HP 70908A, Service Manual

Page: 231 / 335

T est 12. IF Sub-Harmonics
The purpose of this verication test is to measure the DUT 21.4 MHz OUT response due to
subharmonics of the DUT IF . (A subharmonic is an input signal oset from the DUT center
frequency by 1/2 the DUT internal IF frequencies .)
In the 1H0 band the subharmonics are one-half the rst IF (3621.4 MHz/2= 1810.7 MHz)
and one-half the last IF (21.4 MHz/2 = 10.7 MHz) . F or the 1L0, 2L0, and 4L0 bands the
sub-harmonics are one-half the rst IF (321.4 MHz/2 = 160.7 MHz) and one-half the last IF
(21.4 MHz/2 = 10.7MHz)j .
This is a nal test.
1. Connect the equipment as shown in the test setup illustration for this procedure .
2. Set the mainframe line switch to ON.
3. Load and run T est 12. IF Sub-Harmonics . Refer to Chapter 2 for information about running
the software .
When performing this verication test, the full microwave source connects to the RF INPUT .
The synthesized source provides the DUT with a 300 MHz, 02 dBm, 300 MHz IN reference
signal, which ensures worst-case sub-harmonic response in the DUT . The calibrated spectrum
analyzer connects to the 21.4 MHz OUT . The following procedure repeats twice in each band
(once for each subharmonic of interest).
The local oscillator source tunes to center the RF INPUT . The calibrated spectrum analyzer
stores the amplitude of the 21.4 MHz OUT , along with a atness correction, as the
measurement reference , thus ensuring results that are independent of module gain.
T o simulate a subharmonic response , the RF INPUT signal frequency remains constant, while
the local oscillator source retunes an oset amount equal to the separation between the input
signal and the subharmonic . This causes the 1st IF to dier from normal by the oset amount,
thus causing the 21.4 MHz OUT frequency to dier from 21.4 MHz by the oset amount. The
calibrated spectrum analyzer subtracts this oset output amplitude from the previously stored
21.4 MHz reference . A routine compares the dierence with test limits .
8-26 Module V erication T ests