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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:
1) Design a Butterworth, etc. low-pass filter for
a cut-off frequency well above the TX frequency - The cut-off frequency
needs to be high because the input impedance of the filter becomes highly
reactive near cut-off. Fortunately, push-pull PAs produce very little second
harmonic, so the filter can have a fairly high cut-off frequency and still
attenuate the 3rd and higher harmonics. The G3YXM design appears to have a
Butterworth response with f-3dB at 245kHz - it gives about 30dB
attenuation at 411kHz, depending on the PA output impedance. The filter
input impedance is nearly ideal at (50.2 - j0.2)R.
2) Design an
odd - order Chebyshev or elliptic filter with ripple in the passband, such that
the TX frequency coincides with the peak of the last ripple before the cut-off
frequency, where the insertion loss is exactly 0dB. This condition ensures that
the input impedance of the filter will be exactly 50R at the TX frequency. It
will be possible to get higher harmonic attenuation this way, but will require
more accurate components to get the desired response, with higher ratings due to
the higher reactive currents.
3) Design the filter as a matching network with
equal 50R input and output frequencies. For example a symmetrical pi-network
with XC = Ro/Q, XL = 2QRo/(Q^2 +1). Two of these put together makes a good
filter; for example if Q = 1, XC = 50R, XL = 50R, and you get a filter that is
23.2n/58.1u/46.5n/58.1u/23.2n, and gives about 45dB attenuation of the 3rd
harmonic (and about 25dB of the second harmonic). This is the "half wave" filter
that has been used a lot with HF transmitters. You can get higher harmonic
attenuation with greater Q, with the drawback of higher circulating currents /
more critical tolerances. You can get quite adequate results with Q as low as
0.5.
4) Design an output network to achieve particular
time-domain waveforms, e.g. the class E designs by G3NYK.
5) errm... that's all I can think of now
:-)
The 47nF----->
54uH----->82nF------54uH----->47nF filter does give higher harmonic
attenuation than the G3YXM design, and has an input impedance of around
(75-j6)R at 137.5k. But reactance and resistance vary rapidly around this
frequency, between 50 and 95R and j0R and -j85R, so small component value,
load, or frequency changes could lead to the impedance varying a great deal.
This is perhaps why it causes some problems.
With a linear PA that delivers a sinusoidal voltage
or current to the load, you mostly only have to worry about getting the right
impedance match at the TX frequency - but with non-linear amplifiers like class
D, you also really need to think about the waveforms at the PA output - the
filter effectively is part of the PA. Ideally, in a current-switching class
D design (as most are), the load should be a high Q parallel-tuned tank circuit
- this requires a filter with a shunt capacitor at the input, allthough in
practice the Q is not very critical, and can be 1 or less with only slight
loss in efficiency. Voltage-switching class D circuits (e.g. Decca) require a
series-tuned tank. Also, some amplifiers need a load that is inductive or
capacitive at high frequencies for stability. For example, the modified audio
PAs are often unstable with a capacitive load, so require a filter with a series
inductor at the input, so a T network instead of a pi network.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
Sent: Thursday, November 03, 2005 4:21
PM
Subject: LF: low pass filter for power
use
Hi Lf's
in all circuit
diagrams I find filters like
12nF---->54uH---->27nF----->54uH----->12nF
here is the 3db
point at 250khz.
Has anyone test a filter in this configuration
47nF-----> 54uH----->82nF------54uH----->47nF
here is the 3db
point at 147khz.
The 2. filter gives a better suppress of the
harmonics!
Wat is the reason that all use the fist one?
best 73' Tom
DL4EAU
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