[R-390] R390 Dave N3DT, ask and then what is the input Z of the IF?

Bob kb8tq kb8tq at n1k.org
Fri Mar 15 16:09:00 EDT 2019


Hi

Well unless your generator is broken / not a resistive source, there will be a 6 dbv difference (2:1 voltage) 
between open circuit (full voltage on the terminals) and terminated ( half the voltage across the source R and
half the voltage on the load R). 

Since at the levels we use them, modern generators have big stacks of attenuators going to the output, it’s a 
really good bet that the output is a resistive source. It may or may not behave that way on the highest output 
range. It very much should everywhere else. 

Bob
KB8TQ



> On Mar 15, 2019, at 2:46 PM, dog <agfa at hughes.net> wrote:
> 
> Yes, my gen is only calibrated into 50Ω. I really wasn't aware that the IF input was hi Z, I really didn't look at it, but it makes sense given the filters are around 2K as I remember. There is a difference if I terminate the sig gen or not, but it's not a huge difference, certainly not 3dB as I remember. I understand where Roger is going with this though, it makes a lot of sense, but one can come at a baseline that can be used in a particular shop for reference for the IF modules. It's probably not worth the effort to try measuring the input of the IF, I may try the RF input one of these days.
> 
> I never did understand the diff in EMF and uV, it sort of escapes me because I've always worked in dBm where I've worked. Then there's EMF p-p too isn't there? I sort of remember that somewhere.
> 
> Thanks all,
> 
> Dave N3DT
> 
> On 3/15/2019 14:07, Jacques Fortin wrote:
>> Roger, when you write: I suggest 150 uv into the IF deck at 455 and set the DC gain for - 7 volts on the DC load.
>> Would you mean 150µV EMF (Electro Motive Force) or 150µV if the signal generator is terminated in 50 ohms ?
>> This is because there is a 6 dB difference between the two....
>> And I believe it was the intent of the question from Dave, N3DT.
>> 
>> Many RF generators are calibrated regarding their output amplitude only if they are terminated in a 50 ohms load.
>> In that context, if the output amplitude is adjusted to 150µV and left open-circuited (or connected to a very high impedance) 300µV will appear at the output, as being the voltage of the internal equivalent source of the generator, right ?
>> And this is what we call 300µV EMF...
>> In the case of the R390-A IF module, because the input impedance is very high, are we talking about 150µV EMF or 300µV EMF needed ??
>> 
>> 
>> 73, Jacques, VE2JFE
>> 
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