[ARC5] S+N/N ratio results.

[Bob Macklin]

I don't have a lab quality signal generator. I don't make real measurements. 
I just align receivers.

I currently use a Heathkit IG-102. I serves my needs.

Last week I was tweaking my SB-301 and the 2 VHF converters getting ready 
for the VHF contest.

After tweaking the 2M converter when I disconnected the cable from the 
IG-102 from the converter the signal increased by a significant amount. 
Having a good external attenuator probably would not really have helped.

I'm 79 and POOR now so getting a better signal generator is probably not 
going to happen.

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- 
From: "Roy Morgan" <k1lky at earthlink.net>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: "ARC5" <arc5 at mailman.qth.net>
Sent: Friday, June 14, 2013 7:42 PM
Subject: Re: [ARC5] S+N/N ratio results.



On Jun 14, 2013, at 7:31 PM, Richard Knoppow <1oldlens1 at ix.netcom.com> 
wrote:

> …  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.

I've thought about this and thought of taking the idea further:

Make a nice "50 ohm" load for the receiver end of the cable with 50 and 0.5 
ohm resistors, take the signal off the 0.5 ohm point and factor in the 100:1 
voltage reduction.  Almost *any* receiver input impedance or resistance will 
get one hundredth of the voltage at the load end of the cable.

>   Leakage is a real problem.

The 100:1 divider would not help this: you'd be putting 100 times the 
voltage on the cable (and out some of the leakage points) than would be 
needed with an unterminated cable (receiver input 50 ohms) or 10 times the 
voltage if a 10 db pad is used.

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.

> ...   At any rate, getting an approximate measurement on a boat anchor is 
> possible.

Oh, good!  I'd rather not thing that all this is in vain.

> A meaningful measurement on a receiver with fractional microvolt 
> sensitivity is quite difficult.

I think that many reports of fractional microvolt sensitivity are suspect.

Roy

Roy Morgan
k1lky at earthlink.net
K1LKY Since 1958 - Keep 'em Glowing!



______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html