitivity over a 50Ω
termination and to provide proper filter response.
The output of the LPF is amplified by a variable gain low noise
non-inverting operational amplifier. The gain of the amplifier is adjusted to
produce an output phase slope of 4.25 V/rad. The input noise of the op-amp
is about 1.2 nV/Hz. Given 0.5 V/rad minimum phase detector sensitivity, the
signal-to-noise ratio of this block is approximately 172 dB with a 4.25 V/rad
phase detector sensitivity at the amplifier output.
The phase detector amplifier output primarily feeds the 600 MHz loop
integrator. It is also connected to the modules diagnostic multiplexer system
and the 600 MHz PLL phase detector beatnote out-of-lock detector. The
beatnote detector is a window comparator. It monitors the phase detector
amplifier output voltage. An output voltage greater than +2.1 V or less than
–2.1 V indicate an out-of-lock condition which triggers the loop's integrator
13-20 HP 70427A/HP 70428A User's Guide