Ian Jackson wrote:
In message <
[email protected]>, David Farber
Ian Jackson wrote:
In message <
[email protected]>, David Farber
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,
VSWR<1.3, 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!).
I believe the output level dial is calibrated for a 50 ohm load. So
using your numbers if the output dial is set to 10?V then I
calculated a 7.08dB gain to be 22.6 ?V. In effect, the sensitivity
of the tuner at this point is 7 dB worse than what the dial
indicates. Is that right? Thanks for your reply.
The calculation looks OK. However, I suppose it depends on what your
'standard' impedance is.
If you have a 'normal' halfwave dipole at (say) 100MHz, the
impedance at the centre will be about 75 ohms. It would be normal
to connect it (via a 75 ohm feeder) to a tuner with a 75 ohm input
impedance. [Note: It might be more correct to say that the tuner is
designed to work best when fed from a 75 ohms source. In practice,
it might not have a very good 75 ohm input.] Anyway, let us assume
that the level of a received 100MHz FM radio signal level (into 75
ohm) is 1?V.
Now, if you replace the 'normal' dipole with a folded dipole, you
would use 300 ohm feeder and connect it to a tuner with a 300 ohm
input impedance. Ignoring distractions like differences in feeder
loss, the tuner input voltage will be 2?V.
However, despite being fed with twice the voltage, the 300 ohm tuner
won't work any better with the folded dipole than the 75 ohm tuner
works with the 75 ohm 'normal' dipole. In both cases, the input
power is the same. Internally, the electronics will be basically
the same. The only difference will be in the matching circuit
between the input and the RF stage.
So, it is reasonable to conclude that, when you specify the
sensitivity of a receiver, you have to specify the impedance. In
your test, if your standard is 300 ohms, then you would say that
your
tuner was receiving 22.6?V (7.04dB more than indicated on the
generator dial). If it was
75 ohms, it would be 11.3?V (6dB less).
Well, I think I'm correct! What do you reckon?
I don't think you need to mention the impedance of the receiver when
you mention FM sensitivity. If it is has the correct antenna and the
correct impedance matching circuit, the sensitivity of the circuitry
should be enough. No? That way I can compare one receiver/tuner to
another and not care what the input impedance is.