HP 70427A User Manual page 490

Overview
Looking at the Block
Diagram
HP 70427A Intermediate Frequency (IF) Amplifier
HP 70427A Intermediate Frequency (IF)
Amplifier
The HP 70427A IF Amplifier (Assembly A4A1) is a linear, –10 dB to
+45 dB, stepped gain, 1 MHz to 1.5 GHz amplifier, with a 1.5 GHz diplexed
low-pass filter input. The mixer bias source provides up to ±10 mA of bias
current, necessary for odd harmonic mixing in HP 11970 series of external
millimeter mixers.
The amplifier output signal can be monitored (using the IF VIEW selection
of the RF/Microwave Spectrum analyzer monitor port), AM detected (using
the IF amplifier AM detector), or phase noise can be measured on the main
IF Output (using an HP 3048A Phase Noise System).
1.5 GHz Diplexed Low-Pass Filter
The IF amplifier input is filtered by a 1.5 GHz low-pass filter. The input
filter limits the amount of LO feedthrough leakage entering the IF amplifier,
and provides a broadband impedance match to reduce mixer spurs at the
downconversion mixer IF port.
The diplexed filter design is actually two filter inputs in parallel, a high-pass
maximally flat (Butterworth) filter and a low-pass maximally flat
(Butterworth) filter, designed with the same cutoff frequency. The result is a
diplexer with a constant input impedance. Signals below the cutoff
frequency pass through the low-pass part of the filter. Signals above the
cutoff frequency pass through the high pass part of the filter and are
Ω load resistor.
terminated in a 50
The filters were synthesized from 1.7 GHz, 6 pole designs, with 3 dB of
attenuation at the cutoff frequency. The theoretical input return loss is better
than –25 dB at all frequencies. The actual filter provides a return loss better
than –12 dB to beyond 6 GHz, with a 1 dB cutoff frequency of 1.5 GHz.
External-Mixer Bias Source
The millimeter mixer bias source is connected to the filter input through the
low-pass filter section. The mixer bias source provides up to ±10 mA of bias
current, necessary for odd harmonic mixing in HP 11970 series of
millimeter mixers. The bias source is controlled with a 12-bit
digital-to-analog converter (DAC), with 5 μA/step current resolution.
The DAC output voltage range is 0.000 to 4.096 volts, and is used between
0.000 and 4.000 volts. The DAC output is combined with the output of a
precision 10.00 volt reference in a difference amplifier. The result is a
HP 70427A/HP 70428A User's Guide 13-55
Block Diagram