I'm in the market for a used spectrum analyzer like the 7L12
or the HP 141-T. One important use will be to make two-tone
IMD measurements on HF SSB transmitters. I am concerned that
with tone separations on the order of one kHz, the 7L12 may
not have sufficient bandwidth in the 300 Hz mode to resolve
adjacent IMD products expected to differ in amplitude by 40
dB or more.

Has anyone on the group used a 7L12 for this purpose and is
the 300 Hz RBW sufficiently narrow?

There seems little doubt that the 141, with 10 Hz or 100 Hz
RBW, will handle this.

Many thanks in advance.

Chuck
NT3G

On Wed, 18 May 2005 19:04:44 -0400, chuck wrote:

I'm in the market for a used spectrum analyzer like the 7L12
or the HP 141-T. One important use will be to make two-tone
IMD measurements on HF SSB transmitters. I am concerned that
with tone separations on the order of one kHz, the 7L12 may
not have sufficient bandwidth in the 300 Hz mode to resolve
adjacent IMD products expected to differ in amplitude by 40
dB or more.

Has anyone on the group used a 7L12 for this purpose and is
the 300 Hz RBW sufficiently narrow?

There seems little doubt that the 141, with 10 Hz or 100 Hz
RBW, will handle this.

Many thanks in advance.

Chuck
NT3G

I have never played with a 7L12 but do have a 141t. 300hz bandwidth
will work. The best resolution on the 141t is 100hz. There is a 10 hz
video filter but the video filter does no good for resolution. It only
gets rid of noise. The biggest problem that I can imagine with the
7L12 would be whether or not your scope has some kind of storage or
long persistency on the tube. To look at 300 hz bandwidth you need a
very slow sweep speed on the scan. If there is no storage you will not
be able to see it.

73
Gary K4FMX

Thanks for responding, Gary.

Sweep time is something that I had not even been
considering, so I appreciate the info.

From what I can gather about the 7L12, the slowest sweep is
10 mSec/division, or 0.1 second for a complete sweep. That
is the sweep setting Tek says to use for all frequency
domain analyses, regardless of bandwidth. The other sweep
settings go from 10 mSec/division to something like 1
uSec/division, but they are intended for time domain analyses.

As a reality check on a 0.1 second sweep, I looked at HP's
performance test procedure for the 8552B to see what sweeps
they specify for measuring the 8552B's bandwidth. For the
300 Hz RBW, they list a 0.2 second sweep (I assume that's
not 0.2 second/division). Not really too far from Tek's 0.1
second. Of course, at the narrower bandwidths, HP's
specified sweeps get much slower, as you pointed out. And if
it is 0.2 second/division, then there is a profound
difference between the Tek and HP filter designs or I'm
missing something big. As I recall, the slower sweeps are to
avoid ringing in the filters, gaussian skirts notwithstanding.

So maybe the 7L12 doesn't require a storage scope for the
300 Hz RBW? Sure wish I had access to the Tek Op manual for
the 7L12.

73,

Chuck






Gary Schafer wrote:
On Wed, 18 May 2005 19:04:44 -0400, chuck wrote:


I'm in the market for a used spectrum analyzer like the 7L12
or the HP 141-T. One important use will be to make two-tone
IMD measurements on HF SSB transmitters. I am concerned that
with tone separations on the order of one kHz, the 7L12 may
not have sufficient bandwidth in the 300 Hz mode to resolve
adjacent IMD products expected to differ in amplitude by 40
dB or more.

Has anyone on the group used a 7L12 for this purpose and is
the 300 Hz RBW sufficiently narrow?

There seems little doubt that the 141, with 10 Hz or 100 Hz
RBW, will handle this.

Many thanks in advance.

Chuck
NT3G


I have never played with a 7L12 but do have a 141t. 300hz bandwidth
will work. The best resolution on the 141t is 100hz. There is a 10 hz
video filter but the video filter does no good for resolution. It only
gets rid of noise. The biggest problem that I can imagine with the
7L12 would be whether or not your scope has some kind of storage or
long persistency on the tube. To look at 300 hz bandwidth you need a
very slow sweep speed on the scan. If there is no storage you will not
be able to see it.

73
Gary K4FMX

chuck wrote:
Thanks for responding, Gary.

Sweep time is something that I had not even been considering, so I
appreciate the info.

From what I can gather about the 7L12, the slowest sweep is 10
mSec/division, or 0.1 second for a complete sweep. That is the sweep
setting Tek says to use for all frequency domain analyses, regardless of
bandwidth. The other sweep settings go from 10 mSec/division to
something like 1 uSec/division, but they are intended for time domain
analyses.

As a reality check on a 0.1 second sweep, I looked at HP's performance
test procedure for the 8552B to see what sweeps they specify for
measuring the 8552B's bandwidth. For the 300 Hz RBW, they list a 0.2
second sweep (I assume that's not 0.2 second/division). Not really too
far from Tek's 0.1 second. Of course, at the narrower bandwidths, HP's
specified sweeps get much slower, as you pointed out. And if it is 0.2
second/division, then there is a profound difference between the Tek and
HP filter designs or I'm missing something big. As I recall, the slower
sweeps are to avoid ringing in the filters, gaussian skirts
notwithstanding.

So maybe the 7L12 doesn't require a storage scope for the 300 Hz RBW?
Sure wish I had access to the Tek Op manual for the 7L12.

73,

Chuck

Hi Chuck,

I have used both the 141T system, and the 7L13. I ditched my 141T because
the 7L5, 7L13, and 7L18 plugins perform better than the equivalent HP plugins
for the 141T system, and are much more compact.

But,

When I was looking into 7L analyzers, I discounted the 7L12. It is too
primative. The minimum you want to do any real work is a 7L13 with a
7633 storage frame. The 7L14 is much better because it has the digital
storage, but it is also twice the price of a good 7L13.

You *will* need a storage scope frame for either the 12, or the 13. The
narrow bandwidth sweeps must be done really slowly. If you try and rush them,
you will lose most of the amplitude information... the filters just cannot
respond quickly. It is a physical reality of narrow band filters.

-Chuck Harris

From one Chuck to another,

Thanks for the info and the advice! I will definitely look
further into the '13 and '14.

I do understand the need to sweep "slowly" at narrow RBWs.
But I'm still troubled by the fact that the *slowest* sweep
built into the 7L12 is 10 msec/division! That will,
arguendo, degrade the filter response. The storage scope
will surely not sweep the SA at a slower rate, and putting a
distorted SA output signal into a storage scope can't
possibly reshape the response! So there is no cure. If our
assumptions are correct, this is a fatal Tek design flaw
(not a whole lot of them around).

A storage scope would be really important if the SA is
sweeping too slowly for the regular scope's persistence, or
to capture a single-sweep trace. Or for simply storing a
trace for later viewing. But if the sweep rate is 10
sweeps/second, there shouldn't be much flicker with P31.

Something is amiss here, I think. Maybe there is a typo in
Tek's spec sheet? Or more likely, a parity bit error in my cpu!

Chuck

chuck wrote:
From one Chuck to another,

Thanks for the info and the advice! I will definitely look further into
the '13 and '14.

I do understand the need to sweep "slowly" at narrow RBWs. But I'm still
troubled by the fact that the *slowest* sweep built into the 7L12 is 10
msec/division! That will, arguendo, degrade the filter response. The
storage scope will surely not sweep the SA at a slower rate, and putting
a distorted SA output signal into a storage scope can't possibly reshape
the response! So there is no cure. If our assumptions are correct, this
is a fatal Tek design flaw (not a whole lot of them around).

A storage scope would be really important if the SA is sweeping too
slowly for the regular scope's persistence, or to capture a single-sweep
trace. Or for simply storing a trace for later viewing. But if the sweep
rate is 10 sweeps/second, there shouldn't be much flicker with P31.

Something is amiss here, I think. Maybe there is a typo in Tek's spec
sheet? Or more likely, a parity bit error in my cpu!

Chuck

No, your data sheet is wrong. The slowest automatic sweep is 10 secs per
division. The slowest sweep is manual. This specification exists across
the entire 7L line.

The 7L5, 7L13, 7L14, and 7L18 all have monitors built in that will show
"uncal" if you sweep too fast for the filter setting.

-Chuck

On Thu, 19 May 2005 20:15:16 -0400, chuck wrote:

From one Chuck to another,

Thanks for the info and the advice! I will definitely look
further into the '13 and '14.

I do understand the need to sweep "slowly" at narrow RBWs.
But I'm still troubled by the fact that the *slowest* sweep
built into the 7L12 is 10 msec/division! That will,
arguendo, degrade the filter response. The storage scope
will surely not sweep the SA at a slower rate, and putting a
distorted SA output signal into a storage scope can't
possibly reshape the response! So there is no cure. If our
assumptions are correct, this is a fatal Tek design flaw
(not a whole lot of them around).

A storage scope would be really important if the SA is
sweeping too slowly for the regular scope's persistence, or
to capture a single-sweep trace. Or for simply storing a
trace for later viewing. But if the sweep rate is 10
sweeps/second, there shouldn't be much flicker with P31.

Something is amiss here, I think. Maybe there is a typo in
Tek's spec sheet? Or more likely, a parity bit error in my cpu!

Chuck








I just looked at the 141t and it takes 2ms/div for no flicker. This is
at wide bandwidth.

At 2khz/div spectrum width and 300 hz bandwidth it takes 50ms/div
sweep speed max. At 5 khz spectrum width it takes .1 sec/div sweep
speed.

At 100hz bandwidth and 2khz/div spectrum width it takes .5sec/div
sweep speed.
Definitely storage scope area!

The 50ms/div sweep can be viewed without storage on a p31 but it is
not good.

73
Gary K4FMX

Have a look at the 7L14, it goes to 30Hz RBW.

chuck writes:

I'm in the market for a used spectrum analyzer like the 7L12
or the HP 141-T. One important use will be to make two-tone
IMD measurements on HF SSB transmitters. I am concerned that
with tone separations on the order of one kHz, the 7L12 may
not have sufficient bandwidth in the 300 Hz mode to resolve
adjacent IMD products expected to differ in amplitude by 40
dB or more.

Has anyone on the group used a 7L12 for this purpose and is
the 300 Hz RBW sufficiently narrow?

There seems little doubt that the 141, with 10 Hz or 100 Hz
RBW, will handle this.

Many thanks in advance.

Chuck
NT3G
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA