Modifying the Agilent (HP) 85105A for use with OML Millimeter Wave VNA Modules

Instructions for replacing the L.O. splitter in the Agilent (HP) 85105A

Overview: The following changes allow the 85105A to operate with a 2 to 20 GHz L.O. for use with the OML millimeter wave VNA modules. The 2 to 8 GHz L.O. power divider is going to be changed to a Narda #4456-2 2 to 18 GHz power splitter (which works very well up to 20 GHz) by removing it as an assembly with its mounting bracket from the 85105A. The 2 to 8 GHz splitter is removed from the bracket and the 2 to 18 GHz unit is installed on to the bracket. The assembly will then be reinstalled back into the 85105A. The Narda #4456-2 2 to 18 GHz power splitter has a different mounting hole pattern than the original Narda # 4324-2 that Agilent used and will require slight modification of its mounting holes. This modification should be performed by a competent microwave technician.

Carefully save all hardware removed during this modification as it will all be reused.

1) Modify the Narda #4456-2 2 to 18 GHz power splitter by extending the mounting holes of the splitter so that the edge of the enlarged holes is within .025 to .035 to the outer edge of the splitter. This can be accomplished with a small rat tail file or by using a vertical mill with a 1/8" end mill. Do not drill the hole oversize as it is easy to get into the circuit area and damage it. See the illustration below.

2) Remove the three SMA coax cables connected from the L.O. power splitter. These should be pushed back very carefully to avoid causing misalignment of the male connector center pin.

3) Using a short phillips head screw driver, remove the two phillips heads screws from the SMA coax switch on the back side of the L.O. power splitter bracket (ps bracket) and allow it to hang freely in place.

4) Remove the two phillips head screws holding the ps bracket to the 85105A chassis.

5) Remove the ps bracket assembly taking care not to bend or damage any cables, wires, etc. in the 85105A.

6) Remove the 2 to 8 GHz splitter from the bracket and install the modified 2 to 18 GHz splitter in its place using the same two phillips head screws. Larger diameter washers should be used on the screws.

7) Reinstall the ps bracket onto the 85105A.

8) Reattach the three SMA cables onto the power splitter.

9) Reattach the coax switch to the backside of the ps bracket using its original phillips head screws.

10) It is now recommended that the frequency characteristics of the modified 85105A be confirmed.

After reinstalling the 85105A top cover, apply AC power to it.

Attach a 2 to 20 GHz scalar analyzer detector with a 10 dB attenuator to either L.O. output connector and install a 50 ohm load on the other L.O. output connector.

Apply the analyzer's 2 to 20 GHz signal, at a level of +10 dBm to the L.O. input connector on the rear panel of the 85105A. The output power should be greater than +20 dBm across the band. It will be noted that the power is very flat from 2 to over 8 GHz. Above 8 GHz to 20 GHz there may be some slope and 1 to 2 dB of ripple. The L.O. limiting amplifier in the OML millimeter wave VNA module will remove the effects of that slope and ripple. Data taken on two 85105A units will soon be available.

Modification of the Narda #4456-2 2 to 18 GHz power splitter: Modify both mounting holes the same way.

Updated 5/10/2001

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