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perfknee 18th January 2011 05:35 PM

Instek GFG-8255a sweep/rate vs sweep/time
I just bought an Instek GFG-8255a analog function generator used from ebay and I'm testing it out.

I'm having trouble figuring out how to control the sweep upper and lower limits and rate. The obviously translated manual isn't of help.

After playing with the controls for 15 minutes I still can't figure out how to set the lower bound sweep frequency, and I'm especially confused over:

What is the difference between the sweep/time and sweep/rate dials?

Thanks for any help.

perfknee 19th January 2011 12:14 AM

Okay, I figured out how the sweeps work on the GFG-8255a

The Frequency dial controls the upper frequency of the sweep.
The "sweep/time" dial controls the period between sweeps.
The "sweep/rate" dial controls how fast the frequency changes during the sweep, which also has the side effect of controlling the lower limit of the sweep; at slower rates the frequency is so slow that it can't get far from the upper freq set on the Frequency dial, but as you increase the rate the starting (lower) frequency gets lower and lower until it hits the lower limit of the selected frequency range.

The issue I'm running into is that there doesn't seem to be a way of generating a sweep covering the whole audio band (20-20khz). The best I can do is select the 1k range button, which gives me 20-5kHz.

This is annoying because my reason for purchasing this model was for the sweep function, hoping to be able to sweep the whole audio band in one go, for determining frequency response of devices.

perfknee 19th January 2011 01:58 AM

Move to GFG-2104?
Now that I realize this GFG-8255a won't meet my needs, I'm thinking of getting rid of it on ebay, and buying another function generator with a sweep that really works. I'm also not completely satisfied with the analog "jumpiness" of this thing, where in the lower frequencies the rightmost 1-2 digits are always jumping around. It's not as bad as the unit that was reviewed in the eevblog episode, but it does annoy me and I find adjusting the frequency pot dial quite touchy, requiring a few tries each time I want to go to a particular frequency as the frequency shift slightly while I take my hand off the freq pot dial.

So I'm considering the DDS version of this, the instek GFG-2104, which costs $324. I just read the manual (much better than the manual for the 8255a) and it looks like because there are no frequency ranges it should serve my need of generating 20-20khz sweeps. The manual doesn't give quite enough info for me to be sure though, so if anyone can verify, or suggest other audio sweep capable generators in this price range please let me know.

The only feature I with the 2104 had was more memory slots (it has ten) so that I could program all 30 or so frequencies that cover 1/3 octaves of the audio range, and then move directly between them without having to fine-adjust the freq dial each time.

One question I have for those who own the GFG-2100 series is how precise and/or touchy is the frequency encoder... is it relatively easy to dial in a particular frequency in one go, without scrolling through and adjusting each digit of the frequency?

Conrad Hoffman 19th January 2011 02:36 AM

IMO, Instek is pretty decent for the money, but that "for the money" is an important qualifier. What you describe just isn't a great design; it's been known for decades how to design oscillators with 20-20k sweep capability and it isn't like it costs any more to do it well, than to do it poorly. I'd be saving my pennies for a new Agilent generator, probably an arb, some of which are surprisingly affordable, or I'd be looking for a good deal on a used Wavetek 185. It was the two-dial model where you can set top and bottom independently. They made many others that were a bit weirder to set up, but also less expensive. They tend to have stupid issues with connections and contacts as they get older, but once fixed up they serve well.


macboy 19th January 2011 06:15 PM

The GFG2104 has a numeric keypad so you should be able to punch in the desired frequency just as fast as you could recall a saved setting. You should have those 1/3 octave frequencies memorized by now: 20 25 32 40 50 63 80 100 125 160 200 250 ...

The GFG2104 uses DDS which is also how those nice Aligent ones work. Of course the quality of their implementation is another matter.

perfknee 20th January 2011 03:06 PM

Since I'm mostly going to be using the function generator for audio, the $335 NTI MR2

looks like it might fit my needs nicely. It has a nice LCD display and audio-oriented UI, with features like being able to adjust frequencies in 1/3 octave increments, infinitely adjustable sweep limits, and has a cool stepped sweep function that the DDS units don't have. The portable form factor makes it more flexible, although the battery power means output power is limited to +8dbu (2V RMS if I got my calculations right) , so I should probably keep a bench generator for doing things like overload/clipping testing.

They spec the THD+N at:
22 Hz - 22 kHz, average, @ 1 kHz, typical-90 dB (0.0032%) @ 8 dBu,Noise floor typ. 25 μV

which sounds like it puts it into the ballpark of the DDS generators, although it's hard to tell since DDS gens list their THD differently. E.g. the GFG-2104 lists:

Harmonics Distortion -55dBc,0.1Hz~200kHz; -40dBc,0.2MHz~4MHz; -30dBc,4MHz~10MHz

The NTI MR2 listed flatness is 0.5 dB, presumably over the audio range, which is a bit on the loose side for my taste, but probably adequate.

The GFG-2104
by comparison is more flat:

Flatness(Relative to 1kHz) < 0.3dB,0.1Hz~1MHz; < 0.5dB,1MHz~4MHz; < 2dB, 4MHz~10MHz

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