Author Topic: 60 db attenuator help  (Read 14433 times)

0 Members and 1 Guest are viewing this topic.

Offline GK

  • Super Contributor
  • ***
  • Posts: 2607
  • Country: au
Re: 60 db attenuator help
« Reply #25 on: August 07, 2013, 04:39:04 am »
Just for interests sake, a common 1/2W, 1% tol., 2.5mm diameter metal film resistor has 1/3 pf parallel capacitance in free air according to my measurements. I went about measuring it because an amplifier prototype for my analog computer using a 1M ohm feedback resistance was suffering undue reduction in phase margin due to the 0.33pF of capacitance, which corners with 1M at about 480kHz. I got around the issue by making 1M with five 200k in series instead, lowering the net parallel capacitance to one fifth of one third of a puff.

BTW, I'm not sure what you mean by "higher thermal noise of the 25k resistor". If the characteristic impedance of the attenuator is 50 ohms, the noise generated will be equivalent to 50 ohms, regardless of how the attenuator is made.
     
« Last Edit: August 07, 2013, 04:43:41 am by GK »
Bzzzzt. No longer care, over this forum shit.........ZZzzzzzzzzzzzzzzz
 

alm

  • Guest
Re: 60 db attenuator help
« Reply #26 on: August 07, 2013, 08:28:27 am »
Depending on the type of resistor, you might expect some minor differences between values. For example metal film resistors are cut in a serpentine pattern. This serpentine pattern tends to vary between values. This will probably have a minor effect on the parasitic capacitance.

The book Analog SEEKrets (EEVblog PDF mirror) might also have some further information on this. I remember it discusses the parasitic capacitance/inductance of the various resistor types.
 

Offline robrenz

  • Super Contributor
  • ***
  • Posts: 3035
  • Country: us
  • Real Machinist, Wannabe EE
Re: 60 db attenuator help
« Reply #27 on: August 07, 2013, 12:43:06 pm »
Thanks everyone for your patient input and guidance.

AFIK Analog secrets does not go into much detail about stray or parasitic capacitance of resistors. It is mentioned in a few of the examples. 

But I did find this interesting. Quote from Analog Secrets:
"Historically, putting several lower value resistors in series produced one larger value resistor with a reduced time-constant of the shunt capacitance. This scheme fails spectacularly if the stray capacitance to ground has any significant value and is definitely not workable for surface mount components. The resultant distributed time-constant ruins the attenuation characteristic of the overall network. The best practice is therefore to use as few resistors as possible in an attenuator design. This means pushing resistors right up to their power and voltage limits."

Offline GK

  • Super Contributor
  • ***
  • Posts: 2607
  • Country: au
Re: 60 db attenuator help
« Reply #28 on: August 07, 2013, 02:07:19 pm »
But I did find this interesting. Quote from Analog Secrets:
"Historically, putting several lower value resistors in series produced one larger value resistor with a reduced time-constant of the shunt capacitance. This scheme fails spectacularly if the stray capacitance to ground has any significant value and is definitely not workable for surface mount components. The resultant distributed time-constant ruins the attenuation characteristic of the overall network. The best practice is therefore to use as few resistors as possible in an attenuator design. This means pushing resistors right up to their power and voltage limits."


It is perfectly workable with surface mount components; just do not pour a ground plane around, near or underneath and lay the series resistors out in a straight line. But any way, it is only a valid approach when the net resistance is rather large, as then the added inductance is irrelevant, or rather much less a bother than the parallel capacitance (the inductive reactance is small in comparison to the resistance until very high frequencies). Not necessarily an applicable, necessary or workable approach to your application.

 
« Last Edit: August 07, 2013, 02:21:21 pm by GK »
Bzzzzt. No longer care, over this forum shit.........ZZzzzzzzzzzzzzzzz
 

Offline GK

  • Super Contributor
  • ***
  • Posts: 2607
  • Country: au
Re: 60 db attenuator help
« Reply #29 on: August 07, 2013, 02:20:43 pm »
Depending on the type of resistor, you might expect some minor differences between values. For example metal film resistors are cut in a serpentine pattern. This serpentine pattern tends to vary between values. This will probably have a minor effect on the parasitic capacitance.

The book Analog SEEKrets (EEVblog PDF mirror) might also have some further information on this. I remember it discusses the parasitic capacitance/inductance of the various resistor types.



I measured from 200k to 1M and the ~1/3 pF seemed pretty consistent with the resistors I had at hand. I'd estimate my indirect measurement method to be accurate to within +/-20%.
Bzzzzt. No longer care, over this forum shit.........ZZzzzzzzzzzzzzzzz
 

Offline KJDS

  • Super Contributor
  • ***
  • Posts: 2442
  • Country: gb
    • my website holding page
Re: 60 db attenuator help
« Reply #30 on: August 07, 2013, 02:29:41 pm »
Nothing wrong with using SMD resistors for RF attenuators. I've designed dozens, though all in 50 ohm applications.

More than about 12dB causes higher roll off above 1GHz. There'll be some, but usually less than 1dB at 1GHz using 1206 resistors. For best results, the series and shunt resistors should be touching. There will be some extra capacitance to ground where the resistors meet. This can be partially tuned out by using a thin section of track. Grounding the ground ends of the shunt elements needs via holes very close to them. If you really need the attenuator to be very flat above 1GHz then plugged via in pad technology may be necessary, or use coplanar waveguide instead of microstrip to reduce the ground inductance.

Offline robrenz

  • Super Contributor
  • ***
  • Posts: 3035
  • Country: us
  • Real Machinist, Wannabe EE
Re: 60 db attenuator help
« Reply #31 on: August 07, 2013, 02:43:42 pm »
I plan to build this free standing without a pcb inside a shielded bnc box. so capacitance ground will only be ground wire connecting the shunt legs of the attenuator and the box itself.  I currently only care about going to 600kHz
« Last Edit: August 07, 2013, 04:55:59 pm by robrenz »
 

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 2166
  • Country: ca
Re: 60 db attenuator help
« Reply #32 on: August 07, 2013, 11:08:50 pm »
Robrenz,

Here is a way to construct the attenuator using co-planar wave guide.

Take a piece of double sided FR4 about 1 inch wide and 2 inches long, not critical dimensions.

Using a 20 thou end mill make an 80 thou trace with 20 thou isolation on either side down the middle of the board.

Drill a few holes on either side of the trace to allow the ground planes on either side of the trace to be connected to the solid copper layer on the bottom.

This is a 50 ohm co-planar transmission line.

You make cuts in the 80 thou line for the attenuators.

Here are a couple of  pictures showing a single stage of attenuation.






I used this technique when I made my Avalanche pulse generator.

https://www.eevblog.com/forum/projects/transmission-line-avalanche-pulse-generator/msg186236/#msg186236

This technique has good RF performance.

Jay_Diddy_B
 

Offline robrenz

  • Super Contributor
  • ***
  • Posts: 3035
  • Country: us
  • Real Machinist, Wannabe EE
Re: 60 db attenuator help
« Reply #33 on: August 08, 2013, 01:53:47 am »
This is a 50 ohm co-planar transmission line.

Are not the pcb thickness, dielectric constant and copper thickness part of the equation?

Offline Jay_Diddy_B

  • Super Contributor
  • ***
  • Posts: 2166
  • Country: ca
Re: 60 db attenuator help
« Reply #34 on: August 08, 2013, 02:16:02 am »
Robrenz,

Yes, they are. I assumed that the board is 0.062" FR4. At 600kHz the wavelength is 500m, so there is no concern about reflections etc on a 2 inch board.

Jay_Diddy_B
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf