Return Loss Test Set

[Brian Aase]

I'd like to collect enough equipment to fashion together a return loss
test set suitable for measuring such things as CATV amplifiers and
splitters, as well as little RF-centric home projects. The H/P 141T
spectrum analyzer system seems readily available and affordable, but
there is no "catalog" or other guidebook of the various modules and
options any more. (Yes, I know that I could search out an older HP
catalog, and likely will.) But in the meantime, could someone possibly
suggest the most likely combination of modules that would do the job I
want?

Also I need a recommendation for a suitable companion RF generator.
(The 141T has no tracking generator, right?) And finally, will I need
some sort of external bridge/detector unit, or do the 141T modules
have that functionality already built in?

Suggestions from those famailiar with such a setup would be very much
appreciated -- TIA

 

[John Miles]

Also I need a recommendation for a suitable companion RF generator.
(The 141T has no tracking generator, right?) And finally, will I need
some sort of external bridge/detector unit, or do the 141T modules
have that functionality already built in?

Suggestions from those famailiar with such a setup would be very much
appreciated -- TIA

The 141T (display) / 8552B (IF) / 8554B (RF) sounds like what you want.
It will cover (approximately) 10 MHz - 1.2 GHz, and you can use an 8444A
tracking generator with it.

The 8502A and 8502B return-loss test sets are probably usable with an
8554B / 8444A combo, but I can't swear to that. They are meant for use
with the 8505 network analyzer. I believe the 8502B is 75 ohms and the
8502A is 50 ohms; you may end up wanting both, if you are interested in
CATV work.

Here's the HP Journal issue that described the 141T and its initial set
of plugins:

http://www.speakeasy.net/~jmiles1/hpj141t.pdf (16 pages, 2 MB)

-- jm

 

[Chris Jones]

I'd like to collect enough equipment to fashion together a return loss
test set suitable for measuring such things as CATV amplifiers and
splitters, as well as little RF-centric home projects. The H/P 141T
spectrum analyzer system seems readily available and affordable, but
there is no "catalog" or other guidebook of the various modules and
options any more. (Yes, I know that I could search out an older HP
catalog, and likely will.) But in the meantime, could someone possibly
suggest the most likely combination of modules that would do the job I
want?

Also I need a recommendation for a suitable companion RF generator.
(The 141T has no tracking generator, right?) And finally, will I need
some sort of external bridge/detector unit, or do the 141T modules
have that functionality already built in?

Suggestions from those famailiar with such a setup would be very much
appreciated -- TIA


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To measure return loss, what you really want is a network analyser, not a
spectrum analyser. Unfortunately network analysers seem to be more
expensive. As a starting point you could get a diode detector and a signal
generator for the frequency of interest, and a directional coupler. You
would also need a known short circuit with the right connector type to
determine what 100% reflection looks like on your meter, and also a known
resistive terminator to determine what an ideally matched device looks
like. I think this is about the minimum setup you need to measure return
loss. The accuracy will depend greatly on the quality of the directional
coupler. You could substitute a spectrum analyser such as the 141T in
place of the diode detector, but it will not bring you a lot of
improvement, except perhaps that the level of harmonic content in your
signal generator will be less important. If you want to do much better
then you really want a Vector Network Analyser (VNA).

If you are only interested in low frequencies then you could look at the
N2PK network analyser. (try google) It would be nice if someone would
design something similar to cover up to 2.5GHz (e.g. for WLAN antennas
etc.), but the first stumbling block is the need for a good signal source.
There are suitable mixer chips e.g. AD8343, which could be substituted with
little pain into the N2PK type design, but the synthesisers are the hard
part, especially if you want the same instrument to cover the range of
frequencies down to low RF or audio. A decent 4-receiver 3GHz VNA would be
a very popular project if someone came up with a good design.

Chris

 

[Bill Janssen]

Brian Aase wrote:




To measure return loss, what you really want is a network analyser, not a
spectrum analyser. Unfortunately network analysers seem to be more
expensive. As a starting point you could get a diode detector and a signal
generator for the frequency of interest, and a directional coupler. You
would also need a known short circuit with the right connector type to
determine what 100% reflection looks like on your meter, and also a known
resistive terminator to determine what an ideally matched device looks
like. I think this is about the minimum setup you need to measure return
loss. The accuracy will depend greatly on the quality of the directional
coupler. You could substitute a spectrum analyser such as the 141T in
place of the diode detector, but it will not bring you a lot of
improvement, except perhaps that the level of harmonic content in your
signal generator will be less important. If you want to do much better
then you really want a Vector Network Analyser (VNA).

If you are only interested in low frequencies then you could look at the
N2PK network analyser. (try google) It would be nice if someone would
design something similar to cover up to 2.5GHz (e.g. for WLAN antennas
etc.), but the first stumbling block is the need for a good signal source.
There are suitable mixer chips e.g. AD8343, which could be substituted with
little pain into the N2PK type design, but the synthesisers are the hard
part, especially if you want the same instrument to cover the range of
frequencies down to low RF or audio. A decent 4-receiver 3GHz VNA would be
a very popular project if someone came up with a good design.

Chris

How about using a slotted line long enough for the frequency of
interest. Or a slotted
waveguide if the frequency is high enough.


Bill K7NOM

 

[Scott Townley]

Brian,
The 141T does indeed have tracking generators as part of the "lineup".
The 8443 is a counter/tracking gen up to 110MHz (pairs with the 8553
RF plugin), and the 8444 tracking gen up to 1.3GHz with the 8554B RF
plugin. Neither are particularly difficult to find.

As-is, they will easily permit the measurement of swept transmission
(thru) loss/gain measurements without any additional equipment.
However to do return loss you will need a directional bridge of some
sort. I believe that there is an accessory 6dB directional bridge
that I can't recall the model number of. Or you could use any
frequency-appropriate directional coupler with a reference adjustment
for the coupling in dB.

 

[Brian Aase]

Chris, Bill, Scott,
Thanks so much for your comments. You're right in that I really would
LOVE to have a VNA... maybe one of those Rhode & Swartz (sp?) jobs
that sell for about $40K on ebay ;-)
It's great to know about the 8444/8554B plugins. I wasn't aware of
them, and will now start looking. I did know about the need for a
directional bridge or directional coupler, thanks for confirming my
understanding of these.

Brian Aase

 

[John Miles]

Chris, Bill, Scott,
Thanks so much for your comments. You're right in that I really would
LOVE to have a VNA... maybe one of those Rhode & Swartz (sp?) jobs
that sell for about $40K on ebay ;-)
It's great to know about the 8444/8554B plugins. I wasn't aware of
them, and will now start looking. I did know about the need for a
directional bridge or directional coupler, thanks for confirming my
understanding of these.

Brian Aase

Look into the HP 8505. They are in the $1K range, which is a real
bargain if you have the space.

-- jm

 

[Scott Townley]

hey again Brian,
First, a minor point, the tracking generators I mentioned aren't
plugins but full rack-width equipments, probably 5" tall.
Someone else mentioned the 8505 VNA...which is the same vintage (up to
1.3GHz like the 141T), but *by itself* it's just a multi-channel
tracking receiver. To make up a VNA you still need a sweep generator
and a bridge, which were conveniently manufactured as the 8501
(sweeper) and 8503 (s-parameter test set; a fancy name for a
dual-directional bridge). $1k or so sounds right though.

If you need more bandwidth there's the 8405 VNA system, which
depending on the pieces works from 110MHz to 18GHz. That also is a
multiple-box deal, but with a lot more possible combinations involved.
You can get set up for .11-2.4GHz VNA measurements with that lineup
for right around $1k too, maybe a bit more, but with the flexibility
of going further up if microwave is your thing.

 

[Brian Aase]

Scott, thanks again! I'll research all of these and see what I can
come up with at a reasonable (okay, cheapskate) cost.
Would anyone care to offer suggestions of good equipment
sources... other than the "T" company and the "E" website?
;-)

Brian Aase