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 example: Needed value Actual value 215pf 212pf (200+12) 505pf 500pf 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 cutoff > > 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 > kevinpurcell@pobox.com > > The LC product is really the important thing. If you use a lower value capacitance you 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 <http://wb6dhw.com>
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 ... JR