[ARC5] S+N/N ratio results.

[Richard Knoppow]

----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: <kgordon2006 at frontier.com>; "ARC5" 
<arc5 at mailman.qth.net>
Sent: Sunday, June 16, 2013 7:43 PM
Subject: Re: [ARC5] S+N/N ratio results.


Unfortunately, when the load on the input is significantly 
lower than the actual design impedance, the apparent 
sensitivity of the unit will almost always require a higher 
level of signal. That is, a stronger signal is required with 
the lower impedance than would be required if the input 
impedance is properly matched.

This fact is quite evident in receivers like the Collins 
75A-1 when a coaxial cable fed antenna is attached. Adding a 
balun with a 6:1 (even a 4:1 TV balun works very well) or 
some sort of "antenna tuner" between the coaxial cable and 
the receiver antenna input improves the apparent sensitivity 
of the receiver greatly.

Glen, K9STH


Website:  http://k9sth.com


     Glen, I understand this to mean that the receiver 
impedance is higher than the source impedance, not the other 
way around. If so, I generally agree when it comes to an 
antenna. For a signal generator the voltage at the generator 
output will not be changed a lot by the receiver. The 
question with the generator is how to know what the actual 
voltage is at the receiver terminals.  When one has a 
matched condition, say a 50 ohm generator into a 50 ohm 
receiver (that actually looks like 50 ohms) the output 
system of most generators will read correctly.  Where the 
input impedance is high (say 400 ohms) its probably better 
to terminate the generator in its characteristic impedance 
and calculate how much the voltage will be pulled down by 
the receiver impedance in parallel with the generator and 
termination.  Since I have a generator which will put out a 
fairly high voltage and have a sensitive RF voltmeter I just 
measure the level at some convenient level to determine the 
correction for lower levels.  This does seem to work.
    My Hewlett-Packard 606A is calibrated for the voltage 
into a 50 ohm termination and has a true 50 ohm source 
impedance. That is, the open circuit voltage is double what 
the attenuator and meter say.  However I also have a General 
Radio 1001A generator which has a much different 
arrangement.   The output attenuator presents a 10 ohm 
impedance at the panel connector for all except the highest 
level on the step attenuator where it is actually 50 ohms. 
For the lower steps its necessary to use a 40 ohm coaxial 
resistor in series with the output.  The resistor is one of 
the accessories delivered with the generator. Because of the 
way the output circuit is arranged it reads open circuit 
voltage so one must divide the reading for the actual 
effective voltage. Where its set up for 50 ohms into a 
termination or matched load the voltage is, of course, half 
of the indicated voltage. This is a PITA but I got used to 
it before I got the -hp- generator.  Another problem with 
the GR generator is that it has rather poor amplitude 
modulation and a _lot_ of incidental FM at higher 
frequencies. Its also damn heavy.


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