Products > Test Equipment
MHS-5200A function generator teardown / review / reverse engineering
SaabFAN:
I just did some reading on the subject: Apparently, only inverting configurations can be used as mixers to properly sum voltages, which, I guess, is necessary in this case to add/subtract an offset from the DAC-Voltage. It is then fed into the variable gain Amplifier, and then again inverted to give the correct output-polarity. Actually not a bad idea, but in this case not up to the task.
What (I think) they should have done: Mix the signals with the first inverting amp, invert the signal again (to have correct polarity), feed it into the variable gain amplifier and then drive an amplifier configured as a non-inverting amp.
Would have added two ICs (or one if one doesn't care about crosstalk too much^^) to the BOM, plus some passives, and required some more space on the PCB.
I guess, this is how they do it on the High-Power Option that is available for these units, which fits in the large empty space next to the PCB inside the case.
devttys0:
That's true, but I didn't see any summing amplifiers in the circuit. From what I've seen, they achieve the DC offset by adjusting Vref on the non-inverting input of the first AD812 op-amp; I don't see any reason why they wouldn't be able to do that in a non-inverting configuration, but perhaps someone can enlighten me.
SaabFAN:
Refitting a Battery and a boost-converter didn't help much, though it appears as if the square-wave at 1 and 2Mhz looks a little bit better...
Installing the Battery was more difficult than anticipated btw. The signal-gen needs about 6A for almost 11ms, before the current-consumption goes down to about 250mA. It can increase up to 800mA with both channels terminated into 50Ohms at 15V and 12Mhz. I therefore had to modify my original circuit quite a bit - First putting in large caps and after that didn't work, I added coils. Now the unit powers on with a 80% charged battery or if the external PSU is connected. I probably could improve that by adding a SuperCap, but I don't have one here and don't plan on buying one specifically for this unit.
I estimate the cost to modify the device in this manner to be about 15€, including shipping.
Attached: Screenshots of my measurements.
All Waveform-Measurements done with 15V Amplitude
Sine-Waves:
Quickprint22 - +15% Offset, 75Ohms
Quickprint23 - 0% Offset, 75Ohms
Quickprint24 - 0% Offset, 1MOhms
Square-Waves: All measurements with 0% Offset
Large Amplitude - 1MOhms
Small Amplitude - 75Ohms
SaabFAN:
And here are some pictures of the modified unit + Current-Draw when turning on (Measured with Scope-Probe over the Current-Shunt of my Multimeter, PSU: KA3005P)
Quickprint18: Current-Draw
Quickprint19: Voltage-Drop
The last 2 Pictures show the first, rather clean version of the Battery-Conversion and the quite messy one with all the modifications necessary to make it work^^
devttys0:
I managed to significantly improve the sine wave distortion and square wave overshoot/ringing with a couple of simple fixes:
The first was to replace the AD812 with a THS3092. They're pin compatible, but the THS3092 has a much higher slew rate, and I can now output a 15dBm sine wave at 25MHz and the harmonic content is actually within spec (highest harmonic is at -33dBc).
If you look at the attached HarmonicDistortion image, the output of the AD812 is shown in the purple trace, while the output of the THS3092 at the same frequency/power is shown in yellow. The yellow trace is a bit hard to see since it's underneath the purple trace, but you can just make it out; significant improvement!
I confirmed that the ringing/overshoot was caused by the elliptic low pass filter used on the output of the DAC. I replaced it with a Bessel filter designed for a 3dB cutoff at 30MHz, and there's practically no ringing on the waveform. The SquareWave1 and SquareWave2 images show the Bessel filter output on channel 1 (yellow) and the unmodified filter output on channel 2 (green).
The noise level on the output of the Bessel filter is a bit higher when viewed on the spectrum analyzer, especially near the carrier, but it's not too much worse and to be honest the noise performance of this function generator didn't seem to be all that great to begin with. I decided to stick with the Bessel filter for the cleaner square waves, but a Butterworth might split the difference in performance between the Bessel and elliptic filters.
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