Modifying an old BKP 3010 Function Generator Output Impedance

[CodyShaw]

Hey all!

So I picked up an old BK Precision 3010 function generator that my university sold off from their aging labs.

I have it cracked open at home, and it is actually quite interesting to look about inside. No digital in that sucker.

The problem is, it has an odd output impedance of 600 ohms. I feel like it would be more useful to give it a high impedance output instead, or maybe rig up a hack to have it switchable from 50 ohm to high Z.

I feel like I should just be able to simply put in an op-amp in a buffer configuration onto the output (internally there is a crusty wire going from the output BANANA JACK (no BNC here!) to the PCB that I can hack into). I believe a modern op-amp should do the trick, the generator can only make waveforms up to 1 MHz and 20 VPP, so slew rate shouldn't be a big issue. Overall precision is also not a big issue, the damn thing probably hasn't been calibrated in its whole life.

For 50 ohm output, I'm thinking that I may just be able to add a single pole switch to the output of the Op-amp with a 50 ohm resistor down to ground.

What do you guys think? Standard procedure? I was also thinking of doing this task with a BJT, but decided to go with a simple Op-amp configuration.

[w2aew]

A high output impedance?  Really?  I think you really meant to say that you'd rather have a LOW output impedance, or a low impedance switchable to 50 ohms. 

Yes, I simple op amp buffer should be suitable to do the job.  Look for one that can drive a 50ohm grounded load.  Personally, I'd simply put a 50 ohm resistor in series with a op amp that can drive it, and leave it that way.  This will give you short circuit protection, and properly drive 50ohm coax and loads, and have no problem driving high impedance loads.

[CodyShaw]

A high output impedance?  Really?  I think you really meant to say that you'd rather have a LOW output impedance, or a low impedance switchable to 50 ohms. 

Yes, I simple op amp buffer should be suitable to do the job.  Look for one that can drive a 50ohm grounded load.  Personally, I'd simply put a 50 ohm resistor in series with a op amp that can drive it, and leave it that way.  This will give you short circuit protection, and properly drive 50ohm coax and loads, and have no problem driving high impedance loads.

D'oh! Of course that's what I meant. It's early and I haven't had my tea yet

Edit: Did some searching, something like this seems like it would be suitable:

http://www.linear.com/product/LT1010

It can drive +- 10 V into 75 ohms, so I assume there would be a little bit of loss jumping down to 50 ohms.

Any recommendations that may be easier/cheaper to nab?

This looks good too, but also "expensive":

http://www.ti.com/lit/ds/symlink/buf634.pdf

[fcb]

600 ohms is not 'odd' - it's a very common o/p Z for audio work.

Yes, you can buffer it, but why bother. It's unlikely that 50ohms is going to be *much more* useful - depends on what you plan on driving with it.

20Vpp into 50R is 1W (short circuit conditions), so you might struggle with a DIP8 packaged opamp - that said I use the BUF634 in a similar application (audio analyser).

 

[CodyShaw]

600 ohms is not 'odd' - it's a very common o/p Z for audio work.

Yes, you can buffer it, but why bother. It's unlikely that 50ohms is going to be *much more* useful - depends on what you plan on driving with it.

20Vpp into 50R is 1W (short circuit conditions), so you might struggle with a DIP8 packaged opamp - that said I use the BUF634 in a similar application (audio analyser).

I'm doing this mostly as a random project, just something to do. I don't plan on working with much audio equipment, but this would allow me to have more options if I ever were to need a specific output impedance. This is a pretty junky old waveform generator, mind you, so it's not going to be used for anything super serious.

I've never seen a generator with a 600 ohm output before, interesting it has a specific use (obviously it does if it is engineered that way). I'll have to do some reading on it.

I seem to have found your recommendation buffer at the same time you suggested it 

[fcb]

Unless you care hugely about 1:1 gain matching, then i'd run the BUF634 openloop.  Check your supply rails though - these parts aren't that forgiving w.r.t over-voltage.

[edavid]

This is actually a difficult project to do right.  You would like your amp circuit to have/be:

- DC accuracy
- low distortion to 1MHz
- 20V PP output
- short circuit proof
- unconditionally stable

Unfortunately, you can't do that with an op amp, even a fancy one.

If you had a specific application, you could relax some of those requirements, but since you don't know what you're going to do with it, it's hard to say where you can cut corners.

Attached is the schematic for the HP 3312A output amp... you can see it's a class B amp, like a low power audio amp, with an op amp servo for DC accuracy.  The output devices are TO39 transistors with clip on heat sinks, which is a minimum for 50 ohm output.  It's much more robust than an op amp, but it's still possible to blow transistors if you try.

[CodyShaw]

This is actually a difficult project to do right.  You would like your amp circuit to have/be:

- DC accuracy
- low distortion to 1MHz
- 20V PP output
- short circuit proof
- unconditionally stable

Unfortunately, you can't do that with an op amp, even a fancy one.

If you had a specific application, you could relax some of those requirements, but since you don't know what you're going to do with it, it's hard to say where you can cut corners.

Attached is the schematic for the HP 3312A output amp... you can see it's a class B amp, like a low power audio amp, with an op amp servo for DC accuracy.  The output devices are TO39 transistors with clip on heat sinks, which is a minimum for 50 ohm output.  It's much more robust than an op amp, but it's still possible to blow transistors if you try.

Great post, thanks for the help. I really enjoy analyzing and looking and analog electronics such as this.

[Odysseus]

Yes, I simple op amp buffer should be suitable to do the job.  Look for one that can drive a 50ohm grounded load.  Personally, I'd simply put a 50 ohm resistor in series with a op amp that can drive it, and leave it that way.  This will give you short circuit protection, and properly drive 50ohm coax and loads, and have no problem driving high impedance loads.

This will work.

Alternatively, use a power buffer chip likethe BUF634 (or build your own discrete buffer, which can be as simple as 6 transistors and two resistors) in combination with an opamp of your  choice.  Include the 50 ohm resistor outside the feedback loop and you will have met all the requirements posted by edavid, except for the output swing, since most opamps only go up to a ±16V supply.    The MC33072 will get you up to a ±22V VDD.

Something like the attached screenshot is what I'm imagining.

[vk6zgo]

Yes,600 Ohms is a very common output impedance,as it was,& is,to some extent,still,the standard impedance for interfacing Broadcasting Audio equipment.

In a lot of cases,you won't see much difference ---for instance,if you are driving 50k Ohms from 1 volt in 50 Ohms  or  1 volt in 600 Ohms,the load will still see,near as dammit,the same level.

If you really need 50 Ohms output in the matched case,& don't mind the inherent attenuation,you can always use an impedance matching pad.

[fcb]

Alternatively, use a power buffer chip likethe BUF634 (or build your own discrete buffer, which can be as simple as 6 transistors and two resistors) in combination with an opamp of your  choice.  Include the 50 ohm resistor outside the feedback loop and you will have met all the requirements posted by edavid, except for the output swing, since most opamps only go up to a ±16V supply.    The MC33072 will get you up to a ±22V VDD

Give the BUF634 a nice +/-15v supply and it will swing 10Vpp all day long into 50R. The 33072 is a nice part, but does not have the drive for a 50R load at these swings - it will also slew-rate limit way before 1MHz.

Also, I imagine the 20Vpp/50R is being specified into an open circuit - if not, the OP will need 40Vpp...

I wouldn't bother with any of this, 600R is quite livable with.