[ARC5] S+N/N ratio results.

[Richard Knoppow]

----- Original Message ----- 
From: "Geoff" <geoffrey at jeremy.mv.com>
To: "Roy Morgan" <k1lky at earthlink.net>; "Richard Knoppow" 
<1oldlens1 at ix.netcom.com>
Cc: "ARC5" <arc5 at mailman.qth.net>
Sent: Saturday, June 15, 2013 1:42 PM
Subject: Re: [ARC5] S+N/N ratio results.


> I've read that the Collins lab procedure to check 
> sensitivity was to hook the (50 ohm) generator output 
> directly to the receiver with no terminating resistor or 
> pad.  Included in what I read was the statement that the 
> receivers had actual input impedances close enough to 50 
> ohms to make this work out right.
>
> ** Interesting, do you have any citations to that?

     If you mean the input impedance I've measured it on my 
R-388 using a General Radio 1606A bridge. When the antenna 
trimmer is exactly peaked its 50 ohms or very close.  I made 
some measurements the other day on an RCA AR-88. There is 
really no input impedance specified but for all but the 
broadcast band a 200 ohm resistor is specified as the dummy 
antenna.  This, of course, does not mean the input impedace 
is 200 ohms only that the receiver is designed to work from 
this impedance.  I made a number of measurements using a 
Boonton Radio 250A RX-Meter.  For those not familiar with 
this somewhat ancient instrument it is a self-contained 
impedance bridge with a stable oscillator, modified Schering 
bridge and tunable detector. The Schering bridge measures 
_parallel_ impedance.  The impedance of the AR-88 antenna 
input was not constant varying from around 200 ohms to 
around 550 ohms at different frequencies. I peaked the 
antenna trimmer capacitor for each measurement both for 
level as indicated on the S-meter (my receiver has one) but 
also to obtain as nearly as possible a non-reactive reading. 
Quite some time ago I measured the input impedance of my 
Drake R4B. I can't find my notes but believe this also 
measured very close to 50-j0 ohms.
    When checking the sensitivity and noise level of the 50 
ohm receivers I used the output of the 606A directly.  For 
the AR-88 I used a 50 ohm termination.  I also made some 
measurements using the 50 ohm termination and a 180 ohm 
build out resistor to obtain nearly 200 ohms.  I measured 
the voltage across the antenna terminals using a 
Hewlett-Packard 400E voltmeter to determine the actual 
reading vs: the value from the attenuator and meter on the 
generator. I got very much the same values from both 
methods. I made a number of measurements between about 4 Mhz 
and 30 Mhz. there was little fall of of sensitivity or 
increase in noise at the higher frequencies.   This is a 
surprizingly low noise receiver; I was able to get 
consistent readings of about 0.8 uV using the widest crystal 
filter setting, about 3khz and about 1.8 uV using the 
narrowest non-crystal IF setting, about 8 khz. This receiver 
uses 6SG7 RF tubes and a 6SA7 mixer and has very low loss 
coil formers and insullation througout.
   I no longer remember the numbers for the R-388 but it was 
not quite this good.  I found my receiver had about the 
right AVC threshold sensitivity, about 2.5uV, this is what 
results in the S-meter beginning to deflect.  However, Snr 
is measured in all cases with the AVC turned off and maximum 
RF gain. My memory is that the R-388 had Snr of under 2 uV 
for 10 db Snr.
   The 6AK5 and the 6SG7 have similar Gm and equivalent 
noise ressitance so the two should be similar.  Again, this 
measurment is subject to the variation of IF bandwidth and 
shape factor so its not really possible to compare receivers 
of different designs.  For that N.F. is necessary.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com