Hello all,
After getting an FG-7002C and finding the output quite awful looking, I developed the following "procedure" through trial and error. The results are good, the sine wave is clean now. I believe this would work fine for the FG-7005C. Also, if anyone has a 7005C (the 5 MHz version), I would like to see the insides and determine if I can upgrade my unit. I suspect it is merely a resistor change. These units may be branded "EZ Digital" or LG or possibly others. They look like the Tenma 72-410A bench DMM.
Initially, the sine output looked very "pointy". But this procedure will also address an overly "round" sine as well as a sine where the slope looks very wrong. You can also align the amplitude of the square wave output relative to the sine wave, should you wish for equal Vpp or VRMS. There are other adjustments that I didn't get to. The main controller chip can be wired to accept an external reference (10 MHz), which could be a nice thing to do if you like the frequency counter and want a little more accuracy. For the low audio signals, this thing reads dead-on in my case already. Remember, the frequency reference here is only for the readout. The
stability of the
generated frequency is entirely dependent on the VRs and Rs and Cs, and mostly on the single turn pot on the front of the unit, which really ought to be a 10 turn with a more limited range, but I digress...
Procedure:
Observe normal safety procedures. Not for unqualified personnel. At your own risk. etc etc.
Unscrew the four screws on the bottom of the unit.
Separate the top and bottom case shells apart at the rear of the unit. The front will be held together a little bit with plastic slots, but it will come apart. The rear plate is held in a slot and may slide up and down while you pull the top and bottom apart.
Allow 20 minutes to warm up.
Adjust in SINE mode. Press 1 KHz button, tune to 2.1 KHz. Attenuator off. All knobs pushed in, not pulled out. Set amplitude to ~1.6V RMS (no termination). Counter to internal measurement mode.
Ideally, allow 20 minutes to warm up after adjusting the frequency and amplitude, to remove annoying variations. Settings need not be re-adjusted if the drift is small.
Do not adjust at too high or two low output level. Too high you will be viewing the output amplifier's distortion which can cancel or add with the harmonics. Too low and the harmonics you wish to tune are in the noise. The output amplifier is not great, so do not load it down during these adjustments, as the function generator output amplifier's own issues will make this adjustment difficult. Set the amplitude to approximately 1.6 volts RMS, unterminated.
Connect an oscilloscope, DC coupled, 20 MHz bandwidth (even lower is fine really). High-Z mode. Set to view ~2 periods of the waveform initially.
DC offset: Adjust SFR13 for equal max and min above and below zero.
Vertical symmetry: Adjust SFR10 so that the bottom of the waveform looks like the top. Re-visit the DC offset if needed.
For these next adjustments, an FFT (frequency domain) plot is very helpful. Dial the FFT in so that you can see the 2nd and 3rd harmonic clearly. Without an FFT, you can still improve the sine wave purity if you are careful and look closely at the sine wave shape. It can help to pull up a reference sine wave (perhaps from another less-messy function generator) to compare the outputs. A good soundcard and an appropriate computer program might be good enough here.
You may need to adjust the time base of the scope to result in a good FFT image.
Adjust SFR11 for the lowest 3rd harmonic. Alternativly, look at the top of the sine wave and adjust for a proper sine shape -- not too pointy and not too broad.
Adjust SFR12 for the lowest 2nd harmonic. Alternativly, observe the aparent slope of the sine wave near the zero-crossing and adjust for proper sine wave shape.
Adjust SFR8 for the lowest 2nd harmonic. SFR8 is a trim on the SYM adjustment on the front panel, but it also sets the "not pulled out" position. Alternate between SFR8 and SFR12. Observe the slope from troff to crest of the sine wave shape.
SFR9 adjusts the square wave amplitude. You may want to adjust it to match the same peak to peak of the sine wave setting.
Good luck. Would like to see your FG-7005C photos if you have one open.