0
\$\begingroup\$

I have designed a 7th-order elliptic filter using this online tool.

Cut off frequency - 15 MHz
Pass band ripple - 0.1 dB
Stop band attenuation - 70 dB

I've asked it to use tge standard inductor value of 2.2 μH.

The designed filter is shown below:

enter image description here

The Kicad schematic: enter image description here

The Kicad PCB: enter image description here

The actual PCB: enter image description here

This gives a stop band attenuation of around -65 dB which is good enough for my purpose.
I've also simulated this in LTspice which showed similar performance.

I implemented this filter on a homemade PCB. The main requirement of this filter is to cut a 50 MHz noise from an analog signal. In addition, I expected it to cut 2nd and 3rd harmonics of a signal which has a maximum fundamental frequency of around 10 MHz.

When the filter is tested however, the results I got were very different. The 50 MHz noise was still present considerably. So, I tested this filter using a function generator and oscilloscope. Simply, I connected the filter input to my AWG. Two channels of the oscilloscope were connected to the input and output of the filter. Then I changed the frequency from 1 MHz to 100 MHz gradually in the AWG. It showed good attenuation in 15 MHz range. However, after around 30 MHz the attenuation gradually reduced and stayed almost constant. I measured the peak value of the input and output sine from the filter and calculated the attenuation and it was only around -17 dB instead of -65 dB which is shown in the design.

I would like to understand possible causes for this discrepancy. A few that come to my mind are:

  • Deviation of actual component values (bought from AliExpress), wrong approach in actual measurement/impact of oscilloscope probes/etc.

  • Problems in PCB layout causes input to leak to output at high frequencies, issue in the design itself which is not evident in the simulation.

\$\endgroup\$
9
  • 2
    \$\begingroup\$ What is the self resonant frequency of the inductors as stated in the inductor data sheet? Are they shielded inductors or, are they placed to demote inductor magnetic cross-talk? \$\endgroup\$
    – Andy aka
    Dec 27, 2022 at 10:41
  • \$\begingroup\$ @Andyaka Unfortunately inductor was purchased from aliexpress and a datasheet does not exist. \$\endgroup\$
    – peter
    Dec 27, 2022 at 10:54
  • 2
    \$\begingroup\$ That's problem #1 = never buy components that don't have a data sheet and don't come from a reputable supplier with a reputable quality assurance standard. Problem #2 is why are you using (what looks like) 180 ohm resistors on input and output? \$\endgroup\$
    – Andy aka
    Dec 27, 2022 at 11:00
  • 1
    \$\begingroup\$ OK try shorting the input resistor out; it appears to represent your source impedance and, presumably you are already using a 50 ohm source impedance signal generator? \$\endgroup\$
    – Andy aka
    Dec 27, 2022 at 11:43
  • 2
    \$\begingroup\$ Understand that the higher the filter order, the higher precision you need in the components. By the time you get to a 7th order filter, you'll need something like 0.1% in the highest Q section. Use trimmable inductors, trimmer capacitors about 2x the tolerance of those ceramics (e.g. instead of 100pF 10%, use 91pF in parallel with a 20pF trimmer) and spend a happy hour lining it all up on a spectrum analyzer. \$\endgroup\$
    – user16324
    Dec 27, 2022 at 13:25

3 Answers 3

4
\$\begingroup\$

However, after around 30 MHz the attenuation gradually reduced and stayed almost constant

This measurement indicates that the inductors are likely beyond self-resonance. It results in a cascade of capacitive voltage dividers that have an attenuation that is independent of frequency.

Real passive elements have a tolerance that make targeting a specific frequency difficult. Use low tolerance parts <1% for good accuracy.

Use COG/NP0 ceramic capacitors. Other ceramics are very voltage dependent. Capacitors have a self resonant frequency above which they become inductive.

The inductors have a self-resonant frequency above which the become capacitive. They also have a series resistance that is usually significant.

When you buy components that have no data sheet, you must measure them yourself to characterize them within the bounds of your application. Even if there is a data sheet, measure them anyway to verify that they are consistent with the claims in the datasheet.

Rs is the source resistance not usually installed on the board. The AWG already has the 50 internally. Use a 50 ohm cable (coax) from the AWG to the board. R1 on the board should be 0 ohms.

RL is the load resistance not usually installed on the board. Use a 50 ohm cable (coax) from the board to the 50 ohm load. R2 on the board should be open. For testing, R2 of 50 ohms is fine. If you can set the scope to 50, that is better for loading.

However, a high value resistor (100k) can be used for R2 to ensure the capacitors are discharged when the board is not connected.

Attaching an oscilloscope to the circuit changes the circuit. A low capacitance probe still has 9 to 10pF. The ground clip lead can introduce 0.5nH.

Addition: Avoid using wire-wound resistors. They are inductive. Low inductance winding is available. Carbon film and metal film are better although they too can have significant inductance. I think metal foil is the best. Whatever, choose low inductance resistors and capacitors. end addition

Sorry for being long winded, but all these things got in my way when implementing and characterizing filters.

\$\endgroup\$
3
  • \$\begingroup\$ Thanks for the valuable info. I have already tested the circuit with R1 short circuited (and AWG set to 50 Ohm out mode), R2 removed and connecting my tinySA spectrum analyzer which states that it has 50 Ohm impedance when set to attenuation above 10dB. Then I set the AWG to sweep from 1MHz to 100MHz and plot the input and output of the filter. As expected, input shows no much variation in magnitude. The output also shows very similar profile with no much attenuation at all. Instead, it fluctuates a little and does not look like a low pass filter at all. \$\endgroup\$
    – peter
    Dec 28, 2022 at 7:14
  • \$\begingroup\$ I then built a cascaded R/C filter with three stages. Simulated this also and it shows ok attenuation at higher frequencies. R is set to 1k and C set to 560p. I also measured values of the components and they are within range. Re-did the same frequency plot in SA and again see similar profile - no attenuation at higher frequencies. Simulation shows almost -90DB at 10MHz. Since in this circuit I don't use inductors, any issue must arise due to caps? Or are the long leads of resistors and caps behaving as inductors at this frequency range altering the circuit behaviour? \$\endgroup\$
    – peter
    Dec 28, 2022 at 7:16
  • \$\begingroup\$ Assuming this issue arise due to self-resonance (of inductors and/or caps), what would you suggest? Can I try to alter the values (for example increase inductor values) and avoid self-resonance? \$\endgroup\$
    – peter
    Dec 28, 2022 at 12:02
0
\$\begingroup\$

2 possibilities I can think of:

  1. Please show the schematic of your test fixture too. Your filter was done assuming a 50 ohm source impedance and load impedance. Are you sure that's what you're placing when measuring your circuit?

EDIT:

I see you have added an schematic to your question now. Even though you have a placed a 50 ohm in series with your input, I'm not sure that's the impedance the your AWG will see. It should normally go to ground and the load should be high ohmic so that the level indicator in your AWG is accurate enough.

On top of that, you're adding a parallel RC at the input with the other probe to measure the input.

As a 1st step in your debugging process, I'd suggest comparing the measurement of your filter input to a measurement of your AWG connected directly to your oscilloscope. If the amplitudes differ, then that's one source of inaccuracy. You could also try to vary the frequency and see whether you see some frequency dependent effects when measuring your filter input, which you'll probably not see when measuring the AWG directly.

  1. If your notches are not as steep as you expected, a common reason is poor performance of the inductors (poor Q, your frequency of interest is too close to the self-resonance of your inductor, etc).

If you're doing this for work, do yourself a favor and source components that have a proper SPICE model so that your simulation better approximates the real measurement, and make sure that the frequency of self-resonance is at around 1 order of magnitude higher than your highest frequency of interest.

Unless you have done a very poor job in your PCB design, a circuit track will mostly shift the notch of your frequency and probably steal a bit of depth of the notches you want due to their intrinsic parasitic impedance.

\$\endgroup\$
7
  • \$\begingroup\$ I updated the question with PCB/Schematics. The filter itself contains the otput 50Ohm and I am connecting a high impedance probe to the output. The series 50ohm resister forms the input impedance? I am not sure if I have set the output of the AWG to 50Ohm mode when doing the test. I will check and update the comment. \$\endgroup\$
    – peter
    Dec 27, 2022 at 10:54
  • \$\begingroup\$ I've checked the AWG's output and it is set to drive a 50ohm load. \$\endgroup\$
    – peter
    Dec 27, 2022 at 10:58
  • \$\begingroup\$ Also please note that this filter is a low pass filter. \$\endgroup\$
    – peter
    Dec 27, 2022 at 10:58
  • \$\begingroup\$ @peter, then I'd suggest checking the datasheet of your probe and see what's the equivalent input network of it. \$\endgroup\$
    – Designalog
    Dec 27, 2022 at 11:06
  • \$\begingroup\$ @peter once you figure out the equivalent networks of the different components you're using, I'd suggest you try setting up a quick spice simulation (ideal components for now is fine) and see how that extra circuitry affects your frequency response, I'd do it but I don't have access to a PC now. Maybe later \$\endgroup\$
    – Designalog
    Dec 27, 2022 at 11:14
0
\$\begingroup\$

You should try this, made with "passive filter designer", microcap v12.

enter image description here

\$\endgroup\$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.