[time-nuts] 35601A as stand-in for 11848A ?

Kent seiskent at earthlink.net
Fri Jan 11 03:41:47 EST 2008

Hello Brian,

I bought a 35601A hoping to use it for phase noise measurements but 
discovered that the only way to control the unit is through the HPIB 
interface, presumably using the 3047A software which I was never able to 

Looking inside the 35601A, you find  rather involved signal routing with 
dozens of mechanical and FET switches for routing the various signals 
around. To complicate matters, the control lines from the 12 8-bit registers 
that actuate the switches seem to follow no discernable logic with many 
appearing to be assigned randomly. There must have been some software 
compatibility issue with previous equipment. The 11848A shares many of the 
same control signal assignments but there are enough differences both in the 
control signal assignments and other circuits to make the 3048A software 
unusable with one possible exception.

There are two main parts to the 35601A and the 11848A units, the High 
Frequency circuits and the Analyzer Interface. The High Frequency circuits 
which contain the input mixers, input switching, diplexers following the 
mixers, amplifier chain, gain switching, PLL lead-lag filters and other PLL 
circuitry are almost identical.  What is different is the Analyzer 
Interface, assembly A4. The 35601A was closely tied to the 3585A spectrum 
analyzer, using the 350Khz IF out of the analyzer in the low frequency 
processing circuits.  Although the 11848A also uses the 3585A  for the 
offsets greater than 100Khz, it doesn't rely on the 350Khz IF output of the 
3585A.  They also changed the LP/HP filter arrangement and signal path 
switching in the Analyzer Interface of the 11848A which means that the 3048A 
software will not correctly control the Analyzer Interface and so will not 
properly drive a 3561A spectrum analyzer. So, the bottom line is the 3048A 
software won't fully work with the 35601A.

Now for the possible exception. There is a facility in the 3048A software 
which allows manual control of the 11848A. Since the 35601A High Frequency 
circuit (A4) is essentially the same as the 35601A High Frequency circuit, 
you might (I haven't tried it) be able to use the 3048A software to manually 
control the High Frequency board in 35601A.  Doing this you can manually 
change the PLL gain and filtering to establish lock. Of course, you really 
wouldn't be able to use any of the features of the 3048A software that make 
it so useful.

I also recently acquired a 11848A interface and have written  a small 
program to control the 11848A (and also the 35601A) over the HPIB bus which 
allows the various signal paths and filters to be manipulated using a PC. My 
eventual goal, largely inspired by John Miles Phase Noise program (thanks 
for contributing this great program), is to emulate some subset of the 
functionality of the 3047A/3048A software.  It turns out, there are quite a 
few "details" to be concerned with but John has already done much of the 
heavy lifting in controlling various spectrum analyzers and slogging through 
some of the more problematic noise computations,  which makes the task more 

As you may know, the 11848A Service Manual is available on the Agilent 


The diagrams and schematics are marginally readable, however they are clear 
enough to get an idea of how the unit operates. The block diagram is hard to 
read, but there is another one in the 3048A Phase Noise Measurement System 
Reference Manual - 03048-90002 on pfd pages 296 through 299 (section 5-40) 
that can be printed out and pasted together. Assembly A4 is the  High 
Frequency portion and, as I mentioned, closely matches the 35601A.

The 3048A Operating,  Reference, and Calibration manuals are at:


By the way, this is my first post, although I have been following the 
discussions here with fascination for quite awhile - like many others I 
suspect. What at first was little more than a passing interest in time and 
precise measurement, has now become a much more focused effort to measure 
time, time intervals, phase noise, and frequency, aided and abetted by the 
many interesting discussions on this list. Thanks to everyone for 
contributing to this great resource.


----- Original Message ----- 
From: <BriMDavis at aol.com>
To: <time-nuts at febo.com>
Sent: Sunday, January 06, 2008 4:38 PM
Subject: [time-nuts] 35601A as stand-in for 11848A ?

> John Miles wrote:
>>> How many significant digits are there in a 5120A price quote?
>> $2.0*10^4, last I heard. :)
> OK, I'll be crossing that one off the home lab wish list!
>>  I do have a hardcopy manual for the  35601A but am not
>> sure if I have a .PDF of it or not.  Will check tomorrow
>> and let you know.  I can probably scan it easily enough
>> if not, 11x17 foldouts excepted.
> At this point, just the detailed 35601a system block diagram
> showing the switched signal paths and gain/filter settings
> should be enough for me to figure out how practical building
> a Franken-3048a would be.
> Don't spend time scanning the manual on my account, I'll
> order a hardcopy from Manuals Plus once I sort out why they
> have two different P/Ns listed for the service manual.
>> You either need a 3047A/3048A system, or some custom software
>> that you'll have to write yourself.
> The later discussions of homebrew PN setups may have
> obscured my original question:
> Has anyone ever looked into the feasibility
> of using a 35601a as a stand-in for the 11848a
> in a 3048a phase noise system?
> I have a 9000/340 computer with the 3048a software,
> a 35601a, and can probably pick up the 3561A narrowband
> analyzer needed by the 3048a software for $300-500 USD
> (with some patience).
> The 11848a's usually still have four digit price tags.
>> Keep in mind, though, that without the 11848A's internal sources,
>> you're going to have to supply some kind of reference source
>> at the same frequency as the DUT.
> Yes, I plan to have two (or three) copies of the LO/reference chain
> in question, and pull one of the OCXO's for quadrature lock.
> thanks,
> Brian
> **************Start the year off right.  Easy ways to stay in shape.
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