To: rsgb_lf_group@blacksheep.org Date: Sat, 5 Nov 2005 20:42:56 -0000 Message-ID: <001e01c5e249$80a58bb0$f500a8c0@Dell5150> MIME-Version: 1.0 X-Mailer: Microsoft Office Outlook 11 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 In-Reply-To: <001901c5e0b8$9fac4a40$473c7ad5@w4o8m9> Thread-Index: AcXguLCEd3+iTIL+Ry2pHCZkckR9agBi6sKw Subject: LF: RE: Re: low pass filter for power use Content-Type: text/html; charset=windows-1252 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SpamFiltered: by PlusNet MXCore (v2.00) Content-transfer-encoding: 8bit

Dear all

Following all the discussion of the "G3YXM design" filter, I feel I ought to let you in on the time consuming process I used to arrive at those values:

Stage 1. Look up a topband PA design ARRL handbook.

Stage 2. Copy the 2.02MHz lowpass filter design and multiply all the values by 10.

Stage 3. Try it and see if it works!

I did figure that second harmonic output attenuation wasn't goint to be an issue with a square-wave PA so the 200kHz + cutoff shouldn't be a problem and measurements back this up.

Incidently the Ropex uses exactly the same values (weird eh? Did they work it out themselves, copy me or use the same topband design??) but theirs has a nice little refinement, they placed a 3.3nF C (I think) across the second inductor to produce a third-harmonic trap.

I have tried other filter values but those are the ones that work best for efficiency and that's what was important for the absence of smoke in the shack.

KISS

Dave G3YXM

From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of James Moritz
Sent: 03 November 2005 20:53
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: low pass filter for power use

Dear Tom, LF Group,

There are 2 main requirements for a TX output filter

a) Attenuate harmonics to acceptable level

b) Provide the right load impedance for the PA - usually a pure resistance at the output frequency.

Usually, the impedance at the input of the filter is made the same as the load, I assume 50R. There are a number of ways of doing this: