[R-390] R-390A IF deck input impedance, sig gen reading

[Roy Morgan]

The URM-25 uses a ladder resistive attenuator NOT a waveguide beyond cutoff. 

Roy sends.

> On Mar 15, 2019, at 9:04 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> Hi
> 
> If the output changes very little with a change in load resistance, then the output impedance 
> is very low. If it was very high and you put a 50 ohm load on it …. you get close to nothing at 
> all out. 
> 
> The URM-25 (and many early generators) use a “waveguide beyond cutoff” sort of attenuator. 
> That takes the attenuator out of the equation. They mismatch the load coil in this approach. 
> The net result is an output impedance that may be in the vicinity of 10 ohms. 
> 
> Bob
> 
>> On Mar 15, 2019, at 8:54 PM, Larry H <larry41gm at gmail.com> wrote:
>> 
>> I was curious about the IF deck input impedance, so I checked the loading
>> it has on my URM-25.  It had very little affect on the input voltage, so
>> the impedance must be high.  IE: I set my sig gen for 150 uv and terminated
>> it with a 50 ohm load and it measured 152 uv volts RMS with my HP 400FL AC
>> voltmeter.  I removed the 50 ohms and connected it directly to the IF deck
>> input and centered the URM-25 frequency to the IF deck.  I then measured
>> the voltage and it was 304 uv.
>> 
>> For making the IF gain adjust -7 vdc on diode load, the tm-11-5820-358-35
>> book says to terminate the sig gen with a 50 ohm load and then connect that
>> to the IF deck input.  Now we know why.  They want a real 150 uv to go into
>> the IF deck.
>> 
>> Regards, Larry
>> ______________________________________________________________
>> R-390 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:R-390 at mailman.qth.net
>> 
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
> 
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html