[R-390] EXT: For the curious

[Fisch, Michael]

The URM-25D  manual (NAVSHIPS 0967-187-5010, from the BAMA site) section 4 starting at section 4.2 seems to say what we have all been saying and is written for non-electrical engineers.  Maybe it can provide some clarification.
Mike


-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of Jim Whartenby via R-390
Sent: Tuesday, October 8, 2024 1:34 PM
Cc: R-390 Forum <r-390 at mailman.qth.net>
Subject: EXT: [R-390] For the curious

OKAs an experiment, just for the curious, set your signal generator frequency to what the R-390 is set to and connect the signal generator, which is set for about 10 uV, to the unbalanced RF input BNC.  Set the R-390 for normal operation with the function switch set to MGC, adjust the antenna trim and R-390 KC dial for a peak carrier level meter reading.  Now adjust the RF gain control for near full scale on the carrier level meter.  Record the carrier level meter reading and the diode load voltage.
Now disconnect the coax from the Signal Generator and insert a BNC "T" connector and again reconnect the signal generator to the R-390, connect a 50 ohm resistor to remaining arm of the BNC "T".
What does the carrier level meter now read and what is the diode load voltage?  Increase the signal generator level until the carrier meter and diode load are back to the previous readings.  How much additional signal is needed from the signal generator to bring the meter reading back to the original readings?  Also curious,Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

    On Tuesday, October 8, 2024 at 10:43:30 AM CDT, Bob Camp <kb8tq at n1k.org> wrote:

 Hi

If you terminate the generator in 50 ohms, that is the expected way to use the generator. When the dial says "1uV" you get 1uV.

If you have the generator unterminated, that is *not* the expected way to use it. You set the dial to "1uV" and you get 2uV. You are not using it in the intended fashion (at least not calibration wise).

Simply put: That 6 db does not count if you are looking for 1uV.

If you look a 50 ohm termination and say it's a 6 db drop, that's not a drop that counts in this case.

Bob



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