Products > Test Equipment
MHS-5200A function generator teardown / review / reverse engineering
devttys0:
After seeing WD5GNR’s work on reversing the serial protocol for the cheap MHS-5200A function generators, I picked one up off of eBay.
I hadn’t seen much info in terms of how well these units meet their claimed specifications, so I started doing a short review/teardown video, and ended up reverse engineering the analog front end trying to track down the source of harmonic distortion that happens at higher output amplitudes and higher frequencies:
TL;DW: it looks like the main issue is that they aren’t providing a large enough supply voltage to the final output stage (an AD812 dual op-amp), and it’s causing clipping/distortion. This is backed up by the datasheet, although it is also clearly a frequency dependent issue.
Here's a schematic of channel 1's analog front end (channel 2 is identical) for anyone interested.
biot:
Great video, thanks a lot!
albert22:
Very good video. I liked the detailed analysis and the rev eng. schematics.
Thanks and regards
Lightages:
That was very well done.
gby:
Impressive work getting all the way down to a reverse engineered schematic and building those separate test circuits.
I suspect a lot of the distortion is explained by the slew and current limits of the AD812 amplifier used in the output buffer for the generator. To make a 15 MHz 7.5 V0-p signal you need an amplifier slew rate capability higher than 7.5*2*pi*15e6 = 710 V/uSec. Although the headline slew rate number for the AD812 is 1600 V/uSec that is at +/-15V supplies and optimum feedback resistance. At +/-5V the data sheet lists only 425 V/uSec slew rate for +2 gain. It would not surprise me if at +/- 7.5V and 1K feedback R if the typical slew rate was still less than the required 710 V/uSec.
Note also that the AD812 was design for video with net terminated load R of 150 Ohm. In this generator with 50 Ohm load you are down to only 100 Ohm which means the AD812 does not have enough output current capability before current limiting. See Figure 6. Output Voltage Swing vs. Load Resistance in the data sheet.
For another low cost (< $85) you could consider the FY3224S generator. This one is almost the same as the MHS-5200/MHS-3200. Key differences are 12 bit R, 2R DAC, +/-12V supply, and a better output buffer amplifier. The main drawback to the FY3224S is the current limit of only about 70 mA before clipping. So, 10+ MHz open circuit looks fine up to 20 Vp-p while with 50 Ohm load the current limit clips you to only about 12 Vp-p set point and 6Vp-p across the 50 Ohm load. Still much better than the MHS-5200 result.
See the FY3224S thread for lots more info. A specific MHS-5200 and FY3224S comparison table is given in that thread at:
https://www.eevblog.com/forum/testgear/feeltech-fy3224s-24mhz-2-channel-dds-aw-function-signal-generator/msg697718/#msg697718
gby
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