A signal/function generator/scope question...

[matta]

Hey Guys,

Going to be honest here and say although I have owned a scope for a couple years now I have very rarely put it to use, sad, but true...

I understand how to operate its basic functions and can take basic measurements. It is an oldy, but a goody, a Tektronix 422, Dual Trace. I had it calibrated some time back by the local Tek agents and they found it to be in great condition and tuned up where needed.

I don't however own a signal generator, partly my lack of using it, so I bought an app for my iPhone called Sig Gen, with the volume set to full the amplitude should measure the same, so 2Vpp in the app should ready 2Vpp on the scope... it doesn't, I had to attenuate to 1.74Vpp to have it read 2Vpp on the scope, wrote to the software house and looking into it..

EDIT: Got this response from the developers:

The amplitude in the app is really a "best guess", but you're using it correctly.  It may vary from device to device.  Since this is an audio port a 15% error is not meaningful, so I doubt the amplifier circuit is spec'd very tightly.  It may also not be especially linear.  Also the load can influence the amplitude, obviously.

It however get me thinking... MOST hardware signal generators are unbalanced right? Most have BNC outs, so a shield and a signal wire? How does this effect us in the studio 'balanced' world. If at all?

For example you will see in most service notes something along the lines of "Apply a 1kHz tone at 1Vpp to the input"... what happens if that input it balanced, for example, a mic pre? Do you then apply the signal on Pin2 and tie Pins 1+ 3 together? How does it differ from unbalanced devices?

On the topic of signal/function generators, are there any good/decent/inexpensive yet fairly accurate hardware units around that anyone can recommend, even if they are older ones, there is little to no used market here but I'll be going to the UK next year and may be able to pick up something there. Is it worth getting one? Are they any better than software ones?

Thanks in advance

Matt

 

[rodabod]

Good audio signal generators made by people like Lindos or Neutrik, etc. will have balanced outs that will compensate when either hot or cold is shorted to ground by pulling up +6dB like when you reference one side of a transformer to ground.

Certainly believe your 'scope before your IPhone!

 

[Mark Burnley]

Yep,

I doubt the i-phone output is very durable to short circuits and accidental voltages applied to it!

My main bench generator is an Advance/Gould J3B which is great as it has an output attenuator switched at -20dB and -40dB (so ideal for making lower line and mic levels) but also balanced floating outputs (via a substantial output transformer)

I've also got a few unbalanced units, which I drive balanced using a hefty ex-BBC 600R o/p transformer mounted in an aly box with 4mm binding posts in/M XLR out. LF is good to about 12Hz into 600R load.

My portable friend is the NTI Minirator, which is just too handy to not have- probably paid for itself many times over and is very durable for its price/size. The new Minirator also has the facility to upload .WAV files of your choice, and comes with a selection of entertaining samples! Also has usual Sine/Sweep/Square/White/Pink.

Lots of DIY wien bridge circuits on the net- either small bulb or glass thermistor in FB loop to help stability. Buffer this with pot or switched attenuator, and output driver IC to transformer, and you've got a nice DIY generator.

There's always eaby! Farnell did some good ones- LFM2 IIRC?

Hope this helps,

Mark

 

[Mark Burnley]

There's a Farnell LFM4 up at the moment.

Has the word "nice" in the description which is always, er, nice...

Search for "signal generator" or "farnell signal generator" to come up with the goods.

I also collect all-valve generators, but that's just for my Silver Apples projects!

Let me know if you need a mailing address in UK- I can meet up in New Year for our long-awaited pint!

Mark

<edit>

Here's a pic of a J3B-

Gould J3B

 

[matta]

Hey Mark,

Sorry man, missed this one! Thanks for all the info... still searching, will def. keep you in mind if I need a shipping address, I have a bunch of family in and around London as well so I'll try NOT add to your already burdening workload, though I'd love to get that pint when I'm over in May and meet face to face!

I found a generator locally, but it is an AM/FM one and was used to service TV's, me thinks it is not what I'm looking for  8)

Cheers

Matt

 

[substitute]

I've been thinking about signal generators also.  Part of me doesn't see the point, Logic comes with a test tone plug-in and I can meter what's going out and what's coming in.  Even if it's not NASA quality calibration, it's probably pretty accurate, plus it's all sort of within the system that the gear is going to be used in anyways.  But then I think real techs use real tools, and maybe it has more to do with some kind of state of mind than a bottom line kind of functionality.  It would be nice to not have fire up my computer and mixer and all in order to test/calibrate something.