On Dec 29, 12:05*pm, Fred McKenzie wrote:
In article
,

*K7ITM wrote:
To be sure I'm not trying to blow smoke some bad place, I just tried
this, with a 1N4148 as detector.

Tom & Larry-

Perhaps it is understood but not mentioned. *If a test point has a DC
level, connecting a diode might disturb the circuit.

If an oscilloscope is not available, I would use a 0.001 MFD series
capacitor from the circuit, to a 10K Ohm resistor shunted by a 1N4148
diode to ground. *The junction of the capacitor and diode/resistor
combination, becomes a new test point that can be read on the lower
ranges of a DC multimeter.

Of course the reading is only relative, but is quite handy when you need
to tune for maximum output. *In some cases I've permanently wired these
test points into the circuit. *That way you don't change tuning by
removing the circuit.

Fred
K4DII

Yes, I was thinking that I probably should have shared the circuit I
used. With proper construction the load on the RF circuit is mainly
the diode's capacitance. For the test circuit, I used a 100pF cap
from the RF to be measured, to the diode's cathode. Diode's anode to
ground. I put a 10k resistor from the junction of the cap and diode
off to an 0.1uF cap to ground, and monitored the voltage across that
0.1uF cap with the volt meter. I'd have used an RF choke instead of
the resistor if I were trying to optimize it for some specific range
of frequencies, or just a high value resistor. But since my source
was a nice low impedance, there wasn't any need to optimize that part
of the circuit further.

Sensitivity for very low RF voltages is greatly enhanced by using a
high impedance meter. I would NOT put a 10k resistor to ground there
if I wanted sensitivity to low millivolt RF, since the diode average
current at low RF voltages is very low indeed. You can see that from
my reported results with the 1N4148 detector, with a 1 megohm meter
load versus with a 10 megohm meter load. With the high impedances
involved, the 100pF cap allows response down to well below what would
normally be considered RF frequencies. In the circuit described
above, the low-side 3dB corner frequency will be about that of the
100pF cap and the 10k resistor.

Cheers,
Tom

K7ITM wrote:
SNIP


Sensitivity for very low RF voltages is greatly enhanced by using a
high impedance meter. I would NOT put a 10k resistor to ground there
if I wanted sensitivity to low millivolt RF, since the diode average
current at low RF voltages is very low indeed. You can see that from
my reported results with the 1N4148 detector, with a 1 megohm meter
load versus with a 10 megohm meter load. With the high impedances
involved, the 100pF cap allows response down to well below what would
normally be considered RF frequencies. In the circuit described
above, the low-side 3dB corner frequency will be about that of the
100pF cap and the 10k resistor.

Cheers,
Tom

You might find this interesting:
http://www.cliftonlaboratories.com/d..._rf_probes.htm


Charlie.

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