Al3 Second Converter
The Al3 Second Converter down-converts the 3.9107 GHz 1st IF to a 310.7 MHz 2nd IF. The converter generates a 3.6 GHz second LO by multiplying a 600 MHz reference. Bandpass filters remove unwanted harmonics of the 600 MHz driving signal. First IF and 2nd LO signals are filtered by cavity filters.
A15Al Second IF Distribution Amplifier (P/O A15)
The A15Al SIFA (Second IF Distribution Amplifier) amplifies and filters the second IF. (Option 001 instruments provide the pre-filtered signal at the rear-panel’s 2ND IF OUTPUT.) Factory select attenuator A15U802 ensures that the gain provided by the SIFA is 12 dB
f2 dB.
The external mixing input from the front-panel’s IF INPUT connector is also directed through the SIFA. A dc bias is placed onto the IF INPUT line for biasing external mixers.
Third Converter (P/O A15)
The third converter down-converts the 310.7 MHz IF to 10.7 MHz. A PIN-diode switch selects the LO signal used. For normal operation, a 300 MHz LO signal is used. The signal is derived from the 600 MHz Reference PLL. During signal identification (SIG ID ON), the 298 MHz SIG ID Oscillator is fed to the double balanced mixer on alternate sweeps.
Flatness Compensation Amplifiers (P/O A15)
The flatness compensation amplifiers amplify the output of the double-balanced mixer. The amplifier’s variable gain (8 to 32 dB) compensates for flatness variations within a band. Band conversion loss is compensated by step gain amplifiers in the IF Section.
Control for the amplifiers originates from two DACs on the A3 Interface assembly. (DAC values are interpolated approximately every 17 MHz based on data obtained during the Frequency Response Adjustment.) A15’s flatness-compensation control circuitry converts the RF GAIN voltage, from A3, into two currents: RF GAIN1 and RF GAIN2. These currents drive PIN diodes in the flatness compensation amplifiers.
Synthesizer Section
The first LO is phase-locked to the instrument’s internal 10 MHz standard by four PLLs. See Figure 6-5.
The Reference PLL supplies reference frequencies for the instrument. The three remaining PLLs tune and phase-lock the LO through its frequency range. To tune the LO to a particular frequency, the instrument’s microprocessor must set the programmable feedback dividers (N) and reference dividers (R) contained in each PLL.
Sweeping the First LO
The analyzer uses a method called Lock and Roll to sweep the first LO (All YTO). This applies to all frequency spans and involves phase-locking the analyzer at the start frequency during the retrace of the sweep. The sweep ramp, generated on the Al4 Frequency Control assembly, is applied to either All YTO’s main coil, All YTO’s FM coil, Roller Oscillator PLL’s Main Oscillator, or Roller Oscillator PLL’s Offset Oscillator. The frequency/span relationships are as follows:
