On 15 Apr 2004 21:38:49 -0700, (Tom Bruhns) wrote:

Hi Jan-Martin,

Doesn't seem like anyone has answered this--hope you're still
monitoring it for followups.

There was an article in the Nov 2001 "Electronics World" starting on
page 842, "Active RF Scope Probe," using a Maxim MAX4005. It's a
high-input-impedance unity-voltage-gain amplifier with a built-in 75
ohm source resistor. The probe design puts a little attenuation ahead
of that amplifier, probably mostly for protection in case you connect
to too large a voltage, and there's 6dB loss in the termination on the
far end of the transmission line; so the probe circuit adds a 10dB op
amp gain stage. I haven't read all the details but believe it's
designed for 100MHz bandwidth. He has it AC coupled, but it could
probably be DC coupled if you wanted.

Cheers,
Tom


http://home.online.no/~la8ak/m2.htm

Tom,
thanks for reply.

I received a few replies replies via mail, but they seem to have
misunderstood the points in some degree, one outlined a theoretical
circuit which should cover up to 900MHz using general purpose devices
and made no notes to whether it was an improvement over my suggestion
or not.

It seems your is the one I'd been looking for. It has 6dB loss, and
when you add 4dB for protection it makes 10dB which is easy figure for
measurements. It is not really so imprortant which attenuation you
get as long you know what you are doing. In the first case I need only
20MHz bandwidth, while 100MHz or more seems interesting for other
later purposes like RF millivoltmeter. The first instrument is used
with a selective voltmeter, which might be connected to converter for
larger frequency range that 0.005-17.2MHz later.

I've already made one mV-meter using germanium diodes (AA119 or
1N270's) which covers up to 2GHz, but minimum reliable reading is for
10...30mV RMS, and an improvement here is very much desired.. The RF
millivoltmeter is described as item 10 on the same page. A problem is
that it was built 20 years ago, and the mechanical construction now
seem to cause some problems and a new meter seems to be required soon.

73
Jan-Martin
----
Jan-Martin, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/