HP/Agilent 53131A, 53132A, 53181A Channel 3 Guide

Agilent 53131A - 10 GHz - front panel

This guide, will attempt to answer a couple of questions regarding the different channel 3 options on the market and their compatibility with the counters in this series.

The optional channel on the HP/Agilent 53131A series, which are some of the best frequency counters you can get, allows you to measure frequencies in the GHz range (originally, up to 1.5 GHz, 3 GHz, 5 GHz and 12.4 GHz).

Now there are several aftermarket options available that offer various degrees of performance, quality and compatibility, but unfortunately, they cannot be fitted on all frequency counter models, so not all options are interchangeable.


How many counter and option board combinations are there?

There are four different counter configurations, that differ both in hardware and firmware, and there are two types of original 3rd channel boards, that each work with two counter configurations:

  1. The 3 GHz option, which works with 1.5 GHz and 3 GHz counters.
  2. The 12.4 GHz option, which works with 5 GHz and 12.4 GHz counters.

The frequency counters configured for 1.5 GHz, 5 GHz and 12.4 GHz options, have this value specified on the front panel, both on the window of the display and next to the 3rd channel connector, while the 3 GHz variants can have it specified or not.

The 1.5 GHz option was only available on the 53181A.


How come the same option board works for two counter configurations?

The 1.5 GHz and the 3 GHz/vanilla counters are practically the same, with the exception that the lower frequency one is firmware limited. The same thing applies to the 5 GHz and 12.4 GHz counters.

It is unknown whether or not the options were assigned to a particular counter configuration, based on the production testing results.


Which aftermarket options work with which counters?

Like mentioned earlier, there are many aftermarket options available, that offer various degrees of quality, performance and compatibility.

At this time, almost all of them are for the 1.5 GHz and 3 GHz counter configurations, which make up the majority of frequency counters. That includes the Cojotech 3 GHz8 GHz and 10 GHz options.

Cojotech also offers variants of the 8 GHz and 10 GHz options that are compatible with 5 GHz and 12 GHz frequency counters (option E).

For a comprehensive list of all the available Cojotech options, please see the selection guide.


What happens if I install a 3 GHz option on a 1.5 GHz counter, or a compatbile 8 or 10 GHz option, on a 5 GHz counter? 

You get to measure up to 1.5 GHz or 5 GHz as per your counter’s configuration. The maximum frequency will be limited by the counter.


What is the difference between the various options?

The original 3 GHz board comes with a BNC connector, while the 12.4 GHz one comes with an N-type connector. Other notable differences are that the 3 GHz board has a minimum specified input frequency of 100 MHz, while the 12.4 GHz one has a minimum of 200 MHz. Sensitivity wise, they’re almost the same.

HP 53131A/53132A Channel 3 BNC mounting hole
BNC mounting hole
fig. 1
HP 53131A/53132A Channel 3 N-type mounting hole
N-type mounting hole
fig. 2

In the aftermarket corner, you have more variety, with boards covering 2.7, 3, 4, 8 and 10 GHz, but most of them are very loosely specified, the amount of QA they go through is uncertain and they come with no warranty, which often backfires, because many of them have no input protection.

This is where the Cojotech options come into play. The Cojotech HPO-030, HPO-080 and HPO-100, offer better sensitivity than all the other ones available at this moment (including the originals), they have a well defined specification, and every board goes through an extensive testing procedure that verifies if the specification is met as well as verifying the correct operation of the board. On top of that, they come with a 2-year warranty and the right to return them within 30 days if you don’t like them.


Be warned about chimera counters!

As of recent (2021), there has been an increase of chimera counters on the market. These are counters that are made from parts from other counters. It is usually not a problem, but it can become one if the front panel doesn't match the mother board.

The headaches come if you're in the market for a new 3rd/2nd channel option, and you think you have a vanilla counter because that's what the front panel indicates, but instead the motherboard is that of a 5 GHz counter, which means options made for the vanilla counter will not work.

Similarly, you might think you have a 12.4 GHz counter, but when you install the original 12.4 GHz option or a compatible one, you find out that it's not working.

What is there to do? One thing you can do before buying any 3rd channel option is to check the back of the mother board for the presence or absence of R334. This resistor is factory installed on counters that had the original 5 or 12.4 GHz options, so it's presence should be a pretty good indication that the board is that of a 5 or 12 GHz counter. If it is missing, but the solder on the pad doesn't look factory, you should investigate further.

To make matters worse, U14, which is the EEPROM that holds the calibration data and the 3rd channel configuration, can be altered. Normally, when it is being modified a socket is installed in its place (but it is not necessary), to make programming, testing and reinstallation easier. This means that if your counter has a socket, someone has altered the EEPROM and might have converted it from 1.5/3 GHz to 5/12 GHz or the other way around. In this case you need to look for the presence/absence of R334 and make an educated guess.


Is BNC good enough for an 8 GHz option?

Yes and no. They’re relatively ok up to 5-6 GHz, but after that the insertion loss increases a lot, which in turn degrades the rated sensitivity of the prescaler. It won’t completely ruin it, but it won’t be what you see in the datasheet either. On the other hand you get the benefit of having a BNC connector, which is extremely common and useful.

* As of January 2020, Cojotech has introduced an optional BNC cable assembly that is available for the HPO-030 and HPO-080, option BNC-2, which offers excellent perforamnce to 6 GHz, and has only about 1 dB of loss at 8 GHz.

To get an idea on the effect of various cable types, take a look at the cable section in the selection guide, which compares several cable options: selection guide#cables.


Any other gotchas?


If improperly designed or manufactured they can start to oscillate, often outside of the 3 GHz bandwidth and usually at the full power of the gain stage. That signal won’t be stopped by the chassis of your counter, nor by the little on-board shield that is present on some of the boards, and it will radiate in your lab and into your experiments. The exception is made by a few Chinese designs which don’t have any gain (or input protection for that matter).


To be continued/updated...