SERVICE

MODEL 8559A

holes, in the aluminum block housing t o filter the first IF. A fourth cavity is used as the resonant circuit for the 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 frequency 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 2.6711 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 bandpass 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 t o 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 (.01 to 3 GHz) the incoming signal is below the frequency of the LO. If the incoming signal is 2 GHz, the LO 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 + " mixing mode. In this band, the incoming signal frequency is higher than 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 GHz) of the first LO. Adjusting the dc bias of the first mixer diode enhances operation at these frequencies. As with the fundamental mixing mode, each harmonic mode has two possible frequency bands creating a total of six bands: 1+, 1- ,2 +, 2 - ,3 +, and 3 - .Section 3, Figure 17 shows the tuning curves for the six mixing modes and the LO fundamental. The mixing equations for the harmonic mixing modes are:

F,, - NFL, = F,, (for plus modes)

and

NF,, - FI N= F,, (for minus modes)where N is the harmonic number of the mode.