[Elecraft] Optimum Receiver Performance

Wed Oct 30 21:19:22 EDT 2019

I just had a graphic demonstration of the fact Bob mentioned when I was 
just rebuilding an old R-390 receiver a few weeks ago. I was pleased 
that I saw an AM sensitivity (30% modulation in a 4 khz BW) of 0.49 
microvolts for a 10 dB S/N ratio at 26 MHz. I then realized that I had 
the IF gain (an internal pot) set way too high. I re adjusted it to drop 
the gain and saw 0.28 microvolts in a 4 kHz BW and 0.14 microvolts in a 
2 kHz BW .  More isn't always better.

Dave K1WHS

On 10/31/2019 1:03 AM, Bob McGraw K4TAX wrote:
> In writing this, I do so, saying that I find the method to be 
> excellent and reliable and repeatable with regard to HF radio receiver 
> operation for all modes.
>
> In general it seems that hams have the more incorrect notion or idea 
> where more gain is needed to hear a weak signal.  To some degree in 
> certain conditions that is true, but not understanding the why and the 
> how can actually lead to worst receiver performance issues.  In my 
> communications with Rob Sherwood, {Sherwood Engineering, Inc.}  both 
> face to face and in writing, I've learned that most if not many hams 
> use too much RF Gain with their receivers.   All legacy receivers are 
> typically designed for 10 meters, thus attenuation is desirable on the 
> lower HF bands. I've adopted this recommend practice and I'm amazed at 
> the weak signals that I would often miss hearing that I now can hear 
> and pull out of the noise.
>
> If the no signal band noise is reading upscale on your S meter, add 
> some attenuation and/or reduce the RF Gain.  There is no point in the 
> noise running the AGC.   You would like the no signal band noise to be 
> about 10 dB above the noise floor of the receiver.
>
> If the receiver noise floor is 10 dB below band noise, the receiver is 
> contributing less than 0.5 dB of the total noise. Thus more gain means 
> more receiver noise.    So how does all of this work?    As an 
> example; the value taken from the manufactures specifications we find 
> the receiver noise floor to be -130 dBm. Therefore, the target of 10 
> dB above the receiver noise floor for the no signal band noise would 
> be -120 dBm.   Band noise, by band, varies as much as 30 dB over the 
> bands 160M - 10M.    In a noisy urban environment it is anybody's 
> guess at to you band noise level. The point being, you must adjust 
> receiver gain, and attenuation based on your antenna and your location 
> to attain be best results.   It will most assuredly be different for 
> each band and antenna to antenna.    And with directional antennas, 
> some directions are noisier than others.
>
> Here's how one is able to determine band noise.   On a clear frequency 
> for that band and with the chosen antenna, with no attenuation, no 
> preamp and RF gain a maximum, observe the S meter value.   For our 
> example; lets say the S meter reads S-5 on no signal band noise.  This 
> is equivalent to -97 dBm.   There are charts readily available to 
> convert S units to dBm.  {see ARRL Handbook}  We accept the standard 
> of 6 dB per S unit.   Now for the math; our target of 10 dB above 
> receiver noise floor is -120 dBm and our band noise is -97 dBm.  The 
> difference is 23 dB. This says we need to use some value of 
> attenuation, example 15 dB, and 8 dB of RF gain reduction for a total 
> of 23 dB.  This then places the band noise at 10 dB above the noise 
> floor of the receiver. Depending on your receiver, you may have 
> different values of attenuation choices.   Also we must realize with 
> directional antennas that band noise will likely vary with direction.  
> Usually worst case and best case conditions must be observed in order 
> to resolve a nominal value.
>
> As to the method for adjusting RF Gain, most receivers' S meter will 
> move up scale as RF Gain is reduced.  At 6 dB per S unit, reducing the 
> RF Gain such that the receiver S meter indicates 3 S units would be 
> equivalent of 18 dB.   This is easily seen when the receiver is 
> connected to a dummy load.
>
> Another way to assure you have this correct is to connect an AC 
> voltmeter, one that reads dB to the audio output of the receiver. 
> Adjust the receiver gain such that the meter reads -10 db when the 
> receiver is on a dummy load.  Then connect the antenna.  If the noise 
> goes up 3 dB that says the the receiver is contributing 1/2 of the 
> noise.  If you can barely hear your antenna connect, you will need 
> more RF Gain or less attenuation.
>
> As to the use of the receiver preamp, again that is some amount of 
> gain added to the receiver chain and must be considered accordingly.  
> As a rule, the preamp should only be engaged on the higher bands, 6M 
> and 10M and to a lesser extent on other bands. Also one must take into 
> consideration of the noise floor of the receiver with the preamp engaged.
>
> For further study on the subject, visit Sherwood Engineering for one 
> of his papers, videos, or PowerPoint presentations  for Contest 
> University.  Look  under the heading of "Presentations, Tips and How 
> to, and White Papers" . http://www.sherweng.com/
>
> 73
>
> Bob, K4TAX
>
>
> On 10/30/2019 7:33 PM, Bill Frantz wrote:
>> There is no reason to always run RF gain at maximum. Otherwise there 
>> wouldn't be a control.
>>
>> I have always set up FT8 with the gains about 12 o'clock and adjusted 
>> the RF gain so the signal strength meter in wsjt-x is about 60 to 70. 
>> This adjustment gives the maximum level for the weak signals without 
>> overloading the analog to digital converter. The whole idea is to get 
>> every stage in the RF and AF chains in their linear range.
>>
>> Sometimes when trying to copy a weak CW or SSB signal, dropping the 
>> RF gain so the band noise is barely audible will help.
>>
>> 73 Bill AE6JV
>>
>>
>
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