At 21:29 14/08/2003 +0100, you wrote:
Now the bitter bit. Things were now even worse, with it proving impossible
to get anything like the previous output figures or a decent waveform on
136 or even 73 at any over a couple of watts so I hastily put things back
the way they had been and opened the Pommeau.
So todays puzzler is;
How come? Anyone got any ideas? I've gone into sulk mode.
Ian GI8AYZ/MI0AYZ
Dear Ian, LF group,
The main effect of the mods I did to the Maplin audio amp (very similar to
the B&K modules) is to increase the slew rate by increasing the bias
current in the input stage - this increases the output voltage swing at
high frequencies that the amp can achieve and hence the output power for a
given load. It is not easy to see how that would reduce the output.
One thing to check is that the DC bias voltages in the circuit should be
roughly the same after the mods as they were before. If there is a wrong
component, it is most likely that the voltage at one or more nodes in the
circuit will be stuck near one supply rail or the other. In what way is the
output voltage waveform not "decent"? I found that my amp had to be
overdriven to get reasonable output power and efficiency, so the output
voltage waveform was not very sinusoidal, although with a high Q resonant
load as most LF antennas are, the current waveform will always be a nice
sine wave even with really horrible driving waveforms.
One possibility that springs to mind is that speeding the amplifier up has
resulted in instability problems - this type of circuit, with wide
bandwidth (a few MHz), and an overall negative feedback loop, is sensitive
to the load impedance over a wide frequency range, not just at the
operating frequency. In particular, a load which is capacitive at HF will
often cause the amplifier to be unstable at some frequency due to excess
phase lag introduced into the feedback loop. A series tuned antenna by
itself will be inductive at frequencies above resonance, but if there is a
fairly long transmission line between loading coil and TX, the shunt
capacitance of the line will cause the overall load impedance to become
capacitive above a few 100kHz. If matching is done using a tapped loading
coil with one end grounded, this will also introduce additional resonances
at higher frequencies. If the amplifier output voltage waveform is OK with
a resistive dummy load (or no load at all in the case of the audio amps)
but not when the antenna is connected, this is probably why.
With my audio amp, I found capacitive loads either resulted in severe
"ringing" at the output, or caused a blurred waveform due to the thing
taking off at some unrelated frequency. My cure was to add a low-pass
filter between the matching transformer at the amp output and the coax
feeder - the usual pi-section designs won't help due to the shunt
capacitors at their input, so I used a tee configuration instead with a
series inductor at the filter input. The low-pass filter effectively
isolates the amplifier from the ipedance variations at the load. After
this, I had no trouble. I have occasionally also had problems with class D
amps and peculiar loads - these were also solved using a low-pass filter at
the TX output.
Cheers, Jim Moritz
73 de M0BMU
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