Your experience with function generators

There's a service manual if you think there is something wrong with it?

You don't say how bad of stability ? I wouldn't think it would be bad enough to be noticeable in a hour or two use. You could have bad caps in the power supply with intermitting leaks causing some voltage instabilities?

Jamie

Thanks Jamie.

I already have the service manual, but it is not of too much help (it is very difficult to locate components on the boards, moreover components bear a proprietary HP identification code which does not help).

Setting the generator at say 10 MHz and listening the produced carrier on an SSB communications receiver, the tone sounds VERY rough (this indicating a lot of phase noise).

However, more than phase noise, what worries me is frequency stability. Even though I have not done precise measurements, I would say that the 10-MHz carrier steadily shifts downwards in frequency at a rate of say a couple of kHz per minute. Moreover it is a bit "jumpy", i.e. frequency does not vary smoothly, though not to the extent of indicating some instability in components. Knocking on the boards produces no discernible effect.

Waiting for the generator to get thermally stabilized, the situation improves but not as much as I would have expected.

As that is the only function generator I ever possessed, I cannot be sure whether instability is inherent in the principle by which carrier frequency is produced (i.e. by charging a capacitor at constant current).

So, knowing the experience of other people would be of great help to me.

Regards

Tony

There's a service manual if you think there is something wrong with it?

You don't say how bad of stability ? I wouldn't think it would be bad enough to be noticeable in a hour or two use. You could have bad caps in the power supply with intermitting leaks causing some voltage instabilities?

Jamie

Thanks Jamie.

I already have the service manual, but it is not of too much help (it is very difficult to locate components on the boards, moreover components bear a proprietary HP identification code which does not help).

Setting the generator at say 10 MHz and listening the produced carrier on an SSB communications receiver, the tone sounds VERY rough (this indicating a lot of phase noise).

However, more than phase noise, what worries me is frequency stability. Even though I have not done precise measurements, I would say that the 10-MHz carrier steadily shifts downwards in frequency at a rate of say a couple of kHz per minute. Moreover it is a bit "jumpy", i.e. frequency does not vary smoothly, though not to the extent of indicating some instability in components. Knocking on the boards produces no discernible effect.

Waiting for the generator to get thermally stabilized, the situation improves but not as much as I would have expected.

As that is the only function generator I ever possessed, I cannot be sure whether instability is inherent in the principle by which carrier frequency is produced (i.e. by charging a capacitor at constant current).

So, knowing the experience of other people would be of great help to me.

Regards

Tony

There's a service manual if you think there is something wrong with it?

You don't say how bad of stability ? I wouldn't think it would be bad enough to be noticeable in a hour or two use. You could have bad caps in the power supply with intermitting leaks causing some voltage instabilities?

Jamie

Thanks Jamie.

I already have the service manual, but it is not of too much help (it is very difficult to locate components on the boards, moreover components bear a proprietary HP identification code which does not help).

Setting the generator at say 10 MHz and listening the produced carrier on an SSB communications receiver, the tone sounds VERY rough (this indicating a lot of phase noise).

However, more than phase noise, what worries me is frequency stability. Even though I have not done precise measurements, I would say that the 10-MHz carrier steadily shifts downwards in frequency at a rate of say a couple of kHz per minute. Moreover it is a bit "jumpy", i.e. frequency does not vary smoothly, though not to the extent of indicating some instability in components. Knocking on the boards produces no discernible effect.

Waiting for the generator to get thermally stabilized, the situation improves but not as much as I would have expected.

As that is the only function generator I ever possessed, I cannot be sure whether instability is inherent in the principle by which carrier frequency is produced (i.e. by charging a capacitor at constant current).

So, knowing the experience of other people would be of great help to me.

Regards

Tony

There's a service manual if you think there is something wrong with it?

You don't say how bad of stability ? I wouldn't think it would be bad enough to be noticeable in a hour or two use. You could have bad caps in the power supply with intermitting leaks causing some voltage instabilities?

Jamie

Well, I started to type something up and then went and looked at the owners manual for what you have online, and realized that most of what I was typing about was a moot point. Specifically, the FG I bought uses a quartz crystal timebase - it could have a better one, and it can take an external to allow use of as spiffy a timebase as you might like. Leaving it on for a few hours greatly improves stability as the crystal warms & stabilizes.

The FG you have, by contrast, uses a capacitor charged by a constant current as its time base, and both "presumably" and also "from a quick look at the manual" has a fair number of potentiometers (trimmer as well as front panel) in the circuit (presumably, these affect what the constant current is.) Those tend not to age well, IME. You can certainly try a can of contact cleaner on the pots. I don't know what they'd use for a timing capacitor, but I would assume it would be something (a type) more stable than an electrolytic that would be prone to drying out, so the pots is what I'd mostly suspect. They tend to corrode and get weird with old age, so the moving contact does not make reliable contact, and may be making some sort of contact but with a film of corrosion that affects the contact value.

Then again, a dried out electrolytic elsewhere in the circuit could be the culprit (the other standard old age issue) or something completely else could be wrong.

Since there does not seem to be a method to feed the thing a different timebase, the "simple in my case" diagnostic of putting a different timebase into it does not appear to apply.

I had used some of the "similar in concept" Beckman FGs in the past, which is one reason I went looking for a quartz-crystal based FG when buying my own (HP3325A, a few years past yours but still quite retro from these days.) I hated fiddling with the dial trying to get a precise frequency. Contrariwise, some folks hate the 3325A precisely because it does not have a dial....

Dial or no dial, I think stability and quartz crystal run together in these things. An ovenized crystal is better, and with an external reference input, you can get as precise as you can afford with GPS-clocking, rubidium references or the like (or just one really good ovenized crystal that you use for all your local reference inputs. At least your counter will agree with your generator in that case.)

--
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Please don't feed the trolls. Killfile and ignore them so they will go away.

I think you better do a check on the electrolytic in that unit. It sounds like you have bad power supply caps. Bad caps can cause a lot of instabilities because they clean the power rails, keep the DC smooth, which could the source of your noise you're hearing and the fact that it's drifting that bad tells me you may have leaky caps that are sitting on voltage references.. These caps are going through the forming cycle and changing the references.. They're most likely to old to reform and should be replaced.

I would get a cap meter and start checking the electro's and any tantalum units..

Also, get a can of spray coolant for components and let it warm up and then start spraying some resistors and even caps to find a sudden jump in activity.

Jamie

Well, I started to type something up and then went and looked at the owners manual for what you have online, and realized that most of what I was typing about was a moot point. Specifically, the FG I bought uses a quartz crystal timebase - it could have a better one, and it can take an external to allow use of as spiffy a timebase as you might like. Leaving it on for a few hours greatly improves stability as the crystal warms & stabilizes.

The FG you have, by contrast, uses a capacitor charged by a constant current as its time base, and both "presumably" and also "from a quick look at the manual" has a fair number of potentiometers (trimmer as well as front panel) in the circuit (presumably, these affect what the constant current is.) Those tend not to age well, IME. You can certainly try a can of contact cleaner on the pots. I don't know what they'd use for a timing capacitor, but I would assume it would be something (a type) more stable than an electrolytic that would be prone to drying out, so the pots is what I'd mostly suspect. They tend to corrode and get weird with old age, so the moving contact does not make reliable contact, and may be making some sort of contact but with a film of corrosion that affects the contact value.

Then again, a dried out electrolytic elsewhere in the circuit could be the culprit (the other standard old age issue) or something completely else could be wrong.

Since there does not seem to be a method to feed the thing a different timebase, the "simple in my case" diagnostic of putting a different timebase into it does not appear to apply.

I had used some of the "similar in concept" Beckman FGs in the past, which is one reason I went looking for a quartz-crystal based FG when buying my own (HP3325A, a few years past yours but still quite retro from these days.) I hated fiddling with the dial trying to get a precise frequency. Contrariwise, some folks hate the 3325A precisely because it does not have a dial....

Dial or no dial, I think stability and quartz crystal run together in these things. An ovenized crystal is better, and with an external reference input, you can get as precise as you can afford with GPS-clocking, rubidium references or the like (or just one really good ovenized crystal that you use for all your local reference inputs. At least your counter will agree with your generator in that case.)

------------------------------------ I tried to lubricate all contacts and potentiometer with no success. A capacitor? It could well be, but it is very difficult to locate the components on the board. Also, the schematic diagram drawing is of poor quality.

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 my 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

Looked at as change per 15 minutes (rather than total change in total time, which I believe is what you've reported above) this suggests that a 1 hour warmup makes the drift 10 times better, and a 2 hour warmup might be plenty for many purposes. If not enough for your purposes, shop for a newer unit.

15m 98 kHz (1.9%) 30m 18 kHz (0.36%) 45m 16 kHz (0.32%) 60m 9 kHz (0.18%) 75m 6 kHz (0.12%)

I doubt that yours can be expected to do better - my general understanding for capacitor-timebase performance is that 0.10% is about as good as you might expect, so I'd extrapolate from what you report that at 90-120 minutes of warmup time, it would be doing as well as it can, and you should certainly avoid doing anything that you wish to remain stable in the first 30-45 minutes it's on. Whether it was any faster to warm up when it was new is best left to anyone that happens by and used one when it was new, but I doubt it was greatly better.

Might be interesting to leave it on overnight, tune it in the morning, and see how well it sits at that point. That's assuming it's neither too noisy nor too much of a power drain to leave on all night.

If you desire more stability in less time, a newer unit is probably in order, and if you desire considerably more stability (parts per million is not hard to get) make that a quartz-crystal based unit.

--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

The manual is copyright 1974, and equipment of that era had to stay on 24/7 for any stability. Check the ripple of the two power rails, and for open zeners in the power supply. If you simply need something to be on frequency, check out a DDS signal generator.

is a 0Hz to 50 MHz signal source with simple pushbutton setup. It isn't as fancy as the HP3325, but it works well for