[ARC5] Receiver Sensitivity - Power Supplies

[Richard Knoppow]

    There are a couple of standard methods of measuring 
sensitivity. One is to measure the audio output level for a given 
input of modulated RF. This is useful to checking if a receiver 
is operating normally but tells you nothing else about its 
quality or performance.
    The second method is to measure the output noise against a 
30% modulated signal. The method is to set the output at some 
arbitrary level with the input from a signal generator modulated 
with a 400Hz tone at  30%, then turn off the modulation and 
measure the noise. Then adjust the RF level until the ratio 
between modulated output and unmodulated noise is a specified 
amount, often 10db. This is stated as sensitivity in many 
receiver specs. Typically with vacuum tube receivers of good 
quality it is around 2.5 uV. Since this is done with modulation 
there is a variation for CW. This measures the noise of an 
unmodulated carrier against the level of output with a BFO. The 
BFO is switched on and off until the ratio of the output is some 
specified level, again often 10db. The sensitivity is often 
greater this way. It is common for high quality receivers to have 
1.0 uV sensitivity for CW but also many receivers can have a much 
narrower bandwidth for CW than for AM. Comparing the S/N ratio 
for AM with, say, a 7 Khz bandwidth to CW with, say, a 500 Hz 
bandwidth will vary inversely with the bandwidth and may be on 
the order of twenty db for the above. So, if we are talking about 
  a receiver with very narrow bandwidth, say a communications 
receiver with a single crystal filter, a sensitivity of 0.3uV is 
reasonable. If, like many simpler receivers there is only one IF 
bandwidth the sensitivity will not be increased so much but will 
still be on the order of about 1.0 uV for the receiver above. It 
could be greater of course depending details.
     Please note that you really can not compare the signal to 
noise from different designs because of the difference in 
bandwidth and shape of the pass band. A noise factor or noise 
figure measurement eliminates these variations and will give you 
a more directly comparable value for comparison of designs.

On 4/26/2018 5:23 PM, John Hutchins wrote:
> Scott -
> 
> I always thought, that I was imagining that I was hearing things 
> better,  with the BFO on!
> 
> Turn on the BFO;  and throttle back on the RF gain when dealing 
> with an AM station/contact.
> 
> On the BC-348 with the BFO on; it only drops the HV rail ~200 V 
> by  < 1 volt but it surly enhances the reception.
> 
> Thanks for the proof.
> 
> Hutch

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL