[R-390] R-390A sensitivity measurements

[Bob Camp]

Hi

One or two minor items:

When you put an 82 ohm shunt resistor on the generator that gives you a 
31 ohm source impedance for the combination (50 ohms from the generator 
in parallel with the 82 ohms you just put in).

When you sick a 75 ohm resistor in series with the receiver you get a 
106 ohm source impedance.

The 75 ohms is in series with the assumed 125 ohm input impedance of 
the receiver. That gives you 200 ohms.

The 200 ohms in parallel with the 82 ohms gives you 58 ohms. That's 
pretty close to a 50 ohm match to the generator.

The net result is that you are almost matched to the generator, and 
sort of matched to the radio.

If you refigure the resistors you can hit both terminations at the same 
time. In this case you have put in a second termination on the 
generator. You need to subtract six db from the generator output to get 
the correct value for the driving voltage relative to the generator 
output.

The real question is weather any of this will make the radio work 
better.

Any time you put a resistor on the input of a radio you degrade it's 
sensitivity. This is true weather the resistor is is in series or in 
parallel with the antenna input of the radio. If sensitivity is your 
goal stay away from resistors. They can improve overload performance 
but you will always trade off sensitivity.

When you use the radio you peak up the sensitivity with the antenna 
trimmer. As soon as you move it off of the "straight up" position you 
are changing the radio's input impedance to something other than the 
best setting for the impedance you aligned it with. Assuming the signal 
goes up when you do this then your antenna is not providing the same 
source impedance as your generator. Most all of the time I seem to run 
with the antenna trimmer set to one side or the other of straight up.

The antenna is what we care about. If you could set up the generator to 
duplicate the antenna then you might be able to directly measure what 
is really going on. If you run a vector network analyzer into the 
antenna you can get a pretty good idea of what it looks like. That 
sounds like a lot of work though ....

The easy test is to hook the antenna to the radio. See if the noise out 
of the radio goes up. If it does then the whole front end match thing 
is not an issue. With a reasonable R390 and even a fairly short antenna 
I pretty much always seem to pass this test. The only time it can be a 
problem is above about 16 MHz after the band dies.

	Take Care!

		 Bob Camp
		KB8TQ



On Mar 2, 2005, at 11:02 PM, DJED1 at aol.com wrote:

> A shield room is not a bad idea, but it may be too expensive for most 
> of us.
>  I played around with a couple of items tonite, and the issue of 
> shielding
> came to the fore.   first, I made an autotransformer on a toroid which 
> matched
> 125 ohms to 50 ohms (both unbalanced).   I soldered it to a piece of 
> coax and a
> connector and put it on the back of the receiver... the external noise 
> coming
> in on it was 10 dB higher than that of the receiver.   Definitely not 
> useful
> unless built in a well-shielded case.   So I've put that on the back 
> burner.
> I thought about the very good comments from you guys, and did a few
> calculations which turned up some interesting results.
> First, I concluded that adding an 82 ohm resistor in parallel with the
> receiver, to give the generator a 50 ohm load, and adding a 75 ohm 
> resistor in
> series, to give the receiver a 125 ohm load, both form 2 to 1 voltage 
> dividers
> between the generator open circuit voltage and the voltage across the 
> receiver
> terminals.   Thus either will allow us to use the generator meter to 
> read the
> sensitivity.   I haven't tried a pad which matches both, but requires a
> correction to the meter reading.   That also awaits a shielded box.
> I also did some calculations on the noise floor assuming a 125 ohm 
> resistor
> at room temperature, and the 9 dB noise figure I measured on my 
> receiver.   It
> comes out to a voltage of 0.4 microvolt for 10 dB S/N in a 4 Kc 
> bandwidth.
> I then reran some of my measurements using both the URM-25 and the 
> 8660.
> For measurement with the carrier turned on or off, either with the BFO 
> on or
> with 30% modulation, I got measurements of 0.3 to 0.4 microvolt.   So 
> everything
> seems to hang together, including agreement between the URM-25 and the 
> 8660.
> The only difference is that below 0.5 microvolt on the URM-25 the 
> readings
> don't drop down much as I turn the attenuator.   The 8660, on the 
> other hand,
> just keeps going down into the noise.   So shielding is definitely an 
> issue for
> the old generator.   However, I think I'll keep her.   I was pleasantly
> surprised at the accuracy- the two generators agreed within 1 dB at 
> levels of 1 and
> 5 microvolts.
> I got sensitivity of 1 microvolt in 4 KC bandwidth when I measured 
> with the
> modulation turned on and off.   This is the specified procedure for 
> AM.   I'm
> now satisfied that I understand the methods and results.   The only 
> unexplored
> issue is whether a balun will make any significant difference.
> Ed
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