Electronics > Projects, Designs, and Technical Stuff
Homebrew spectrum analyser high-impedance probe
philpem:
I'm not sure whether this belongs here or in Test Equipment, but here goes anyway :)
A while ago I picked up an old Advantest R3361A spectrum analyser. The old girl works a treat, despite being near enough twenty years old now. Or rather - she works a treat when probing anything which provides a standard 50-ohm output impedance. I'm sure you can see where this is headed :)
I'm currently debugging a low-distortion receiver for the phase-modulated time signal on BBC Radio 4 (198kHz) and a separate but similar receiver for the Anthorn / MSF time signal on 60kHz. The output impedance of the ferrite loop is high enough that loading it with 50 Ohms is enough to completely kill any signal it picks up. Probing the output of my first-stage RF amp (a common-source JFET amplifier with Zo of ~10k-ohms) works to some extent, but the JFET clearly isn't happy driving it. Probing with an oscilloscope is a complete waste of time - even the amplified signal from the JFET is well below my oscilloscope's noise floor (although admittedly my oscilloscope is a Tek TDS2000B series machine which doesn't have particularly good noise characteristics to start with...)
My question is thus: how would I go about building a high-impedance probe for my spectrum analyser?
I've looked into MMICs (I have a couple of MAR-6 and ERA-2SM chips dotted about) but these appear to be internally terminated for 50 Ohms and will load the 'target' circuit down far too much.
JFETs are another possibility, but the output impedance of such devices is rather low. They might work as a preamp stage, but for a 50-ohm driver? Forget it.
Can anyone suggest anything else I could look into? I'm only after ~150MHz bandwidth at the most - heck, even 20MHz would be a nice start.
Thanks,
Phil.
olsenn:
Active probes that only need a few hundred MHz bandwidth are easy to make, even with just carefully selected op-amps. If you have the equipment to measure/calibrate its linearity and gain, you should be fine
pauln:
--- Quote from: olsenn on April 03, 2013, 01:38:26 am ---Active probes that only need a few hundred MHz bandwidth are easy to make, even with just carefully selected op-amps. If you have the equipment to measure/calibrate its linearity and gain, you should be fine
--- End quote ---
I made an active probe several years back, using a J310 followed by an MMIC, voltage regulator and a handful of other bits all SMD - its not Agilent but it works well . The original article was from a very early 73 magazine.
Happy to send circuit, photo and description via PM if required. I may well have a PCB as well.
Let me know.
Paul
KJDS:
The lazy way of doing it is to just use a length of 50ohm cable with a 470R resistor as the probe. This will reduce the signal seen at the analyzer by (about) 20dB but means the circuit is loaded much less.
The proper way of doing it is with a FET input stage followed by a buffer amp. The main problems with these are stability, probe near a high Q resonance where the JFET is a little marginal and it can oscillate and also the risk of static damage.
fpliuzzi:
Maybe the "Poor Man's 1GHz Active Probe" project by Elektor magazine would help you out...
http://elektrotanya.com/files/forum/2009/10/e04a036.pdf
This high impedance, active probe is on my to do list (need it for my RF Explorer-3G analyzer).
I have the components already, but I need to make the PCB for it.
Specs listed in article for the DIY probe:
Input-Z: 0.75pf//10Mohm
Output-Z: 50 ohms
Bandwidth: 100KHz - 1.5GHz (+-2.5dB)
Gain: -20dB nominal
Noise figure: 1dB (using SOT143 BF998 MOSFET)
Regards,
Frank
Edit: Updated the broken link.
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