Dear all,
a former collegue of me had to design an amplifier in the frequency range 10 to
20 kHz delivering a few watts. For the output filter he had used tunable pot
cores. He was wondering that he could tune the filter just for a specific
output level only. Changing this level would change the performance of the
filter and required a retune of the output filter.
I could just help him by explaning that, as far as I know, all iron powder or
ferrite coils show a certain dependance of AL value versus excitation of the
coil. This can even be shown in a simple VFO employing either air coils or iron
powder or ferrite coils. When the supply voltage of the oscillator is changed,
changing oscillator power too, the influence on frequency is much higher when
using iron powder or ferrite coils.
Therefore in the case of the low pass filter I guess the rf power will change
the desired inductivity values slightly - I confess I do not know in what
direction. But there should be an influence.
Assuming that the inductivity values of the output filter have been determined
correctly, I would propose to prove this thesis at VY1JA if the annoying phase
effect would become less when reducing power.
If this should be true, the filter coils should be retuned slightly, not the
antenna.
HW?
73 Ha-Jo, DJ1ZB
"Talbot Andrew" <[email protected]> schrieb:
> 1)
> Slight Saturation in the core causing a loss in inductance at high power
> perhaps?
> Assuming the air wound and cored inductors were adjusted to exactly the same
> values initially.
>
> Or
>
> 2)
> Direct coupling in the near field from your original air wound coils
> influencing the coupling into the scope of your V and I pickoff, that is not
> present in the closed torroidal cores.
>
> Can you wind the power down and measure the phase difference which would
> show up 1) but not 2)?
>
>
> Andy G4JNT
>
>
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]] On Behalf Of J. Allen
> Sent: 13 November 2006 12:57
> To: [email protected]; LowFER at LW Cafe
> Subject: LF: LPF coils and VY1JA to 1W ERP
>
> Gentlemen,
>
> During the amp repairs at VY1JA, the coils in the LPF were changed from air
> wound to torroid core at the suggestion of one from our group as a possible
> way of removing ringing from the transmitter. There was no difference in
> the operation of the two, except that now there is a very slight phase shift
> with the current waveform lagging the voltage waveform. The bottom line is
> that the air wound coils produced a better LPF only because of this very
> slight lag which is not apparent in the LPF using them. The inductances of
> each coil was measured using an MFJ 257 analyser and was wound to the same
> value. The current and voltage traces were picked up for the scope using
> the same scope and scopematch coils, and measurements were made at the same
> power level with the same transmitter, coax and dummy load connection.
>
> Have any others seen this very slight lag on LPFs with torroid cores? Do I
> now begin tuning the antenna slightly to correct for the phase shift which
> comes from the LPF, so that the transmitter sees a purely resistive load?
>
> Repairs to the interim transmitter at VY1JA are complete and the station is
> transmitting now at license limit 1W ERP on ~137778.8 QRSS60. Again,
> reports and captures are appreciated.
>
> Thanks,
>
> J.
>
>
>
>
>
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