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Amp module suggestions

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brandon7861

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Nov 28, 2018
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As mentioned in my gerber thread, I am looking to make a few active probes. The BF998 mosfets are great for the probe front end with their extremely low capacitance, but their output is about 18-20dB down the way the circuit is set up, so I want to follow that up with an amp that can be switched in to bring that level back up to normal. This is what I am thinking about using because it is already matched to 50ohm internally, which is what the BF998 circuit is already outputting. My thinking is that I can make the amp selectable without dealing with mismatches, just bypass it when not needed. Any thoughts, suggestions or other recommendations? i will probably use two of these since they are about 8dB each, but I am not familiar with all the options out there. Hoping someone knows a better part for this project.


Edit maybe one is enough, I suppose it don't have to be a 0dB probe.
 
Last edited:

The MAR series are 50 ohms in, 50 ohms out. Your MOSFET front end will need to drive into 50 ohms.

For my money, the best use for a preamp like this would be to use with a pickup loop. Just hold it close to the circuit and make a non-contact measurement. No risk of loading an oscillator circuit and introducing error.

73
 
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That was the point, since it is already 50 in and 50 out, it would pair up with the active probe that already outputs 50ohm. The idea is having the option to have the MARS after the BF998 circuit (which outputs 50ohm as is) and have a switch that bypasses the MARS for when I need less signal.

I already made one of these diy probes and absolutely love it. I can probe right on a crystal and it only shifts a few Hz, so probing at the output of an oscillator doesn't change the frequency at all. Only issue is that the probe is about 18dB down, and for something thats only half a volt, it is almost too low for some frequency counters, so I need the amp to bring it back up for weak signals.
 
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The front end is going to be 5v as the pdf shows and the MARS will have its own 9v regulator. Availability of recommended MARS bias resistors in the rating/type I want and stage isolation are the motivation for having two regulators. Excessive, maybe.

I want to put clip leads on the probes so it can be connected to the radios power socket/protection diode leads instead of needing a separate bench supply just to run the probe.
 
Here is how I measure and adjust an oscillator frequency without
loading down the circuit. I have my frequency counter connected
to the vertical output of an oscilloscope, using the amplifier in
the oscilloscope to boost the level to my counter. Turn the scope
sensitivity all the way up and look for the weak signal.
The X1 or X10 probe setting, and a piece of wire near the oscillator circuit to
pick up signal. Or the probe can touch the metal can of a crystal.
or sometimes have the probe set to X10 and have a 1k ohm resistor
in series before probing the circuit. Good for below 30 MHZ circuits.
 
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I've never seen a scope with an amplifier in it, thats cool. My first and only scope has been a USB Picoscope 3206D. That one does not have an amplifier in it. i would love a good bench scope, this USB one is a hassle. I will watch for that feature next time I am scope shopping.

I've used all sorts of improvised probes with and without resistors and capacitors at the tip, but nothing comes close to the convenience of being able to touch the circuit without blasting past the tolerance listed in the alignment procedure. I don't think many people realize the effect a 10x probe or an improvised probe has. Either its loading down the circuit, or the signal is too small to detect and an amp is needed. That Mickey Mouse BS gets old real fast, it turns a quick alignment job into a royal PITA. Good active probes are worth their weight in gold.

I went with the MAR4's for the second stage, got them ordered with enough stuff to make 10 probes, pogo pins and quality clip leads included.

I know not everyone can afford a good active probe, nor does everyone have the skills to make their own, but too many people think their 10x probe is giving them accurate results. They don't. My friend swears by a very well known radio shop (that I won't mention) and his stuff comes back about 80Hz off frequency every time. Can't tell him that though lol. AM people don't care, but SSB operators should.
 
yes, these oscilloscopes have a linear amplifier section
following the probe. calibrated gain sections. to make it viewable
on the CRT. I have 3 or more old CRT display scopes around now.
two were given to me. you can get them cheap/free nowadays.
Find an old B and K, or Hitachi scope, going to 100 MHz if possible.
they usually have a vertical output on the back for one of the channels.
a BNC connector, to your frequency counter.
Also some have a Z-axis BNC on the back (modulate the trace brightness)
but not really used for much.

I also have an old Motorola 2001 Service Monitor, from an employer.
I actually had 3 of them here at my house, but took two back to the
shop. too much junk here. test equipment that is not needed anymore.
I also get power supplies, components from various employers.
I have a nice stack of Astron Power supplies, 20 to 70 amp.
all free for the taking. some were not working, but easy to repair.

so electronics manufacturing or repair companies that are going
out of business, or want to get rid of "junk" knowing that I
am a tech/engineer at home will often let me take stuff out
the back door, knowing I want it, and will use it.

at one employer, we had an expensive Tektronix portable LCD ScopeMeter.
The battery would not charge much anymore, so I was told I could
take it. But I didn't. I don't really like the digital scopes
and their display. Love the old all analog CRT.


so on craigslist or eBay when you see some old HP test gear
for sale this is often the case. or the guy stole it from the back
room at work. then try to sell it for some good money.

I always wanted one of them black and white colored
WaveTech signal generators, that have one watt output,
up to 512 MHz. I have seen these listed on them
websites, as well as big screen TV's "new in the un-opened box"
that were stolen from their employers. sometimes for
kicks, I send a craigslist reply of "so which Wal-Mart do
you work at?" to the big screen TV ad. (I get no reply!)
 
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nomadradio said:
A college buddy put a pirate AM radio station on the air from his dorm room using one of those.
Click to expand...

from eBay: WAVETEK 3000. to 520 MHz.
WAVETEK_3000.jpg

.
ha ha! we did the same thing while bored at a lab, at various
employers in southern California. these WAVETEK's were
all over back then. (1978 era) Connect to an antenna. Modulate it, or just
turn the internal tone modulation. would jam or talk over the popular
top-40 radio station, and the girls working at the local sandwich
shop, would get mad. or jam out the popular "born again Christian"
radio station in the area and at the employer I was at. The people
into that stuff were always judging and criticizing me, so I jammed
their station while at work. I would snicker and laugh, and probably
be heading out them evenings to go see Black Flag (before Henry joined),
Circle Jerks, Agent Orange, Weirdos, etc.
 
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