I need to make some trimmer cap replacements to an IC-735 and then do
a PL adjustment after replacing them. To do this properly (according
to the service manual) I need an RF voltmeter capable of measuring
the range of 40-400mV.

I have a DMM with an input impedance of 10M ohms. I was hoping to
construct a simple RF probe to the DMM and thus measure RF volts.
However, the way I understand it, most of the suggested RF probe
schematics (eg - the one in the ARRL Handbook) would only work in the
range of volts, rather than millvolts, due to the voltage drop on the
detector diode.

I have found a solution in the RSGB "Test Equipment for the Radio
Amateur" book, which unfortunately requires building 4 transistor
array to amplify the signal. A bit more work than I want to get
involved in unless necessary!

So my question is if anyone knows of a simpler solution for an RF
probe that can measure RF millivolts without an amplifier array?

Any suggestions greatly appreciated.

Larry VE7EA

Well, the old Boonton RF millivolters are dirt cheap these days...

Do you have a good scope on hand? Something along the lines
of more modern Tektronix? That would be the easiest way to
do it. You can make a cheap RF probe using a hot carrier diode
or germanium diode, but the reading accuracy would be
questionable, especially at very low RF voltage levels.

Are you looking for a peak while tuning, a relative indication?
Or, do you need to know the exact RF voltage level?

Pete

On Dec 28, 1:41*pm, lagagnon wrote:
I need to make some trimmer cap replacements to an IC-735 and then do
a PL adjustment after replacing them. To do this properly (according
to the service manual) I need an RF voltmeter capable of measuring
the range of 40-400mV.

I have a DMM with an input impedance of 10M ohms. I was hoping to
construct a simple RF probe to the DMM and thus measure RF volts.
However, the way I understand it, most of the suggested RF probe
schematics (eg - the one in the ARRL Handbook) would only work in the
range of volts, rather than millvolts, due to the voltage drop on the
detector diode.

I have found a solution in the RSGB "Test Equipment for the Radio
Amateur" book, which unfortunately requires building 4 transistor
array to amplify the signal. A bit more work than I want to get
involved in unless necessary!

So my question is if anyone knows of a simpler solution for an RF
probe that can measure RF millivolts without an amplifier array?

Any suggestions greatly appreciated.

Larry VE7EA

It's a common misconception that detector diodes (or any other
semiconductor diodes, for that matter) don't turn on at all below some
magic voltage. That's a lie! The conduction current versus bias
voltage is a smooth curve that passes through 0 current at 0 volts
(assuming you don't have light falling on the junction ;-), but
there's more conduction at +1 millivolt than there is a -1 millivolt.
I've built single diode detectors that I've used to detect RF as low
as about 100 MICROvolts. It's QUITE EASY to see the output for a 40
millivolt input even if you use a cheap 1N4148 silicon diode as a
detector and a not very fancy DVM to read it out. Try it! The thing
is, the output at low voltages will be a voltage that is linearly
proportional to the _square_ of the input voltage. That transitions
to being linear with input voltage as you get to RF voltages in the
range of "one diode drop," or about half a volt for standard silicon
diodes. That square thing means sensitivity drops rapidly as you get
to low RF voltages: cut the RF in half and the DC output drops to 1/4
what it was.

If you can get an RF Schottky diode, or a germanium detector diode,
you'll have greater sensitivity, but if your volt meter goes down to
below a millivolt (e.g., a 3-1/2 digit DVM with a 200mV range), you
should be able to easily see the response to 40mV of RF with just a
1N4148-based probe.

To be sure I'm not trying to blow smoke some bad place, I just tried
this, with a 1N4148 as detector. Using a _very_ cheap DVM with 1
megohm input resistance, I got 0.1mV output with about 30mV RF input.
With a better DVM with 10 megohm input resistance, 15mV of RF is
easily detectable at 0.1mV output. This was at 50MHz, but holds down
at lower frequencies too.

Cheers,
Tom

"K7ITM" wrote in message
...
To be sure I'm not trying to blow smoke some bad place, I just tried
this, with a 1N4148 as detector. Using a _very_ cheap DVM with 1
megohm input resistance, I got 0.1mV output with about 30mV RF input.
With a better DVM with 10 megohm input resistance, 15mV of RF is
easily detectable at 0.1mV output. This was at 50MHz, but holds down
at lower frequencies too.

Cheers,
Tom

You ain't blowing smoke or whistling Dixie, otherwise the diode
detector probes for the Boonton RF millivolters would never
have worked!

Pete

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

"Fred McKenzie" wrote in message news:fmmck-2F9622 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

It would be easy to cobble a FET cathode follower to avoid
the loading issues, but that might be more than the OP wants
to get involved with.

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

On Dec 29, 6:00 am, "Tio Pedro" wrote:
Well, the old Boonton RF millivolters are dirt cheap these days...

Do you have a good scope on hand?

DOH! In fact I do have access to one via a friend. Why didn't I think
of that in the first place? Too much wine and egg nog over Christmas I
guess. But also, thanks to v\everyone for all their comments - all
very useful and if the scope doesn't work for me I will try some of
the other ideas out. Cheers...

Larry VE7EA

" DOH! In fact I do have access to one via a friend. Why didn't I think
of that in the first place? Too much wine and egg nog over Christmas I
guess. But also, thanks to v\everyone for all their comments - all
very useful and if the scope doesn't work for me I will try some of
the other ideas out. Cheers...

Larry VE7EA


don't forget the scope is going to give you a very high reading,
since the display is peak-to-peak. You'll have to take the
reading from the waveform's 0 voltage cross-over
poiint to the peak of on one side of the full
waveform, and multiply by .707 to get the RMS value.

Pete