71497Re: [softrock40] Re: SoftRock filters and inductor variability
- Jan 7, 2013
----- Original Message -----
Subject: [softrock40] Re: SoftRock filters and inductor variability
>> I'm not sure there is a design flaw. It seems to me that some of the cores are just not of the nominal permeance resulting in
>> inductance than intended.
>> If the components actually conform to the design then there would be no trouble.
> Perhaps when originally tested, that set of cores had lower A_L values than expected?
The T37-6 has a specified AL_Value of 3.0 +/-5%
The 40/30/20m LPF was designed to be 0.8uH = 16 turns.
I tested two 37-6 cores with 16 turns and the lowest inductance I could achieve with the turns spread was 0.97uH and 0.98uH. I used
two completely different ways of measurement. A PIC meter and the old fashioned way with a 1% capacitor and dip meter and a RX to
confirm resonant frequency.
Working backwards this gives an AL_Value of 3.8. Only 14 turns are needed to get the corectly designed value.
>> The 15/12/10m build is hardly affected by this problem.
> Hmmm. See:
Sorry, I've only just got to terms with Elsie. I'm not quite sure what I am looking at, it's the 30/20/15 as built and with 2 turns
removed? Did you measure the inductors?
All I have checked with Elsie is the loss at the highest frequency, at 20m for the 40/30/20m version. Using the original design
values Elsie showed about -0.3dB but with my measured values about -5dB.
My interest is to find out why, maybe just some, of the 40/30/20s did not produce full power on 20m.
Paul pointed the way, I think it may be due to the cores.
While those like you can play with different designs I was wondering what to tell people who have built a bad 40/30/20.
Do we tell them to rip out the LPF, get new capacitors and rebuild?
I think just removing a turn, or two at the most, from each coil could return the LPFs to near enough the design parameters.
I repeated this for the 30/20/17m version, again T37-6 cores. -2dB at 18MHz with the bad core value. Not so noticeable but one turn
removed would be better.
Elsie showed the 15/12/10 version to be only slightly affected by the high AL_Value.
73 Alan G4ZFQ
> I have replaced the filter with a Cauer design, but have yet to scan it on the VNWA. My 15m 2nd harmonic is now at -40dB and I
> have (according the the spectrum analyzer at least), 1W output on 10m. I wouldn't recommend this change unless you have a way of
> accurately measuring small value capacitors and can set the resonant frequency of the parallel coils/capacitors by tweaking one or
> the other. Not too bad with the VNWA, I could put the toroid and capacitor in parallel in my test fixture.
> Values for the filter came straight from SVC Filter Designer from Tonne Software (same source as Elsie): 5th order, 30MHz
> bandwidth, C-input lowpass, Cauer, 0.044 dB Passband ripple, 40dB Stopband depth. This gives over 26dB 'attenuation' of the 15m
> second harmonic. 3dB to go...
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