For god's sake guys! Why complicate?
Do what power supply people did for 40 years when working on offline power
supplies: disconnect the scope's ground wire and stand on a rubber mat. The
scope transformer is tested at 10 times the line's voltage and the chances
of going bad are really slim!
I know it's not safely safe and all but I'm still alive after having done it
for the 40 years I mentioned. Just be careful. If you can find some old
Tektronix from the 500 series: these had transformers tested at 2500 VAC.
And BE CAREFUL that's all folks!
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Saturday, September 28, 2013 9:22 PM
To: amps@contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to drive the amp, VDD
Supply
Bill,
> That's doing it the hard way. Just set the scope input to AC instead of
DC.
> Problem solved.
I wish it were that easy. But it isn't. For example, let's consider this
case: You want to check the gate drive voltage, with an amplifier that's
floating with the AC line, while your scope is grounded.
So you set up the scope to display the difference between channels, then
connect one channel to the amplifier's common (which is NOT at ground), and
the other channel to the gates. The shields of both probes get joined, but
not connected anywhere else.
Result: Depending exactly how the power supply is configured, and how your
supply is grounded, the scope channel connected to the amp's common can see
anything from zero up to the full 240V RMS swing, at 50 or 60Hz.
The other channels sees that same, plus the gate drive voltage.
The gate drive will only be a few volts, so you might want to set up the
scope for 1V per division, after considering the probe's attenuation.
With 10:1 probes, you set the scope to 0.1 V/div.
That WOULD display the clean, nice, pure gate drive voltage, if and only if
the following conditions are met:
- Each channel can cope with the full 240V swing plus gate voltage, without
saturating or even loosing linearity. That would be about 65V p-p, through
the 10:1 probe. It's asking a bit too much, when the scope is set for 0.8V
p-p full scale!
- The gain and phase of the two channels are matched well enough to really
cancel out the strong 50 or 60Hz signal. This part is actually much easier
to get well.
The vertical amplifiers of my scope, a Tektronix from the late 1970's,
saturate at about 3 times the voltage required for full deflection. I could
use this scope to measure the drain signals by the channel difference
method, without problems, but not the gate voltage. To keep it from
saturating, I could set the vertical gain only to 40V full scale (5V/div),
with 10:1 probes. The next setting, 2V/div, would already saturate. And at
5V/div and 10:1 in the probes, my gate signal might be
0.05 divisions, essentially the same as the width of the trace!
So, it does not work for that purpose. A special transformer-coupled RF
probe would be required, or one of the other techniques.
> And always use at least a 10X probe for RF measurements. A 1X probe
> has too much capacitance.
Yes. And not only that - also a simple 1:1 probe has a termination problem.
The cable might be 75 ohm or a little more, while the scope input is 1
megaohm in parallel with some capacitance. You get standing waves on the
cable, resulting in totally wrong amplitude on screen! So, even when one is
measuring a very low impedance source, for which the high capacitance of 1:1
probes is no problem, teh measurement will most likely be wrong. 1:1 probes
are useful only at low frequencies, where the cable length is a negligible
part of the wavelength.
On the other hand, a good way to go is a simple 1:1 probe made just from a
piuece of 50 Ohm coax, with a 50 Ohm termination resistor right in the BNC
plug. VHF and UHF scopes, and most spectrum analyzers, usually work this
way, and can even have the 50 Ohm loads built in.
Of course, don't connect such a probe to a high impedance point! ;-)
Well, back to my radio now. I'm having some fun today building a "world
receiver", an all-band AM receiver optimized for that mode, that covers
exactly the AM broadcast bands - all of them. It's a fun project, combining
old and new technology - the IF amp uses dual gate MOSFETs biased negative
by a silver oxide cell, in the style radios from the 1930's did with tubes
and mercury batteries, while the local oscillator is a 14 bit direct digital
synthesizer driven from a 400MHz clock.
Tuning is by optical encoder, and the thing has 100 memories stored in
EEROM.
I think I will build a wooden cabinet for it, shaped like a cathedral radio
from the early 30's.
I just had to tell somebody! ;-)
Manfred
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Visit my hobby homepage!
http://ludens.cl
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