Sound Technology FM generator question.

In message , David Farber writes

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!).

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Ian

I believe the output level dial is calibrated for a 50 ohm load. So using your numbers if the output dial is set to 10µV then I calculated a 7.08dB gain to be 22.6 µV. 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.

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David Farber
David Farber\'s Service Center
L.A., CA

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?

--
Ian

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. It's kind of like stating that the power of a speaker is independent of what type of amp it's hooked up to.

I did a little more digging in the owner's manual. There is an intermediate, almost lossless, antenna matching network (which I don't have) that is supposed to be hooked up between the tuner and the generator to make things go together nicely. There is also a simple schematic diagram of how to construct one yourself if you don't have the Sound Technology matching device but the drawback is the ~6dB attenuation. So to go from the 50 ohm unbalanced output of the generator to the 300 ohm balanced input of the tuner would require three resistors connected as follows:

R1 130 ohms from the center terminal output of the generator to one of the

300 ohm inputs of the tuner. R2 150 ohms from the shield side of the generator to the other 300 ohm input of the tuner. R3 62 ohms which goes directly across the generator output.

In this configuration, the readings on the dial are approximately twice the actual output. In other words, if the dial is reading 10?V, then there is only 5?V going to the tuner.

Thanks for your reply.

--
David Farber
David Farber\'s Service Center
L.A., CA

Dave,

Sound Technology offered a 50 to 300 ohm transformer, model 100, that allowed you to direct read the dial on the 1000A. there was also one for 50 to 75 ohm conversion, but I've never seen one.

Regards, Tim Schwartz Bristol Electronics

No. If you have a 75 ohm receiver which requires 'V' volts for a given SNR, a 300 ohm receiver would require '2V' volts to give the same SNR. If you were comparing the two, you would need to mention what impedance applied.

For all you know, the 300 ohm receiver is simply the same model as a 75 ohm version, except that it has an internal 300-to-75 ohm step-down transformer. For the same performance, the 300 ohm model would definitely need twice the input voltage.

If you have 75 ohm signal source and a 300 ohm receiver, you can get 'free' gain by simply using a step-up transformer to give you more source voltage. The limit is when the transformer matches the source to the load. If you try to us a step-up transformer between a 75 ohm source and a 75 ohm receiver, you will actually get LESS into the receiver than you would with direct connection.

Ah, but.... With audio systems, the output impedance of the source is very low compared with the load impedance. The power fed into the speaker only really depends on the source voltage and the speaker impedance.

This will probably OK if the receiver input is completely floating, with no reference to ground (including a centre tap on receiver input coil or transformer - if it has one).

I reckon that you are better investing in a simple ferrite-cored

75-to-300 ohm balun / matching transformer (or make one - dead easy) and allow for the small corrections discussed previously. I see that Tim Schwartz has suggested 50-to-300 ohms but, to me, 75-to-300 ohms is much easier as it is a simple turns ratio of 2:1. If you really want 50-to-300 ohms, it would be simpler to add a couple of resistors to act as a resistive 50-to-75 matching pad. But, in over 40 years in cable TV, I never worried too much about mixing 50 and 75 ohm impedances (but only where it didn't matter, of course!).

--
Ian

If I'm understanding you correctly, this is analogous to the ratings on a volt meter where you would need to know the internal impedance of the meter to be able to factor in circuit loading. Yes?

Thanks for your reply.

--
David Farber
David Farber\'s Service Center
L.A., CA

Hopefully the opposite (I think). With luck, the impedance of your voltmeter will be sufficiently high that has negligible loading effect on any circuit you are likely to come across.

Just to recap, I think that you don't need to worry too much about the difference of impedances between the source and the load. Even if it is balanced, provided that the input of the receiver is floating, you should be able to feed it directly from an unbalanced generator (with or without any additional resistors).

But what you do need to take into account is that, for the same performance, a high impedance input receiver expects a high input voltage, and a low impedance input receiver expects a low input voltage. It doesn't really matter how the voltage gets to the receiver input - as long as you know what it is (whether by measurement or by calculation).

If you are concerned about whether the receiver input is floating, you can overcome this with a simple balun. This can be a 1:1 ratio transformer so, effectively, the receiver will still be fed from a 50 ohm source. You simply use a few corrections (as discussed) if the receiver input is not 50 ohms. However, as you know that you have a 50 ohm generator and a 300 ohm receiver, you might as well use a home-made or an off-the-shelf matching transformer and balun. You can get these with an F-connector for the 75 ohm end and a couple of spade connectors for the 300 ohm end (I have a couple myself). I see that Radio Shack sell them:

Although the technical spec is more-or-less non-existent, the insertion loss should be about half a dB.

--
Ian

Tuner sensitivity is measured in dBf -- dB with respect to a femtowatt. 2uV into 300 ohms is the same power as 1uV into 75 ohms.

In message , William Sommerwerck writes

Ah! That is what I would have expected.

The point is that, in order to measure the sensitivity, you don't have to feed one from a 75 ohm signal generator, and the other from a 300 ohm signal generator. But you do need know what the input voltage really is.

Of course, for this sort of equipment, you probably can't rely on the input impedances being anywhere near the nominal values. What you calculate and what you get could be quite different. For greatest accuracy, it would be best to use a resistive and/or transformer matching circuit.

For matching between 50 and 75 ohms, a cheap cable TV in-line attenuator (the type with F-connectors) suffices. 6dB is enough, and (if my brain is working correctly) the attenuation would be 6db - 1.58dB = 4.42dB. [The 1.58dB is the voltage rise when going from a 50 ohm source to a 75 load.] If you use a transformer to get to 300 ohms, you'll need to subtract the loss of around 0.5dB, which makes an attenuation of around

4dB. So you're 6dB up because of the 75-to-300 ohm conversion, and 4dB down because of the attenuator pad and transformer loss - making a convenient +2dB higher than what the output meter on the signal generator indicates.

--
Ian