dr*audio
Fish fingers and custard!
When troubleshooting tuners, there is a need to check the gain in each rf stage to see where it is being lost, or to check the output of the local oscillator. To do this you need a probe with a high impedance, low capacitance input, so as not to load down the rf circuit and give a false reading. The probe also must have a low impedance output if you are going to use it with a spectrum analyzer, which has a 50 Ohm input impedance. I needed such a probe but they are very expensive, so I decided to design one. I figured an opamp would work if I could find one that had high enough bandwidth. I found such an opamp, the OPA683DBV, with a 200MHz bandwidth, more than enough for AM and FM broadcast bands. I built it on a junk prototype board from work, but you could make your own pcb layout or use perfboard. The opamp in the schematic below is configured for unity gain, however, I measured 20dB of gain, probably because the output impedance of the opamp is lower than 50 Ohms, so going into the 50 Ohm input of the spectrum analyzer you get more voltage swing. The OPA683DBV is designed for +/- 6.5V power supply maximum. I am running mine at +/- 5V. You can run it off a single supply, but then you have to add biasing resistors on the input, to run it at mid supply, and the resistors will govern your input impedance, which you want to keep very high.
Here is the schematic:
Here is what the board looks like:
It's a good idea to shield your board so it doesn't pick up noise or rf to cause false readings. I used brass.
This is just the amplifier for the probe. You could just attach a probe tip right to the board, like a nail, and a ground lead. I made a probe with a cable I can attach to the board. I made it out of a cheap giveaway pen, with a small finishing nail for the tip, epoxy-ed into the top cap of the pen, which I shoved into the end where the tip went. I soldered the cable to the tip and to a ground lead which I inserted through a hole I drilled in the side of the tube.
For the power supplies I used 2 wall wart supplies. Here's what the whole system looks like. Note that I covered the amplifier with electrical tape to keep from shorting something out if it drops into what I'm working on:
Here is the schematic:
Here is what the board looks like:
It's a good idea to shield your board so it doesn't pick up noise or rf to cause false readings. I used brass.
This is just the amplifier for the probe. You could just attach a probe tip right to the board, like a nail, and a ground lead. I made a probe with a cable I can attach to the board. I made it out of a cheap giveaway pen, with a small finishing nail for the tip, epoxy-ed into the top cap of the pen, which I shoved into the end where the tip went. I soldered the cable to the tip and to a ground lead which I inserted through a hole I drilled in the side of the tube.
For the power supplies I used 2 wall wart supplies. Here's what the whole system looks like. Note that I covered the amplifier with electrical tape to keep from shorting something out if it drops into what I'm working on:
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