Hi all
For many years, I've used signal tracers to tracking down problems in larger audio systems as quickly as possible. Where access to equipment is awkward, staring at a scope screen or meter whilst moving about can be hazardous, so before stripping equipment down for a closer look, it's good to narrow the problem area down. Often, all that is needed is a simple pocket size amplifier and speaker but occasionally, an RF detector probe is an asset too.
I have had a few probes for locating RFI, but these are not simple AM diode detectors - one also has a basic JFET preamp with forward bias to get better sensitivity from an HP 5082-2800 series Schottky diode.
My first question is; what diodes, designs or integrated detectors are now considered good for low level RF in the 250 kHz - 250 MHz range? I don't want to waste time and money on experimentation but I do need to improve on this present DIY RF probe setup, which is a little too insensitive.
Also, Is there perhaps a suitable buffer design to use in a test probe for straight line level audio measurements?
Comments and further thoughts welcome too, as you never know the needs of others who may read this.
For many years, I've used signal tracers to tracking down problems in larger audio systems as quickly as possible. Where access to equipment is awkward, staring at a scope screen or meter whilst moving about can be hazardous, so before stripping equipment down for a closer look, it's good to narrow the problem area down. Often, all that is needed is a simple pocket size amplifier and speaker but occasionally, an RF detector probe is an asset too.
I have had a few probes for locating RFI, but these are not simple AM diode detectors - one also has a basic JFET preamp with forward bias to get better sensitivity from an HP 5082-2800 series Schottky diode.
My first question is; what diodes, designs or integrated detectors are now considered good for low level RF in the 250 kHz - 250 MHz range? I don't want to waste time and money on experimentation but I do need to improve on this present DIY RF probe setup, which is a little too insensitive.
Also, Is there perhaps a suitable buffer design to use in a test probe for straight line level audio measurements?
Comments and further thoughts welcome too, as you never know the needs of others who may read this.
Try looking into the Analog Devices AD8307-8310 series of log amp detectors. Very sensitive, with a very large frequency range (sometimes too large). They've have been used for all sorts of RF measurement devices by the amateur radio (ham) crowd, including sniffers, SWR meters and impedance bridges.
Bill
Bill
RSSI IC's are great for this job, but types accepting "low" frequencies (<100 MHz) are becoming rarer and rarer (and more expensive too).
Sometimes ago, I had the occasion to experiment with an LT chip (don't remember the exact number type, the one that does "low" frequencies).
It is indeed very sensitive, but in a modern environment swamped with wifi, GSM and ISM devices, it is near to unusable because of unwanted sources.
Otherwise, better than silicon schottky's you have good old point-contact Ge diodes like OA95 or AA119.
I have built a sniffer probe using a voltage doubler configuration.
The input is a very short pigtail (~1.5cm) of folded insulated wire, and it can be safely inserted anywhere close to components and accurately detect any VHF tendency to oscillation.
I'll try to post a pic one of these days, it is quite effective, and when connected to an oscilloscope it shows the modulation.
It is not extremely sensitive, but when parasitic oscillations break out, their level is normally out of control and easily detectable.
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