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Old 29th July 2005, 01:01 AM   #1
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Question HP 208A Oscillator, any use??

Hi,

I've discovered the limitations of my sound card for testing my amp... good for using with rmaa, and simple sine waves up to about 8Khz but absolutely hopeless for square waves, and anything above about 8Khz....

I can get a HP 208A test oscillator for $95 Aus, but I can't find any info on it, all I have is "Generates from 5Hz to 500kHz into 600ohm" I don't know if it can do square waves, and I'm a bit concerned about the "into 600 ohms"..... anyone use one of these?

search on HP 208A turned up zip!

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Old 29th July 2005, 03:47 AM   #2
clem_o is offline clem_o  Philippines
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Yep, zilch, unless you are willing to buy a manual!

If it says oscillator, it probably is just that. You can tie a high-speed comparator circuit to its output to get square waves, though to get exactly 50% duty cycle may require some tweaking of the reference voltages.

600ohms should be a very useable impedance to work with, why is it a worry?

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Old 29th July 2005, 04:16 AM   #3
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Thanks Clem_o,

wasn't sure about the 600 ohms was thinking about the input impedance of preamps, being in the 10's of K ohms, I guess that 600 ohms is the minimum impeadance it will drive, higher is ok

I sent an email to the seller to find out if it has any square wave output

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Old 29th July 2005, 04:54 AM   #4
Enzo is offline Enzo  United States
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I don't know the 208 off the top of my head, but generally if it says oscillator it is just for sine waves.

The ones that produce sine and square and maybe even triangle are more often called signal generators or audio generators. Also you would probably be happy with a function generator. Those will not only make the sine, square, and triangle forms, but usually will also do sweeps, pulses, and DC offset, and even duty cycle on the square waves.
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Old 29th July 2005, 05:52 AM   #5
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Thanks Enzo I'll keep a lookout for a function generator!

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Old 29th July 2005, 09:57 AM   #6
jaycee is offline jaycee  United Kingdom
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or build your own The MAX038 chip is a good starting point for this.
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Old 29th July 2005, 12:20 PM   #7
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thanks jaycee, interesting device I'll see what I can dig up! little pricy at $52 Aus, but when you look at the price of a new function generator it's pretty cheap, especially if as it says, you need minimal external components.

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Old 29th July 2005, 04:57 PM   #8
jaycee is offline jaycee  United Kingdom
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*cough* samples *cough*
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Old 30th July 2005, 02:36 AM   #9
clem_o is offline clem_o  Philippines
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Yep, MAXIM may be generous... :-)

If I recall, there's several function generator chips on the market, XR2206 comes to mind. Probably cheaper but the sine-wave distortion is just horrible, since the chip actually generates triangle waves; a bunch of diode-resistor pairs are used to mash the wave into something sinusoidal!! In general, function gens don't seem to have particularly good specs as far as sine-wave distortion is concerned. May be a reason why HP has a specialized "sine wave only" product...

Oh one other thing comes to mind - Linear Technology did put out an article or two on the design of a really low distortion sine-wave generator, written by Jim Williams. I think the articles are available on their website...
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Old 30th July 2005, 03:11 AM   #10
PRR is offline PRR  United States
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> I can get a HP 208A test oscillator for $95 Aus, but I can't find any info on it, all I have is "Generates from 5Hz to 500kHz into 600ohm" I don't know if it can do square waves, and I'm a bit concerned about the "into 600 ohms"..... anyone use one of these?

I never met that guy, but he is surely son to the 200AB, the box that Mr H and Mr P built in their garage after studying under Prof Terman. Started a dynasty. A mainstay of the audio industry for 20 years.

Tuned 200ABs will make MUCH cleaner sinewaves than any function generator. The 8038 FG chip says as good as 1% but it is a very ugly kink-wave. A 200AB will always be under 0.5% and the flaws are smooth, invisible on the scope. Tweaked 200s can get below 0.05%. However they only go 20CPS-200KC (20CPS-40KC on the high-power model).

A 200AB will cleanly drive any load from ~200 ohms to infinity. The output impedance is 60 to 100 ohms (rises at the high frequencies) so if you load it hard the output varies a bit: for precision response measurement you put a voltmeter on the 200 and keep it flat. (WHen it was invented, Gain Sets were still common.)

A 200AB does not do square waves: why would it? OK, square testing did get common later. But we usually used dedicated square sources: putting a Schimdt inside a low-distortion sine generator is begging for trouble. My Heath had a square-wave but I sometimes had to un-power that section to get really clean sine waves. FWIW: the Heath also made rated output in 600 ohms, but would drive from a few hundred to infinity ohms just fine.

The 208 clearly has a wider frequency range. I bet the output is ample for any pro line level work (and more than you need for testing sound cards and hi-fi amps). It would not be markedly "less" than the 200AB.

A hundred bucks in Good Working Order is probably fair: I can find 200ABs from US and German test-gear recalibration shops, $49 working but not calibrated and a not-DOA warranty. If you were in the right place when your local engineering school retired their analog-lab gear, you could get working 200ABs for $5 and "dead" (blown pilot light) for $2 or free.

The 200AB is vacuum-tube: the main fault is gummy grease in the tuning shaft (I had to hammer one apart to oil it) but after that they are heir to all the firebottle failings and fixes. I bet that 208 is transistor: my vintage H-P tranny gear needed a good switch-contact cleaning but otherwise everything is perfect. From what I have seen of H-Ps transistor designs, if it does get sick it could be a real headache to figure out.
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