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HP 8664A option 3-6 GHz

Question asked by BoinFinland on Jan 5, 2015
Warranty void if label broken ;-)
Well, at your own risk as usual.

The 8664A YIG oscillator covers 3-6 GHz just as the 8665B. Probably the same part. One difference between the two models is that the 3-6GHz output is terminated with 50 ohm in the 8664A and connected to the Microwave Extender module in 8665B. In my 8664A I also found the original RF switch, assembled and unconnected, used to switch between 0-3 GHz and 3-6 GHz (8665B).

What I need is few semi-rigids and control logic for the RF switch to cover the full range 0-6 GHz with some minor limitations.

Microwave Assembly
Remove the small SMA 50ohm termination at MW LOOP out and connect the 3-6 GHz output to the RF switch (33314-60014) input 2 with a short semi-rigid.
Connect one short semi-rigid between the RF switch output (C) to the step attenuator (33321-60030).
Replace the old coax with a new longer semi-rigid from the Output module J2 to the RF switch input 1
You can swap input 1 and 2 but it will be very tight to connect it that way.

"My favorite programming language is solder" - Bob Pease
I could try to read the proms, disassemble the code and modify it for my purpose but by the time I would be in the debugging phase my hardware hack would be working fine since long so, for a small change like this its time to heat the iron.

The RF switch (15VDC) is driven by U53 on the I/O assembly board and is activated for every change of the step attenuator so the first thing to do is to cut the lines going to input pin 3 and 6 on the bottom side. U53 (ULN2068B) is a darlington driver for the RF switch relays and only needs a short 50-100ms long positive pulse at the input to switch state. Pin 3 high for 50ms enables normal mode 0-3 GHz out and pin 6 high for 50ms enables 3-6 GHz out.
I don`t know, but I think it`s better to control the switch with a pulse rather than continuously, at least it draws much less power.

How you control these pins is not that important. It can be made from the rear panel using a SPDT toggle switch that returns to zero (Momentary-Off-Momentary) or you can use the unused Mode button on the front to enable 3-6 GHz.
I`ve chosen the latter even if it involves more work and more pcb cutting. In my case I used Mode5 for 3-6GHz and Mode4 for normal mode 0-3GHz and the Mode5 LED as indicator for frequency * 2.
Removing the front panel is not difficult but needs careful disassembly and removing the 4 or 5 front panel BNC connectors. These are special type of BNC connectors that can be removed from the backside as there is one extra nut for the cable assembly to open, so the front chassis part can be removed easily via the front.
Don't twist the coax though and be sure to help it through the hole during assembly.

I used one half of 74ALS74 Flip-flop (5V logic) as the "memory" with Q and /Q driving the RF-switch drivers U53. I used 220R in series with 1.5uF capacitor in each branch between flip-flop output and driver input to get the pulse and 100k resistor pull down and in parallel a 1N4148 diode (anode to gnd) to kill negative pulses at the U53 driver inputs. Mode5 switch goes to the preset input with a 100k pullup and Mode4 switch goes to the clear input of 74ALS74 with a 100k pullup resistor. Both Mode4 and Mode5 switches grounded at one end.

The yellow LEDs on the front is current limited to 10mA when connecting a LED to ground so lift pin 2 on U25 and connect a 1N4148 diode from this pin (kat) and pin 19 (anode) in connector J1 (50 pin) to block the high current from U25 output pin 2 active high. This signal is used during the initial LED test.
Add one more 1N4148 diode from J1 pin 19 (anode) and the flip-flop Q output to indicate the new mode.
Connect 74ALS74 D input high (+5V) and clock input to U25 output pin 2 now used as reset of the flip-flop at power on using the normal test LED function. Unused pins on the second flip-flop are all connected high and outputs left open circuit.

Reverse Power Protection (RPP):
I believe the reverse power protection circuitry only works fine up to about 4.5 GHz as it is only used in 8664A (3GHz) and 8665A (4.2GHz) but not in 8665B (6GHz). I think it is safe to remove the reverse power protection circuitry if you
never feed high RF power the wrong way into the generator but you then need a new coax from the attenuator to the generator output connector. I measured my RPP up to 6GHz in a VNA and saw the degradation but it works fine 
in my generator up to 6GHz at least at low power (max +7 dBm).

Calibration (Special 171):
Self Calibration only works in normal mode 0-3GHz and should return the code 0.
In 3-6GHz mode you may end up with error codes as the 0-3GHz output is unterminated in the RF switch (reflective type) and the FW has no control of the switch. This is no problem for the operation though.

Frequency Display:
Mode4 (normal) - The frequency displayed is correct.
Mode5 (3-6GHz) - Set frequency to the half output frequency, valid between 1.5-3GHz.
There are more possibilities to get 6GHz out below 1.5GHz display settings but my analyzer stops at 3.5GHz so I haven`t tried to find them all.

If you enter 100kHz deviation you will get 200kHz in the x2 mode (3-6GHz).

0-3GHz works just as usual in a 8664A.
3-6GHz works fine but the display reads half frequency and half frequency deviation.
       The RF amplitude only change in 5dB steps like the step attenuator dictates. No fine level control.
       Max output power +7dBm up to 6GHz.

Future project:
To get AM and fine amplitude steps working properly you will also need the Microwave Extender module or at least a 3-6GHz amplifier with ALC Automatic Level Control and AM modulation but that will be a future project. However, who needs AM above 3 GHz? At least not me.

So, if you need a 6GHz generator and happen to have a good old two person lift 8664A available in your lab, this is a doable option instead of buying one.
As usual comments and ideas are welcome. Maybe someone even have a SW/FW modification available ;-)

Edited by: Bo in Finland on Jan 5, 2015 8:56 AM