[ARC5] S+N/N ratio results.

[Richard Knoppow]

----- Original Message ----- 
From: "Dennis Monticelli" <dennis.monticelli at gmail.com>
To: "David Stinson" <arc5 at ix.netcom.com>
Cc: "ARC5" <arc5 at mailman.qth.net>
Sent: Thursday, June 13, 2013 8:55 PM
Subject: Re: [ARC5] S+N/N ratio results.


>I think these radios do just fine with weak signals on 40M 
>or lower where
> the atmospheric noise is higher.  I put a passive DBM in 
> front of a BC-453B
> and it will capture whatever is above band noise. 
> Stability is good too.
> The issue is CW selectivity.  You get used to what we did 
> as novices, use
> the analog signal processor between our ears.
>
> Dennis AE6C
>

    I think it depends on the particular receiver. There are 
other characteristics beside noise that affect the 
performance at higher frequencies.  One is the losses in the 
coils and condensers and other insulators. The material of 
the tube sockets and bases can contribute.
    Conventional signal to noise is measured as follows:
Instruments needed; A signal generator with a calibrated 
attenuator capable of putting out very low levels down to a 
fraction of a micro-volt, and with very low leakage, 
adequately shielded cable to attach the generator to the 
receiver. Some means of verifying the voltage level at the 
receiver; an audio frequency volt meter. This will be 
connected at an audio output so does not need high 
sensitivity.

    Most receivers that have a specification for signal to 
noise also specify a dummy antenna or a source impedance for 
the measurement. The voltage at the generator terminal needs 
to be known if the measurement is to be meaningful. Some 
receivers have fairly constant input impedance, some do not 
but specify a dummy antenna to allow for reproduceable 
measurements.
    The generator is modulated 30% at (usually) 400 hz. 
Preferably the generator should be capable of low distortion 
modulation with low incidental FM so that the sidebands are 
symmetrical.
    We will assume the measurement is for a 10 db signal to 
noise ratio. If for some other value set the levels below 
accodingly.
    The audio meter is attached to the receiver.  AVC is 
turned off and RF gain is set at maximum. This is not 
changed. A reference level is chosen for the noise, 
preferably a level where there is no hum or other audio 
noise but enough audio dynamic range to allow at least a 10 
db increase.  Set the generator for a low value of output 
with no modulation, say one microvolt.  Tune and peak the 
receive carefully and set the audio gain for the reference 
you have chosen to be zero level.  turn on the modulation. 
The output will go up. Adjust the receiver audio gain for an 
output exactly 10 db above the noise reference (for a 10 db 
ratio). Now, turn the modulation off again and read the 
noise. If not still at zero (probably will not be) adjust 
the _generator_ output level to bring the noise back to the 
reference.  Now start the modulator again and adjust the 
receiver AF gain for ten db above the noise again.  You may 
have to juggle these two several times to get it to where 
the level with modulation is exactly 10 db higher than the 
noise. When you acheive this read the level of the RF output 
from the attenuator or meter on the generator.  This is the 
RF level for 10 db Snr.  This level is likely to change with 
frequency depending on receiver design. It will also change 
with IF bandwidth and is meaningless without a statement of 
the bandwidth.  It will also change somewhat with the shape 
of the IF filter being less for a given bandwidth if the 
skirt selectivity is greater.
    Many receivers give specs for a 10db Snr but not all. 
For instance, the RCA AR-88 gives one for 6db and one for 20 
db.
    Also note that many receivers give only a minimum RF 
level for a minimum audio output level.  This is meaningless 
other than being a simple test of of the receiver's 
condition.
    Typical 10db signal to noise ratios with typical IF 
bandwidths will vary from around 5uv to less than 1 uV 
partly due to the variation in bandwidth.  Older receivers 
with fixed bandwidths will vary from around 6 khz down to 
around 3 khz.  If the SNR is measured at seveeral 
frequencies over the range of the receiver it will give some 
idea of the consistency of performance. Very good receivers 
will not vary much.
    Single conversion receivers may show higher noise at 
high frequencies because one is seeing the image noise as 
well as the noise at the main signal tuning. This is 
probably significant only for single RF receivers with low 
IF frequencies.
    My AR-88, which is a remarkable receiver for its time, 
requires less than 1 uV for 10db SNR right up to 30 Mhz 
using the widest crystal filter setting, about 3 khz.
    I've measured a number of other receivers this way but 
have misplaced my notes.  By memory the SP-600 did about 2 
uV for the 3 khz bandwidth.  It should have done as well as 
the AR-88 but its possible the bandwidth of the AR-88 is 
narrower. However, the AR-88 gave 1.8 uV at the 8 khz 
bandwidth setting so I think perhaps its actually pretty 
quiet.
    I stress again that this method does not produce a true 
noise measurement. Its mostly useful for trouble shooting. 
A true measurement would measure _Noise Figure_ which is a 
meaasure of the noise from a receiver or amplifier in 
comparison to the theoretical minimum noise. That is, the 
ratio in db of the noise output of a practical circuit to 
the noise from an ideal noisless circuit. Noise figure is 
measured using a noise generator.  Noise generators can be 
made so that the noise output can be known very accurately. 
The measurement is simply a reading of the output on the 
generator to the output with a proper termination on the 
amplifier or receiver.  The ratio in db is the N.F.
    Hewlett-Packard published an application note, actually 
a book on N.F. measurement which is available on line from: 
http://www.hparchive.com/appnotes.htm
   look for AN-57-1 and AN-57-2  while these are oriented 
toward microwave applications the theory holds for lower 
frequencies as well.


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