[ARC5] Quick sensistivity comparison

[Geoff]

----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: "Richard Hankins" <g7rvi at richard-hankins.org.uk>; "ARC-5 Mail List" 
<arc5 at mailman.qth.net>
Sent: Thursday, June 13, 2013 1:04 PM
Subject: Re: [ARC5] Quick sensistivity comparison


Actually, tube-type ("valve") receivers routinely give better LDS / MDS 
(least descernable signal / minimum descernable signal) signals than 
solid-state equipment. Things like phase noise are definitely higher in 
"modern" solid-state equipment. Now, generally for operation on MF and HF, 
the ambient noise levels on the bands negate a lot of the LDS / MDS. That 
is, the noise level is often substantially above the weakest signal that the 
receiver is capable of receiving. As such, the improved sensitivity is not 
needed when an antenna is connected to the receiver.

I routinely get LDS / MDS level of well under 0.1 microvolt (and the leakage 
from the generator is extremely low) on receivers like the Collins S-Line, 
Heath SB-Line, Drake 4-Line, etc. Even on general coverage receivers like 
the HQ-140X, SX-100, etc., the LDS / MDS is often below 0.1 microvolt.

Glen, K9STH



Since you didnt specify mode or receiver bandwith those statements are 
rather ambiguous.
When dealing with vintage receivers that have only fair to poor IF filtering 
I use the AM test for a 10dB SNR at 30 MHz at maximum bandwidth for the 
models mentioned and even .25 to .5uV is a very rare occurance. For CW I 
crank in whatever maximum selectivity is available and of course sets with 
selectable filters will have different results. And then measure MDS which 
is described as 3dB above the noise but many, myself included, can hear well 
below that.

There is also minimal shielding inside most vintage receivers, and impedance 
mismatches that add to the generator and cable issues.
In fact using the lowest range of a generators attenuator is not considered 
good practice and using an external lab quality step attenuator and a much 
higher generator signal results in the expected accuracy with all other 
things considered.

A HQ-140X fully overhauled but unmodified will have a hard time reaching 1 
uV on AM while a slightly modified HRO-60 is an easy .25uV. Radios such as 
the 51J and R-390 families are rather poor at 30 MHz.
I rate my minimally hopped up HRO-60 as the most sensitive on 10M AM of the 
over 100 tube radios I own and a fully overhauled NC-300 with a selected 
6BZ6 and 6BA7 a very close second. I do a lot of 10M AM operating.

I disagree about SS having lower sensitivity at HF and it has been proven 
many times by many people. The HRO-500 and others without PLL phase noise 
are extremely sensitive with very low internal noise. Those with a PLL vary 
considerably but most are very sensitive on a test bench. It is when the 
wide open untuned front ends meet hundreds/thousands of strong signals that 
the PLL becomes a noise generator.
You can hear the noise during a contest by monitoring outside of the ham 
band and the backround noise will rise at the contest start and fall at the 
end.

Carl