In message , David Farber
writes
I have a trusty old Sound Technology ST1000A FM generator and I have a
question regarding the variable FM output level. The owner's manual states
that the output impedance is 50 ohms, VSWR1.3, 200Vdc isolation. My
question is if I am using this device to check the sensitivity of an FM
tuner, will the output level be affected if I connect the output terminal to
an RG-58 cable and then use a 75 ohm to 300 ohm matching transformer to make
it compatible with the old FM tuners? It seems to me the 2:1 step up ratio
of the matching transformer should affect the level not even considering
the fact that the 75 ohm input of the matching transformer does not match
the 50 ohm output of the cable.

Thanks for your reply.

The fact that the generator 50 ohm output and coax has a 75 ohm load
will give you a voltage of 0.6 of the generator open circuit voltage. If
the load had been 50 ohms, you would get 0.5 of the open circuit
voltage. The increase is 0.6/0.5 = x1.2 (+1.58dB).

75-to-300 ohm transformer should give you a 2-to-1 voltage step-up
(+6dB).

The transformer will have some loss. This should not be more than about
0.5dB.

So, the voltage at the FM tuner 300 ohm input will be the generator
output (into 50 ohms) + 1.58dB + 6dB -0.5dB = Vout + 7.08dB.

This assumes, of course, that the tuner input impedance really IS 300
ohms (which it probably isn't!).
--
Ian