HP 8559A Operation And Service Manual page 275

SERVICE
holes, in the aluminum block housing to filter the first
second LO, which operates at one of two fixed frequencies. After mixing with the first IF, the second LO
produces the second IF at 321.4 MHz.
The need for operating the second LO at two separate frequencies becomes apparent when measuring a signal at
or near the first IF frequency, 3 GHz. The signal passes through the first mixer and first IF unaffected by first
LO tuning and appears as an equally strong signal at all frequencies. This response is called IF feedthrough or
baseline lift. Changing the frequency of the second LO shifts the feedthrough response away from the fre-
quency being measured by effectively altering the first IE Two LO frequencies may be selected with the ALT IF
control, 2.6861 GHz (regular IF) and 2.671 1 GHz (alternate IF). The LO shift (15 MHz) is reflected in the first
IF and fits within the 17 MHz to 23 MHz 1 dB passband of the 3 GHz bandpass filter.
Third Converter. The Third Converter Assembly A10 contains the second IF amplifier, the second IF band-
pass filters, the third mixer, the third LO, and the third IF filters and compensation amplifiers. The second IF
amplifier consists of a single-transistor amplifier with a 321.4 MHz bandpass filter at its input. It provides
about 15 dB of gain before passing the signal to a second 321.4 MHz bandpass filter at its output. The net 1 dB
bandwidth is 6 MHz to 9 MHz, narrow enough to reject the second mixer's image frequency. The double-
balanced third mixer produces sum and difference frequencies, as do other mixers, but rejects input and LO
frequencies, simplifying subsequent filtering. Two transistors form the third LO, fixed at 300 MHz, which,
when mixed with the 321.4 MHz second IF, produces a difference frequency at the final IF, 21.4 MHz.
Three conversions or frequency translations are necessary before the input signal reaches the final IF, where the
analyzer's major bandpass filtering and calibrated gains occur. The circuits used in the final IF are more easily
controlled at 21.4 MHz than they would be at the higher input frequencies. The RF section's function is to
down-convert the input signal accurately so the analyzer can control and display it.
Harmonic Mixing. To extend the frequency range of the H P 8559A, harmonic mixing is employed. Instead
of limiting the first mixer input to the fundamental range of the first LO (3.01 GHz to 6.04 GHz), harmonics of
the LO are allowed to mix with the incoming signal. Each of the six FREQUENCY BAND GHz buttons on the
front panel selects a different mixing mode. A mixing mode is characterized by the number of the LO harmonic
used and the relationship of the incoming signal frequency to the LO frequency. For example, in the first band
must tune to 5 GHz to produce a difference frequency at the required IF, 3 GHz. This band is characterized as
the " 1 mixing mode. This relationship is expressed by the fundamental mixing equation:
-
Band two (6 to 9 GHz) uses the "1
the first LO frequency. Now an 8 GHz incoming signal mixes with the 5 GHz first LO, producing an IF
response at 3 GHz. The mixing equation also reflects this change by becoming:
Higher frequency bands are realized by using the second harmonic (6 to 12 GHz) or the third harmonic (9 to 18
with the fundamental mixing mode, each harmonic mode has two possible frequency bands creating a total of
+ +
, , ,
six bands: 1 2 2
-
modes and the LO fundamental. The mixing equations for the harmonic mixing modes are:
and
where N is the harmonic number of the mode.
+
mixing mode. In this band, the incoming signal frequency is higher than
+
.
, ,
and 3
3 Section 3, Figure 17 shows the tuning curves for the six mixing
- -
F,, F,, (for plus modes)
=
-
NF,, F,, (for minus modes)
=
-
IF. A
fourth cavity is used as the resonant circuit for
MODEL 8559A
the