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Homebrew spectrum analyser high-impedance probe

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JackOfVA:
I build and sell to the amateur radio and shortwave listener market a voltage probe active antenna. It's basically a high input impedance source follower, with careful attention to maximizing dynamic range and minimizing noise figure.  The input stage is a BF996 dual gate FET, which drives a BFQ19 follower, and the output is a BFQ19/BFQ149 CS follower. Frequency coverage is 10 KHz - 30 MHz where an input filter starts rolling off the coverage to avoid FM broadcast band overload. Without the input filter, it's usable well above 100 MHz. This is, excepting that low frequency response does not extend to 0, an FET scope probe. (It's also a bit physically large to use as a probe.)

In developing the antenna, some early experiments involved driving a 50 ohm load directly with the BF996 and I found it does a credible job, based purely on amplitude. It does not provide the best intermodulation performance, so a more complex circuit is required, but you should not see more than 6 dB loss into 50 ohm load.

The input capacitance of a BF996, plus stray trace capacitance and, in the case of my active antenna, a disconnect  relay, is on the order of 8-10 pF. To prevent the input blocking capacitor from being a limiting factor (voltage divider effect) it's necessary to make it considerably larger - I use a high voltage (630V) 1000 pF C0G capacitor. 

If you don't want to fiddle with a physically small SMD part like the BF996, a TO-92 style J310 is nearly as good and much easier to throw together in a one-off arrangement. For minimum distortion, set the gate bias to provide about 20 mA idle current through the J310. That's a lot more than most people think of when an FET is used, but the "sweet spot" for the J310's intermodulation distortion is in that range.

philpem:
To be honest, I prefer working with SMD -- it means I don't have to get the Dremel out to drill the PCBs :D

At the moment I'm reconstructing the CAD files for the Elektor probe, to which I'll add a MAR-6 as a test. It turns out I have almost all the parts I need scattered amongst my spares boxes :D

Chances are I'll build up the Elektor probe as-is, then add the MAR-6 and see what (if anything) happens. I'd like to do a board shrink-and-split - move the voltage regulator into a metal can mounted on the back of a bulkhead N plug, then run a cable to the Probe Power socket on the SA. That'd mean the probe board was just the probe, meaning I can machine a nice highlighter-pen sized case for it.

I sense a weekend project coming on :)

I'll let you know how I get on - it should be interesting.

fpliuzzi:
Hi Phil,
Your idea of adding a MAR-6 to the Elektor probe sounds very interesting. Please keep us updated.

Regards,
Frank

P.S. When I went to purchase the tiny $20 PCB from Elektor to build their "Poor Man's 1GHz Active Probe", they wanted to add an additional $30 to mail that puny thing to me, here in the US. $50 total is unreasonable.

fpliuzzi:

--- Quote from: pauln on April 06, 2013, 06:05:19 am ---There seems to some interest in this - I have sent the whole article to others via PM .  Here is the schematic I built and a picture of a prototype.
Originally from an early 73 magazine article.  Works well but is not HP/Agilent standard by long distance.

--- End quote ---

Thanks for the schematic. It's very helpful to see how the MMIC can be incorporated into my active probe (hopefully built soon).

Regards,
Frank

KJDS:
How stressed is that 78L05.

7V across it, and at a guess 40mA? A quick check suggests it'll get warm enough to worry me enough to either lower the supply voltage if practical or if you're going to mount it away from the probe then use a 7805 instead.

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