EMRFD Message Archive 1130
Message Date From Subject 1130 2007-10-30 14:42:53 Ed Almos Capacitor & Inductor Values In Filters When building low-pass filters it is impossible to get the exact
values for the capacitors and inductors*, capacitors are only
available in standard values and inductors are only accurate to +/- 1
turn. So, when building a low-pass filter set for HF I decided to get
close using available capacitors but go to the next lowest value. For
Needed value Actual value
215pf 212pf (200+12)
3310pf 3300pf (3000+300)
As I am using slightly lower C values I assume that the cutoff
frequency of the filter will be a little higher. So, when choosing my
inductor values, do I go for the next highest available value (to
hopefully bring the filter back to frequency) or does it not matter?
Edward Almos HA6SST
* Blame ELSIE, 505pf capacitors are never in stock.
1131 2007-10-30 14:56:21 Leon Re: Capacitor & Inductor Values In Filters ----- Original Message -----
1132 2007-10-31 16:40:31 Kevin Purcell Re: Capacitor & Inductor Values In Filters The real question is what tolerance are your components and how do
they affect the filter.
1133 2007-10-31 17:17:33 brainerd@wildblue... Re: Capacitor & Inductor Values In Filters On 31 Oct 2007 at 16:40, Kevin Purcell wrote:
> > As I am using slightly lower C values I assume that the cutoffThe LC product is really the important thing. If you use a lower value capacitance you
> > frequency of the filter will be a little higher.
> But how much? As others suggested Elsie (or a spreadsheet) can be
> useful in determining the sensitivity of the circuit parameters to
> component tolerances.
> 73 DE N7WIM / G8UDP
> Kevin Purcell
should probably use a higher value coil and vice-versa. I use the AADE Filter program to
calculate the values on my BPF board. I first picked the nearest standard values. The
adjusted up or down to get the best response. Most filter design programs will also let you
display the efforts of component tolerance have. They run through the calculations a bunch
of times each with random values within the tolerance of the components. You see a smear
on the plot representing the different responses.
Dave - WB6DHW
1134 2007-10-31 18:49:08 jr_dakota Re: Capacitor & Inductor Values In Filters Actually 3310 is well within the tolerance range of most 3300pf caps
... for instance if you are using 1% tolerance 3300pf caps you can end
up 33pf high or low and still be within tolerance ... Point is unless
you hand select components chances are a 3310pf cap, if available,
won't be 3310pf anyway
What are you using for inductors? Toroid inductors can be tweaked in
value by expanding or compressing the windings much like you do air
core coil at VHF
Now you know why trim caps and Toko 10 or 7mm 'can' type adjustable
inductors are so popular with designers ...