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Your experience with function generators



 
 
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  #1  
Old March 18th 12, 09:07 PM posted to rec.radio.amateur.boatanchors
Antonio I0JX
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Posts: 25
Default Your experience with function generators

The frequency stability of my HP3312A function generator is poor, very
annoying for any practical use. I am trying to determine whether that
behavior is fairly normal for function generators, or my particular unit has
got a problem.

What is your experience on function generators frequency stability?

Thanks & 73

Tony I0JX
Rome, Italy

  #2  
Old March 19th 12, 12:15 AM posted to rec.radio.amateur.boatanchors
Scott Dorsey
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Posts: 703
Default Your experience with function generators

Antonio I0JX wrote:
The frequency stability of my HP3312A function generator is poor, very
annoying for any practical use. I am trying to determine whether that
behavior is fairly normal for function generators, or my particular unit has
got a problem.

What is your experience on function generators frequency stability?


Most of them aren't terribly long-term stable, because they aren't designed
for that. But with the HP, there will be a specification for drift in the
back of the manual.

If you give it a half-hour warm-up, how much does it drift in the next
half-hour?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #3  
Old March 19th 12, 03:18 PM
k4kqz k4kqz is offline
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First recorded activity by RadioBanter: May 2011
Posts: 8
Default

How do you define "poor" stability? Scott is exactly right- units as the 3312 were not necessarily intended for long term stability. His suggestion of letting the unit warm up then checking is very good advice. Most test equipment is speced based upon a warm up period of usually 30 minutes. I had a couple of 3312s over 25 years ago in my design lab and they were fine for lots of the work we were doing. If you have a unit that is truly drifting badly over a short period of time after warm up then I'd say it has a problem. However, I don't know if you'll find a drift spec for that particular unit. Best I recall we (yes, I worked for HP/Agilent) didn't routinely spec drift on the low cost units until DDS came about. But one thing is sure- we were pretty much always better than anyone else with respect to most any specification you picked! Output jitter was especially bad on some competing units.

John
  #4  
Old March 19th 12, 09:01 PM posted to rec.radio.amateur.boatanchors
Antonio I0JX
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Posts: 25
Default Your experience with function generators

"Scott Dorsey" ha scritto nel messaggio
...

Antonio I0JX wrote:
The frequency stability of my HP3312A function generator is poor, very
annoying for any practical use. I am trying to determine whether that
behavior is fairly normal for function generators, or my particular unit
has
got a problem.

What is your experience on function generators frequency stability?


Most of them aren't terribly long-term stable, because they aren't designed
for that. But with the HP, there will be a specification for drift in the
back of the manual.

If you give it a half-hour warm-up, how much does it drift in the next
half-hour?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Unfortunately the HP manual does not tell the frequency stability. No useful
information found searching the web.

Frequency measurement results:

- Just turned on. Frequency = 5.000 MHz Delta F = 0
- 15 minutes after. Delta F = -98 kHz
- 30 minutes after. Delta F = - 116 kHz
- 45 minutes after. Delta F = -132 kHz
- 60 minutes after. Delta F = -141 kHz
- 75 minutes after. Delta F = -147 kHz

Again the question is: is my generator faulty or all instruments based on
the same (analog) frequency generation principle (charging a capacitor at
constant current) behave more or less the same?

73

Tony I0JX

  #5  
Old March 19th 12, 09:04 PM posted to rec.radio.amateur.boatanchors
Antonio I0JX
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Posts: 25
Default Your experience with function generators



"k4kqz" ha scritto nel messaggio ...


How do you define "poor" stability? Scott is exactly right- units as the
3312 were not necessarily intended for long term stability. His
suggestion of letting the unit warm up then checking is very good
advice. Most test equipment is speced based upon a warm up period of
usually 30 minutes. I had a couple of 3312s over 25 years ago in my
design lab and they were fine for lots of the work we were doing. If you
have a unit that is truly drifting badly over a short period of time
after warm up then I'd say it has a problem. However, I don't know if
you'll find a drift spec for that particular unit. Best I recall we
(yes, I worked for HP/Agilent) didn't routinely spec drift on the low
cost units until DDS came about. But one thing is sure- we were pretty
much always better than anyone else with respect to most any
specification you picked! Output jitter was especially bad on some
competing units.

John

John, I here report the answer I gave Scott.

Unfortunately the HP manual does not tell the frequency stability. No useful
information found searching the web.

Frequency measurement results:

- Just turned on. Frequency = 5.000 MHz Delta F = 0
- 15 minutes after. Delta F = -98 kHz
- 30 minutes after. Delta F = - 116 kHz
- 45 minutes after. Delta F = -132 kHz
- 60 minutes after. Delta F = -141 kHz
- 75 minutes after. Delta F = -147 kHz

Again the question is: is my generator faulty or all instruments based on
the same (analog) frequency generation principle (charging a capacitor at
constant current) behave more or less the same?

73

Tony I0JX


  #6  
Old March 20th 12, 12:53 AM posted to rec.radio.amateur.boatanchors
Michael Black[_2_]
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Posts: 368
Default Your experience with function generators

On Sun, 18 Mar 2012, Antonio I0JX wrote:

The frequency stability of my HP3312A function generator is poor, very
annoying for any practical use. I am trying to determine whether that
behavior is fairly normal for function generators, or my particular unit has
got a problem.

A function generator, by definition, is a generator that can put out
multiple waveforms. They are also generally audio and low RF (ie hundreds
of KHz) generators. They are great as a signal source when you need a
triangle or square wave, not so great if you need a sinewave. And despite
them running at "high frequencies" that doesn't mean they are the best
choice for that.

I see it all the time (or used to) in sci.electronics. basics People
would ask about why their 2206 or whatever function generator IC wasn't
very good at the top of the range (whatever that was, 1 or 2MHz). The
answer was obvious, it was the top of the range, pushing the limits of the
device.

But they had no experience with RF, so they thought they could extend the
nominally audio device to low radio frequencies.

Switch to something that is made for RF, and you immediately get better
results, without any real effort. It's easy to build an LC oscillator at
1 or 2MHz, "way down there", but to stretch an RC oscillator to 1 or 2MHz,
"way up there" is pushing things.

Get an RF mixer, and some crystal oscillators, and use that to mix the
audio or low RF frequencies up to where you need them. Not perfect unless
you can get rid of the image, but the function oscillator stays where it
is stable, and you can use it as the variable element of the "oscillator",
complete with sweep function (if the function generator has a sweep
function) and multiple waveforms.

Michael VE2BVW


What is your experience on function generators frequency

stability?
Thanks & 73

Tony I0JX
Rome, Italy


  #7  
Old March 20th 12, 01:59 AM
k4kqz k4kqz is offline
Junior Member
 
First recorded activity by RadioBanter: May 2011
Posts: 8
Default

Tony,

I don't think there's anything wrong with your 3312. The numbers you've provided, however, don't actually mean a whole lot without specifying exact test conditions. Product specs are provided at a specific temperature(+/-), input power, etc. And, we've not even addressed the issue of the last calibration/service date of the unit. But, for the moment let's assume you are testing under the correct conditions and that the unit is in a calibrated state. After allowing for a 30 minute warm up (and it may actually need to be an hour) the frequency drifted less than one half percent over the next 30 minutes according to your numbers. Considering we're discussing a function generator that was designed in the early 70s using technology available at that time I'd say what you're experiencing is pretty damn good! Again, HP products were considered by the vast majority of those who knew what they were talking about :-) to be the best- the "standard". And the 3312 was an excellent, state-of-the-art, best-in-class product when it was introduced. But it's a function generator, not a high stability, low phase noise RF signal generator.

If you're looking for incredible frequency stability then you are using the wrong piece of equipment. Maybe you should consider a newer DDS unit, or probably more likely a true RF sig gen. If you're wanting a product with good spectral purity and lots of functionality then the 3312 is as good as you're going to find for products that are going on 40 years old as it is. And I bet it will even beat several products introduced way more recently.

John
  #8  
Old March 20th 12, 03:29 PM posted to rec.radio.amateur.boatanchors
Scott Dorsey
external usenet poster
 
Posts: 703
Default Your experience with function generators

Antonio I0JX wrote:

Unfortunately the HP manual does not tell the frequency stability. No useful
information found searching the web.


Let me see if I can dig up the military version documentation, which I know
does.

Frequency measurement results:

- Just turned on. Frequency = 5.000 MHz Delta F = 0
- 15 minutes after. Delta F = -98 kHz
- 30 minutes after. Delta F = - 116 kHz
- 45 minutes after. Delta F = -132 kHz
- 60 minutes after. Delta F = -141 kHz
- 75 minutes after. Delta F = -147 kHz


That doesn't sound too bad. Sounds like you need to let it warm up for
an hour if you care about stability.

Again the question is: is my generator faulty or all instruments based on
the same (analog) frequency generation principle (charging a capacitor at
constant current) behave more or less the same?


You can get a signal generator with a precision oscillator which will be a
lot more stable. But function generators are not designed to be highly
stable or have very low distortion. They're designed to produce a wide
variety of waveforms. You lose a lot with the ability to get that variety.

The HP is just great as a sweep generator for touching up IF strips,
but I wouldn't put a key on the output and put it on the air directly.
The distortion is really too high for that.

I think a drift of 24 KHz/hr after an hour's warmup is not terrible.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #9  
Old March 20th 12, 05:48 PM posted to rec.radio.amateur.boatanchors
external usenet poster
 
Posts: 12
Default Your experience with function generators

Antonio I0JX wrote:
The frequency stability of my HP3312A function generator is poor, very
annoying for any practical use. I am trying to determine whether that
behavior is fairly normal for function generators, or my particular unit has
got a problem.


My HP8116A has the same behaviour, to the point that frequency is specified
with only 4 digits.
Normal for my unit anyway.

73

Frank IZ8DWF
 




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