[ARC5] S+N/N ratio results.

[Richard Knoppow]

----- Original Message ----- 
From: "Geoff" <geoffrey at jeremy.mv.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: "ARC5" <arc5 at mailman.qth.net>
Sent: Friday, June 14, 2013 2:22 PM
Subject: Re: [ARC5] S+N/N ratio results.

     For the most part I agree with your remarks. Except 
for:
Audio meter; the meter may be calibrated for some db 
reference or it may not, however, we are concerned only wtih 
ratio, not absolute level so the impedance of calibration is 
of no consequence. One can make the measurement on a 4 ohm 
output or a 600 ohm output.

     As far as RF matching, its more complicated.  The 
critical thing is to know what the voltage is at the 
receiver terminals. You are quite right that the only way to 
be sure is to measure the actual impedance presented at the 
antenna terminals with a bridge.  OTOH, one can just feed 
from a low impedance and assume the voltage will be close. 
I mean a terminated output from a 50 ohm generator is 25 
ohms and will not be much affected when connected to a 
receiver with 200 or 400 ohms input. At any rate close 
enough for the sort of rough measurement we are making.
    Leakage is a real problem.  Laboratory grade generators 
generally have very low leakage but the connections to the 
receiver must be well shielded and that presents a problem. 
One can have ground loops at RF that screw up any sort of 
sensitive measurement.  The connections to a receiver with 
screw terminals will probably always be leakier than a 
coaxial connector but can be made decent by using coax 
connected to the terminals with almost zero length leads. 
Another problem with the lead from generator to receiver is 
that it may pick up external signals including noise which 
throw off the measurement.  This becomes difficult without 
heroic measures like screen rooms.
     At any rate, getting an approximate measurement on a 
boat anchor is possible. A meaningful measurement on a 
receiver with fractional microvolt sensitivity is quite 
difficult.


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