[Asterisk-Users] OT: Differences between test equipment

Rich Adamson
Thu, 08 Sep 2005 06:16:36 -0000

> Given the current discusison regarding ztmonitor, line testing, etc.,
I've
> been looking into purchasing a used transmission test set.  From my
> research, it seems that there are two items that might fit the bill:  the
> HP 3551A and the HP 4935A.
>
> I know nothing about these specific devices.  I *do* have a good
> background in electronics, and I understand the concept what they're
> measuring and why., but I know nothing about the specifics of how this
> relates to transmission test sets!  :)  In fact, I'm not even sure that
> these are indeed the right devices for the job.
>
> Could someone who is familiar with either of these devices tell me if they
> will fit the bill?  And if possible, which of these has more useful
> features as telephone line test equipment?  I'm handy with an
> oscilloscope, function generator, VOM, etc.  Will I be able to drive
> either of these?

As a reference point, go to http://www.triplett.com and look for
their Model 2 or Model 4 telco test set.

"The Model 4 Subscriber Loop Test Set combines the features of a
transmission test set, a 17-frequency multi tone generator, and a
multi range volt-ohmmeter in a rugged portable unit."

The Model 4 is the most useful as you can connect the tone generator
to one ATA port (as an example) and the Line Loss part of the meter
to another ATA port, and measure the actual transmission loss
between the two ports.

If you are "only" going to use the test set to measure the pstn
loss from a central office milliwatt generator, then the cheaper
Model 2 is all that is needed. This model does not include a tone
generator, so its limited to measuring tone levels sourced from
some other devices. (Note: the Model 2 is nothing more then an AC
voltmeter that is far more sensitive then those found in the
typical electronic repair shop. For example, measure noise on a
pstn line needs to fairly accurately read signals on the order
of .01 volts and below.)

For the most part, you are not interested in measuring power influence,
line current, and lots of other parameters. The key items that you
do want to measure is "line loss" and "line noise".

There are tons of other new/used test sets that include dsl test
facilities, time domain reflectometer (tdm), and other items. You
don't need any of that stuff at all, and those types of test sets
typically start at something around $2k US. I didn't look at the
HP models that you noted, but I'd guess those boxes support far
more then what is needed. HP typically does not manufacturer
anything that is inexpensive; they are far more oriented towards
the high end test equipment.

I prefer an analog meter as opposed to a digital display as the
analog meter tends to indicate fluctuations much better. The digital
ones almost appears like random numbers on the screen since the
sampling intervals are rather lengthy.

There are other companies that produce the same exact type of portable
test set. Use keywords such as "subscriber loop test", "model 4
subscriber test set", in google and you'll find over a million hits.

The last time I looked at ebay stuff, these inexpensive test sets
were not listed.