MODEL8559A

SERVICE

SECOND CONVERTER ASSEMBLY A5, CIRCUIT DESCRIPTION

The IF from the First Mixer Assembly A4 is coupled into the Second Converter Assembly A5 bandpass filter through coupling loop L3. Three circular, slug-tuned cavity resonators, operating as an inductive transmission line, make up the bandpass filter. The filter forms a high-Q circuit centered at 3 GHz with a 23 MHz bandwidth that is required to accommodate the regular and alternate IFs. Coupling loops L4 and L5 provide coupling between the cavities. Loop coupling is also used to couple the 3 GHz IF signal to the second LO output at the mixer diode CR1.

The second LO contains varactor diodes that are controlled by a voltage from the Marker Assembly A8. The diodes shift the frequency of the second LO either 15 MHz (ALT IF) or +1 MHz (SIG ID). The varactor control voltage is always between 1V and 28V and corresponds to the oscillator frequency; increasing the voltage increases the frequency.

Both the second LO and the 3 GHz IF signal are coupled into mixing diode CR1, generating a difference frequency of 321.4 MHz that is coupled through the matching filter (C3, L2, C4) to the Third Converter Assembly AlO. The matching filter is a passive network designed to match the impedance of the second mixer to the 50 ohm impedance of the Third Converter Assembly A10. The match is optimized in both IF modes by adjusting L2 (2nd MIXER MATCH).

SECOND CONVERTER ASSEMBLY A5, TROUBLESHOOTINGVerify that the Second Converter Assembly A5 supply voltages are correct.

If the displayed signal amplitude varies between ALT IF and REG IF, perform and verify the bandpass and second LO frequency adjustments.

Second LO Frequency: A failure in the Second Converter Filter Assembly A5A2 can cause the Second Converter Oscillator A5A1 to oscillate at about 3 GHz. This symptom can occur when the delay circuit in the filter assembly does not delay the application of the +13V bias voltage. To test the delay, observe the +13V bias as the instrument is turned on. There should be a noticeable delay before the +13V is applied to the line. The - 10Vsupply, on the other hand, should rise gradually. If the +13V and the - 10V respond properly, check the varactor voltage, varactor diodes, and the cavity adjustment as the possible source of the second LO frequency error.

Second LO Fails to Oscillate: The Second Converter Oscillator Assembly A5A1 can intermittently fail to oscillate after turn-on. If this symptom occurs, replace the entire assembly. Before removing the defective circuit board, note the orientation of components, leads, and hardware; orientation is critical to proper operation. To prevent damage to the replacement circuit board, do not over-tighten the hex-head antenna screw during instal- lation.

Second Converter BandpassShape: Low signal power from the First Mixer Assembly A4 can distort the second converter bandpass filter shape. Excessive ripple in the bandpass can be the result of a mismatch in the signal path preceding the Second Converter Assembly A5. An input attenuator setting of 0 dB can cause such a mismatch. The second converter mixer diode or Mixer Match adjustment can also affect the bandpass ripple.

Residual FM: Residual FM can originate from the Marker Assembly A8 Second LO Driver, which supplies the varactor bias voltage, or from within the second LO itself.

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