Difference between HP / Agilent 435A and 435B Power Meters

[TimYTTT]

Hello HP/Agilent fans and experts,

Can someone please explain me what is the difference between the  HP/Agilent 435A and 435B Power Meters?
Both power meters are of analog type, looking identical from the outside. For at least one sensor (HP 8484A) the HP recommends use of 435B power meter version only. I checked HP site and did not found any useful info about that.

Thanks for your help!

[johnk0gcj@gmail.com]

Good question. I've had a couple 435A's over the years and had seen but never had 435's, and had
assumed that the difference was the interchangeable scales on the 'A'. However, I recently obtained
another 'A' model, and it has the non-interchangeable scale. At first I thought someone had swapped
knobs with a non-working 435 until I realized that it has the marker where it belongs, just above the
outside circumference of the scale. The two 435a's are identical except one is a 1415A prefix with the
old style range indicator and the other is a 1629A prefix with the interchangeable scales. Apparently
the difference is other than what I've been able to detect without getting inside. Maybe when I get more
curious I'll do an inside examination.

[johnk0gcj@gmail.com]

After giving this a little thought I remembered from years ago that the 435's select the correct
indication by determining which power sensor is connected. If I recall correctly it uses 2 or 3
pins and corresponding wires in the cable to indicate which sensor. The interchangeable scales
remove the need to do the math.
 

[metrologist]

I think mostly you found the difference.

http://www.test-italy.com/Specifiche%20strumenti%5CHP%5CHP%20435A,11683A%5CHP%20435A.pdf

https://xdevs.com/doc/HP_Agilent_Keysight/HP%20435B%20Operation.pdf

[johnk0gcj@gmail.com]

Thanks a lot. Now you've goaded me into doing some more
research to find the difference between the A and B.

[TimYTTT]

First thanks to all of you for your inputs.
In the meantime I did some comparison based on the HP product leaflets for both meters. The differences that I detected are not significant:
1. Frequency Range
A: 100 kHz to 18 GHz (sensor dependent)
B: 100 kHz to 26.5 GHz (sensor dependent)
Conclusion: Anyhow, difference between A and B, being sensor dependent, is questionable.

2. Response Time (0 to 99% of reading)
A:
2 seconds on 3 μW range
0.75 second on 10 μW range
0.40 second on 30 μW range and
100 milliseconds on all other ranges
B:
Range 1 (most sensistive range) 
      <10.0 seconds
Range 2   <3.8 seconds
Range 3   <1.3 seconds
Range 4 to 10   <500 milliseconds
Conclusion: For me not so relevant difference

Enclosed is the document with the feature comparison. I do not understand why HP introduced the B variant of the Power meter. Maybe just the commercial reasons?

[metrologist]

The reason for the B model I highly expect is the painful process of chasing gigahertz. The 8485A is the only supported sensor to 26.5 GHz

[johnk0gcj@gmail.com]

That sounds logical.