Ringing Squarewave - filtering problem

[tOngEh]

Ringing Squarewave

HI all,

Im building a Biastee circuit which consist of a L and C. Attached is the circuit:
a square wave was injected in port2 and port 3 was plug into a oscillascope. Im expecting a nice square wave but what i saw was this:

 

[tyassin]

Re: Ringing Squarewave

Hi

Think you forgot to attach the response.

 

[tOngEh]

Re: Ringing Squarewave

Oh ya! Thank you for the reminder!

 

[tyassin]

Re: Ringing Squarewave

Hi

Is it the inductor at port 2? And what are the values of the inductor and capacitor?

 

[tOngEh]

Re: Ringing Squarewave

Yes, port2 has a L=27nH, Port1 C=100pF.
I did try to isolate the problem by doing a opencct at port 1(removing C) and a short cct at port2(removing L and short the the 2 points using solder). By doign this, the ringing waveform still persist.

 

[FvM]

Ringing Squarewave

I guess, you understand, that the filter characteristic is determined by the L and C values and the impedances at all three ports.
What is it? If it's not 50 ohm, also the respective cable length has to be considered.

P.S.: I see, that the shown resonance frequency is about 10 MHz in contrast to a filter corner frequency of about 100 MHz. So apparently, the resonance is not caused mainly by the said filter elements rather than connected external circuitry.

 

[tOngEh]

Re: Ringing Squarewave

Yes, i do understand. LC affects the filter behaviour. I also designed the filter based on a 50ohm terminated port impedance. I just dont understand why even without the L, the ringing still occurs. By shorting the 2 points is as good as applying to square wave to the oscillascope directly? Is there something wrong with the dimension of the microstrip?

 

[FvM]

Ringing Squarewave

A stub of 10 mm or less shouldn't be designated "microstrip" in the involveld frequency range, it's a short PCB trace. I take a look on your photo and wondered, if the SMA flanges are soldered to the PCB back side at all?

 

[tOngEh]

Re: Ringing Squarewave

Oh.. In another words, is the microstrip too small? Th flanges i assumed to be the grounded contact is not soldered to the backside of the microstrip.

 

[FvM]

Re: Ringing Squarewave

Oh.. In another words, is the microstrip too small?

No. It's not working as a microstrip without a continuous ground connecting all coaxial ports.

 

[tOngEh]

Re: Ringing Squarewave

Hi there,

I did another trail run with a different SMA connectors. THe flanges for this new SMA connector is in contact with the backside ground but not soldered on. With this new setup, problem still persist.

What exactly is the ringing about? My feel is that the cct acts like an oscillator but without continuity.

Added after 8 minutes:

ANyway, i took up this experiment by following very closely to this:

 

[FvM]

Re: Ringing Squarewave

I verified, that the said diplexer dimensioning has a sufficient low Q with all-sided 50 ohm termination, so it would not produce the shown ringing.
Obviously something must be different in the real circuit:
- no effective 50 ohm termination of at least two ports, or a resonant structure at one connected port
- dimensioning of the bias-T is very different from said values
- SMA flanges are still unconnected to the ground plane.
You may want check the latter by pressing a piece of metal against all three flanges (to short them against each other) and observing the waveform.

Do you get a clear square wave when connecting the pulse generator to the oscilloscope without the bias-T?

 

[tOngEh]

Re: Ringing Squarewave

Yes. i did get a nice squarewave when connecting the pulse gen to the oscillascope.
1) Howdo iverify that the 50ohms SMA are really 50ohm? I just request them during the purchase.
2) WHen you mentioned dimension are very different, to what degree of difference are you refering to? IN terms of mm?
3)How do i tackle the resonant structure? As i had mentioned, even without the L at port 2, by shorting the points across the ringing already happens.

 

[FvM]

Ringing Squarewave

When searching the cause of a 10 MHz ringing, you can ignore all striplines, connector impedances and similar.
It must be a serious circuit fault, e.g. a missing ground connection, as mentioned.

SMA connectors aren't made with other impedances than 50 ohm.

 

[tOngEh]

Re: Ringing Squarewave

Hi!
Yes, the fault indeed lies in improper grounding as i did not solder the SMA flanges to the ground. The waveform looks much nicer now! Thank you FVM!