[Drake] Re: Couple of questions
Bob Stein
[email protected]
Thu, 09 Jan 2003 17:14:48 -0800
Barry L. Ornitz wrote:
> Martin Sole, HS0ZED, asked:
>
>
>>Interesting question time. The Drake manual states 10dB
>>signal to noise at 0.25uV. Is that 0.25uV p.d. or e.m.f?
>>I think Americans use terms like soft and hard micovolts,
>>never sure which is which. I understand p.d. (potential
>>difference) being the voltage across one of two loads,
>>50 ohms from the receiver and 50 ohms from the sig gen,
>>with e.m.f being open circuit volts from the generator
>>with no additional external load.
>
>
> I agree with Jim Garland, W8ZR, about this being the voltage,
> measured across the antenna terminals, from a 50 ohm source.
> You should note that the receiver rarely ever produces a 50
> ohm resistive load.
>
> I too have never heard of the terms "Hard" of "Soft"
> microvolts. As Jim again noted, people are often confused by
> the output of signal generators. Rarely, if ever, is the
> output voltage actually measured in a signal generator.
> Typically the voltage is measured upstream of the calibrated
> attenuator. The output meter (and attenuator) calibration
> take into consideration the fact that the output should be
> properly terminated into 50 ohms.
>
> Since receivers rarely present a matched 50 ohm input
> impedance, an attenuator is often added between the signal
> generator output and the receiver. A six dB attenuator is
> commonly used. This provides a reasonable 50 ohm match to the
> generator independent of the actual receiver impedance. Of
> course, this added attenuation must be accounted for when
> doing the actual receiver sensitivity test.
>
> 73, Barry L. Ornitz WA4VZQ [email protected]
>
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Many years ago I had a General Radio 1001-B Signal Generator. This was a
relatively modern instrument, designed to be competitive with the H-P
606A
(only in price - the design was abominable). The variable output
attenuator was calibrated only in *open-circuit* microvolts, without
dBm calibration.
Even more peculiar was its output impedance. On the highest setting
of the attenuator switch (x 100,000 I believe) the output impedance
was 50 ohms nominal. On all other switch settings, the output
impedance was 10 ohms. So GR supplied a 40-ohm resistor in a housing,
similar to their fixed attenuators, which had the typical GR
connectors on the ends. Except on the highest range, this was
attached to the output connector of the signal generator, providing a
50-ohm source impedance.
It is small wonder that GR got out of the rf instrument field.
Needless to say, I got rid of the GR generator as soon as I was able
to acquire an H-P 606A.
An interesting side note comes to mind. The GR signal generator was
sold to me as surplus to their needs by - guess who? -
Hewlett-Packard! It actually came from their QA department - I wonder
why!
Bob, W6NBI