And, success!
With the same prescaler and bias network as used by HP, the option board works!
I bought the prescaler chips from Aliexpress, they came loose in a non ESD plastic bag, s I was a bit dubious if they would be working or even genuine. Closer inspection revealed that they may be salvaged parts. Good for not being fakes at least......
However, on installing one, it turned out to work fine, nice.
There is a selected resistor (R6 in the HP design) that is specified as 316K, I stuck a 500K trimpot there and tweaked it until the counts reliably froze with no input and got a value of 288K. If I continued winding the trimpot until the display showed random numbers with a 3GHz input at it's lowest acceptable input level, I got 190K, so it seems this resistor does need to be selected for optimum operation (Or my more accurate 5.0V vreg has shifted the required value compared to the 5.1V zener regulator in the original design). I think something around 267K (that seems to be a standard value) should be fine.
I then ran the tracking generator from my 3.2GHz Siglent VNA into the input along with an 8481A power sensor hooked to a E4418B power meter using a BNC Tee
A bit of a crappy setup, but it gives a rough idea of performance.
Here's the results:
I need to get an 8482A to reliably test the low frequencies below 100MHz, but the option board locks onto a signal waaayyy down to 15MHz, not too bad! I can get a lock up to 3.1GHz, so the frequency range is a bit better than specs. Happy with that.
I found that my design needs a bit more power in the 2.7 - 3.0GHz range, HP specify -21dBm whereas I need -18dBm. Maybe some tweaking to the layout maybe something like the diode positions as mentioned by Tom Lee, or a third stage of amplification?
Here's a chart showing SWR, maybe someone can suggest some improvements from this too?
Anyway, as it is, I'm pretty happy with it so far. :)
Let me know if there's anything obvious I can improve on the next (hopefully final) revision if needed.
Jared
Jared Cabot