Few days ago, I received my UTG962E ...
Here are the results of my first investigations
1) could confirm the video-loop from IAmBack (Replay #236 June 25, 12:50 pm)
- second reference was my old MHS5200A (26bit DDS-core claim 200MHz, but running at 176MHz)
- next step was a frequency measurement over time (10s gate time)
-> observed instable results
-> did time sequence for UTG962 and MHS5200A picture: 20MHz_frequency_drift_comparison
what can we see
- frequency counter is good enough
- MHS5200A is showing smooth frequency drift during heating up
- UTG962 WTF ...
2) let's do to frequency domain
- FFT on my Rigol scope ... no chance to see anything
- my Perseus SDR (in spectrum mode) ... BINGO
-> could see the unwanted side-bands (spacing approx 720kHz, not a function of carrier frequency)
First conclusion: this is not 200MHz/256!
Second conclusion: can't be a simple PWM for crystal tuning
Third conclusion: must be an "internal algorithm"
picture: spectrum_35MHz
- checked also my old MHS5200A: clean spectrum with reduced SNR (due to 12bit DAC) no side-bands at all!!!
3) check the precision of frequency steps
- how to do with an "instable signal"?
- simple solution: scope in X-Y mode and a hand operated stop-watch
keep in mind: we have a dual channel function generator with same reference for both channels
- set both channels to the same frequency (e.g. 50MHz)
- check the scope for a stationary 1:1 Lissajous figure
- offset one channel by e.g. 0.1Hz -> you should see one period in 1/0.1Hz = 10s
- measure the time for multiple periods with your stop-watch
-> very precise results with nearly no effort!
Conclusion: UTG962 looks perfect!
- final: drift investigation with other frequency ratios: 1:2, 1:3, 2:3, 3:5, 5:7, ...
- you get stationary figures
- no change after 10 minutes -> really perfect!
4) the big question: what's going on? ... let's investigate the carrier and the side-bands
- Perseus in SDR-Radio mode, SSB-demodulation of a 25MHz carrier
- wanted carrier: sounds good, looks good picture: Carrier_25MHz
- unwanted carriers: sound terrible (noise), look terrible picture: first_sideband_25MHz
5) compare the level of the unwanted signals with the spec of the UTG962
- measured the level of the first unwanted side-band against the level of the wanted carrier
- my Perseus is limited to 40MHz (14bit@80MHz ADC)
picture: sideband_dBc-versus_frequency
- my problem: if you claim that the unwanted side-bands are harmonic, then my measurements are "close" to the specs (not really!)
My concerns: "harmonic" distortions should not impact frequency measurements
They claim at 10MHz a phase noise below -125dBc/Hz @ 10kHz offset. What's the value of this statement? The devil sits at multiple of +/- 720kHz!
My summary:
On the positive side
- compact device with impressive functionality and build quality
- easy to work with (graphical human interface with high resolution screen)
- good enough for at least 99.9% of all typical function generator use cases
- this is an entry level device with an excellent price/performance ratio
=> I will use this device (my MHS5200A and a 50 years old Wavetek are now retired and in passive stand by)
On the critical side
- they invested enough to achieve an excellent frequency resolution
- they had limited resources to keep the unwanted side effects of the first point under the visibility limit (noisy side-bands)
- assume that a better (state of the art?) solution would require a more expensive FPGA
- performance could be critical for applications like precise reference clocks or measurements with highly selective filters
and on top
- there are many happy users ... and ... it was hard to catch and analyze the impairments ;-)
PS: comment regarding complaints from Quarlo Klobrigney
- I confirm Labrat101 statement
- had similar problems ... root cause was always lousy grounding (e.g. bad BNC-connections or cables)