SDG2042X square wave 25 MHz wrong????

[conander]

Hello all,
This is my first post. I am an electronics amateur entusiastic and recently I have bought the SDG2042X (40Mhz function generator) + SDS1202X (200Mhz osciloscope).
I saw the review in the youtube channel and I liked but. I wasn't aware of a few thinks regarding the Function generator.
The max, frecuency supposed to be 40 MHz. This is true..but only in sinewave. Not valid for triangle/square waves. Square waves only goes up to 25 MHz
Other strange thing is when I see the signal in my brand new 200Mhz (1Gsample) oscilloscope of the 25MHz square signal I see this.
The square signal is quite sloopy, no sharp edges....
Is that normal?
Thank you in advance.
Best regards.

[wraper]

Max rated frequency is for sine only. Square wave won't be with perfect edges either. Also you need to use 50 ohm terminator, otherwise waveform will be distorted at higher frequencies because of the reflections in coax cable.

[Jwalling]

Have a look in the manual for the rise-time specification on the output. I suspect that's what you're seeing and it's normal.

Jay

[jpb]

Square waves are a sum of odd harmonics, so the next highest harmonic is the third harmonic.

If the frequency bandwidth for sine is 40MHz and you're looking at a 25MHz square wave then the third harmonic will be 75MHz and this will be attenuated by around
1/(1 + (75/40)^2) in power or around the square-root of this in amplitude which I estimate at less than 0.5.

So the next highest harmonic will be down by a factor of 2 and the one after that (the 5th harmonic) will be reduced by much more.

It is not surprising that the square wave is not very square.

[conander]

Hello,
I also tested adapting the impedance of the osciloscope to 50 Ohm (the osciloscope has this option) and nothing changes....
Insted saw tooth wave is perfect. This signal has also armonics, not as much as the square...
Regards

[Electro Fan]

Lengthy discussion of square waves here:
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/

[jpb]

Hello,
I also tested adapting the impedance of the osciloscope to 50 Ohm (the osciloscope has this option) and nothing changes....
Insted saw tooth wave is perfect. This signal has also armonics, not as much as the square...
Regards

As Electrofan points out, there has been a lot of previous threads on this subject - so it is worth searching.

I had a quick look at the specs and the ramp only goes up to 1MHz, so if the bandwidth is around 40MHz you could go up to the 40th harmonic without significant attenuation!

In fact the series of generators goes up to 120MHz so the bandwidth may be more than 40MHz. The datasheet gives the rise/fall times as 9nsecs which corresponds to a BW of around 40MHz.

If you want a square "square wave" then (and this is arbitrary) you might want the rise and fall times to be around 1/4 of the square which is 1/8 of the period which corresponds to a period of around 72nsecs which is a frequency of around 14MHz.

The upper limit of 25MHz is a software set thing corresponding to a period of only 40 nsecs which gives the length of the square as only 20 nsecs (for 50% duty cycle) and given that the rise time is 9 nsecs and the fall time is 9 nsecs (10% to 90% I presume) the wave hasn't time to finish rising before it has to start falling.

[TheSteve]

Just for fun and to show the effect of rise time etc on a square wave I set my pulse/pattern gen to output two 25 MHz square waves. The first (yellow trace) was set to a rise time of 2.8ns, the second(blue trace) was set to a rise time of 9ns. Both outputs are terminated with 50 ohms.
The blue trace demonstrates pretty much what the best case 25 MHz square wave will look like with a generator that has rise/fall times of 9ns.

[conander]

very intetesting, the specifications of the sdg2042x says like 8-9 ns rising time
does any body know if i could set a sinewave of 20mhz with a preset strength of -70dbm to feed one ham radio an test it?
is this possible?

[Howardlong]

very intetesting, the specifications of the sdg2042x says like 8-9 ns rising time
does any body know if i could set a sinewave of 20mhz with a preset strength of -70dbm to feed one ham radio an test it?
is this possible?

Yes, particularly if it's an SSB radio.

-70dBm is a strong signal in narrow band RF receiver terms, so you would not be testing sensitivity at this level. You'd also need a step attenuator to do that.

In addition, typically test signals for receivers use modulation of the test signal as part of the test.

One final thing, don't what ever you do transmit into your signal generator. Remove the mic or any other method of keying up the radio.

[conander]

will this be an option?
http://www.ebay.es/itm/DC-4GHz-PE4302-Digital-RF-Attenuator-Module-High-Linearity-DSA-0-5dB-31-5dB-50-/131854724091?hash=item1eb3276bfb:g:c1YAAOSwNuxXaOm7

[billfernandez]

Other strange thing is when I see the signal in my brand new 200Mhz (1Gsample) oscilloscope of the 25MHz square signal I see this.
The square signal is quite sloopy, no sharp edges.... Is that normal?

My 200MHz scope shows that the squarewave rise/fall times of my SDG2042X is about 8.2nS, which is well within it's specs.  The attached photo shows what that looks like for a 25MHz squarewave on my 200MHz scope.  Looks essentially the same as what you saw on yours.