SDG6022X QAM Mod. displayed on SSA3032X-R

[markus_jlrb]

Dears,

I have the opportunity to play with the  SSA3032X-R device.
I'm using my Siglent SDG6022X AWG to play arround with QAM IQ Signals.
In order to make some tests with the dmod utility of the SSA3kX-R
I have used a QAM64 signal with center at 100MHz and 300ksps.
The SDG6kX generates a fixed message length of 4096 patterns.
But when setting the SSA to different message lengths (128, 256, 512, 1024, 2048, 4096)
I see a significant increasing error rate (EVM%) with the increase
of the message length that I could not fully understand.
perhaps someone can give me some explanation on this issue.

I would expect the same error rate when the IQ signal is of a stable
quality.

So in this specific case my question is if I'm wrong and have to adapt
my QAM knowledge or did the SDG or SSA or both have some issues
in generating or/and demodulating the QAM signal.

For better understanding I attached the SDG 64QAM Setting as well as
the screenshots for the 128 - 4096 of the SSA while demodulating.
The stat values where reseted after message length change.


Many thanks for your explanation in advance.

Markus

[tautech]

Sync instruments together or to a common 10MHz reference.

[markus_jlrb]

Thanks tautech for your reply.

According to my knowledge the receiver of the
QAM signal (the SSA) should recover the clock
properly and sync on the accurate sample point
in the iq data stream when jitter could be neglected.
That was what I had learned 30 years b4 at SIMENS
in the digital lab for  point to point communication
via QAM in the field of radio link science and development.
At that time I was a student doing my practical education
in this field.
Meanwhile I'm not involved in this field anymore so doing
this for hobby purpose and perhaps I had forget some
principles after that long time, so please correct me if I'm
wrong.
I will follow your hint and sync both devices to record the
difference.
For this purpose I could use the clock of my R&S CMU200
that is equipped with a high precision clock source.

Markus

 

 
 

[markus_jlrb]

Thanks tautech,

to point me the proper device config.
As already stated by you the error rate decrease significant
when syncing the clock of both devices.

I have attached only the spectra of the 4096 msg length
QAM64 signal without and witch clock sync as well as  some
additional spectra of the SA and RTSA mode.

I enjoy the time playing around with this device.

Markus

Markus

[nctnico]

Thanks tautech for your reply.

According to my knowledge the receiver of the
QAM signal (the SSA) should recover the clock
properly and sync on the accurate sample point
in the iq data stream when jitter could be neglected.

You are right about that unless the SSA doesn't do clock tracking. But that would defeat the purpose as you can't sync to the clock of an LTE tower (for example) you might want to measure.

[markus_jlrb]

@nctnico

nice to read this - I was doubting my old knowledge.
In the case you would like to receive as LTE sig from the
tower, you need your own clock recovery which is not always
easy to achieve especially when you have propagation
distortions and overlapping due to multi path.
This was the most difficult problem to be solved during
my time in the lab decades ago.
Many genius engineers were working on this problem
to solve for QAM128 and QAM256 during my time at
the company and elsewhere. The had developed adaptive
equalizers to regenerate the clock and data stream.
Was a very interesting time for me as a young student.

Markus   

[nctnico]

Nowadays you can do such things in software on a PC with an SDR. Still not easy but you can receive & track an LTE signal for sure. I've done some high level programming in this area to track the clock synchronisation of LTE towers.

[tautech]

Thanks tautech, to point me the proper device config.
As already stated by you the error rate decrease significant when syncing the clock of both devices.

I cannot know how you did these tests therefore suggested you sync instruments however there maybe something else going on.
Try referencing one and not the other.
Maybe also remove the mains earth between them and rely on the signal cabling to bond them.

But agree it shouldn't be this difficult but unfortunately I do not have an analyzer to do tests right now. 
Bump me for tests at the end of Sept.

[gf]

In the case you would like to receive as LTE sig from the tower, you need your own clock recovery which is not always easy to achieve especially when you have propagation distortions and overlapping due to multi path.

That's where OFDM (with many subcarriers and a low symbol rate per subcarrier) can help.
I thought that LTE would use (a variant of) OFDM anyway, and not raw QAM

[markus_jlrb]

Dear responders  to my thread,

we have not to mix my intention just testing the QAM64 Modulation
generated by the SDG6022X on the SSA3032X-R as a playground I
did while testing the SSA which I get borrowed, with the mentioned
problem of measuring radio signals from a LTA tower mentioned
by nctnico, which is more sophisticated as only connection two devices
(SDG and SSA) via plain coax cable (clock synced or not).

If the SSA3kX-R could be used for this kind of wireless LTE measurement
is not clear to my due to lake of knowledge in this field especially in terms
of LTE.

I'm more keen of collection some knowledge in DVBC/DVBS transmission
and the usage of such RT SSA device for diag purpose.

Markus