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I am using a satellite finder as an RF level detector for a sort of "microwave camera."
It works, for some definition of "works," but it doesn't seem to work like I would expect it to.

This is a schematic of a similar device:
enter image description here

The RF sections are different, but the low frequency section (around and after the detector) are the same - mine uses more SMD parts and some of the values are a little different, but the circuit is the same.

VR1 (lower right corner) is on the front of the device and has markings to suggest it is a gain control (marked dB from -6 to 6.)

I do not understand the section from D2/D3 to pin 7 of IC1 - that is the detector and the first amplifier stage.

I assume D2 and D3 together with C19 and C20 form a fullwave rectifier. If it is not fullwave, then I don't know why there are two diodes.

R8 and R9 are voltage divider to provide ~6V to the positive inputs of the 4558, forming a sort of "virtual ground."

R12 and C17 and C18 form a low pass filter to keep sudden changes in the RF level from slamming the meter too fast.

Now we come to the parts that I don't get at all.

VR1 is in series with R8 and R9. For the "virtual ground," this makes the lower half of the voltage divider a bit higher resistance, resulting in a slightly higher voltage for the positive inputs to the op-amp. That is fine, but what in heavens is the voltage from the wiper used for? It goes to the RF side of D2 and D3. Is it providing some kind of variable biasing for the detector?

What does VR1 really do?
Does the voltage there provide some kind of biasing to D2 and D3?
Does this result in some kind of thresholding effect? What effect does VR1 have on the gain of the first amplifier stage?

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  • \$\begingroup\$ Why not post the exact schematic? What do you mean by "virtual ground" - do you mean mid-rail voltage? \$\endgroup\$ – Andy aka Jan 13 '16 at 8:49
  • \$\begingroup\$ I couldn't find the exact schematic for the one I've got, and tracing the schematic in the one the one I'm using would require a complete disassembly. The way that thing is built, I'd have to unsolder the RF-connectors to get it out of the housing. The soldering iron I've got isn't up to the job of heating that large a mass. The areas I can see match well with the schematic I posted (at least in the low frequency section,) but I can't get at the back side to check all the connections. I'm afraid that if I post a schematic updated with what I can see then it might not be consistent. \$\endgroup\$ – JRE Jan 13 '16 at 8:55
  • \$\begingroup\$ Regardless of whether this is really the correct schematic, I don't understand this schematic. I'd really like to know what is going on with the detector and VR2. If it helps me understand the one I'm really using that would be great, but the real mystery is the circuit as posted. \$\endgroup\$ – JRE Jan 13 '16 at 8:58
  • \$\begingroup\$ "VR2 is in series with R8 and R9" - I don't get that at all? \$\endgroup\$ – Andy aka Jan 13 '16 at 9:09
  • \$\begingroup\$ R8 is connected to one end of VR2, R9 to the other end. VR2 is in series with R8 and R9. It is in the lower right hand corner of the schematic. Is drawn a little strangely, but the three parts are in series. \$\endgroup\$ – JRE Jan 13 '16 at 9:16
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Agreed -- as drawn, that wiper connection doesn't make much sense. (I assume you are referring to VR1, not VR2.) The worst part of the wiper connection is that it will tend to short out the RF signal if set to the left extreme, but VR1 doesn't appear to be configured as an RF attenuator.

Perhaps the schematic was supposed to show the wiper connected to the anode of D3 (not cathode). With an anode connection, VR1 would allow trimming of the offset (or effective threshold) of the first op amp through the DC path of R7 and R20.

When RF is present at the shared node of D2 and D3, the diodes will pump current through R7, which creates a DC voltage that the op amps can detect.

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  • \$\begingroup\$ Yes, the wiper on VR2. That thing bugs me for a couple of reasons. The first being what it actually does, the second being that it is connected straight to a point that has RF on it. I'd have expected it to have at least an inductor between it and the diodes and don't really understand why it doesn't. \$\endgroup\$ – JRE Jan 13 '16 at 9:02
  • \$\begingroup\$ @JRE: There seems to be some confusion over the reference designator (schematic versus your physical unit?), but I think we are both talking about the 500-ohm pot with the strange wiper connection to the RF point. It still just looks like a schematic mistake where the wiper should connect to the anode of D3 instead of the cathode. With an anode connection, the pot could adjust the threshold of the detector without shorting out the RF point. \$\endgroup\$ – Fred Schleifer Jan 13 '16 at 9:34
  • \$\begingroup\$ Sorry, you are correct. VR1, the 500 Ohm potentiometer. \$\endgroup\$ – JRE Jan 13 '16 at 9:39
  • \$\begingroup\$ And, yes, the wiper really connects to where the scheamtic shows it. The one I'm using has the VR1 located less than 1cm from D2 and D3, and the trace is visible. The wiper really connects to the junction of D2 and D3. \$\endgroup\$ – JRE Jan 13 '16 at 9:40
  • \$\begingroup\$ @JRE: With that connection, D2 can still work as a detector -- as long as VR1 doesn't load down the RF point too much by adjusting it to the extreme left. However, D3 could "undo" some of the detection work of D2 as \$\endgroup\$ – Fred Schleifer Jan 13 '16 at 9:50

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