Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: RF Gain/S meter/Noise level  (Read 4458 times)
W0TLP
Member

Posts: 83




Ignore
« on: December 21, 2011, 04:53:27 AM »

Dear Elmers,

I am new to HF and am trying to better understand the relationship among the RF gain control, S meter and band noise.

I am using the Yaesu FT857d and LDG analog meter set to show signal strength on RX.

I have a fair amount of noise on the lower bands and I'm still learning to identify normal band noise and potential EMF interference, which I believe my station suffers but I cannot yet pinpoint it.

Those noises change as I adjust the RF gain control. When I turn the control fully clockwise I hear more noise but the S meter drops. Conversely, when I turn it fully counterclockwise the noise silences but the S meter raises to the top to +60 dB.

This is counter intuitive to me. It seems to me that when I hear more noise the S meter should be at its maximum, and vice-verse.

Secondly, how do I know where to set the RF gain? It sounds as though my RX audio is cleanest when I can set the RF gain so the S meter points to 9 when I am hearing only noise, no voices. Again that seems counter intuitive to me.

Using the attenuator silences some of the noise I suspect is RFI but of course also reduces strength of desired signals. Playing with the DSP sometimes helps, too, but again too much silences desired signals. The noise blanker makes no difference.

Finally, when I have the RF gain set so the needle points to 9 -- again, that gives me the cleanest audio of desired signals -- the needle doesn't move or fluctuate with the modulation being received. I'm pretty sure it used to, and I could see "+10 dB over 9" now and then. That may have been on 10 meters, where I experience little noise.

Feedback and observations will be greatly appreciated.

Thank you all.

Teak
« Last Edit: December 21, 2011, 05:06:04 AM by W0TLP » Logged
NR4C
Member

Posts: 308




Ignore
« Reply #1 on: December 21, 2011, 05:42:11 AM »

You have observed a very useful phenomenon. 

Excessive gain tends to overload circuits.  Turn up the AF gain will overload your ears (and brain) and the sound will sound distorted.  RF gain works the same way.  Turning up the RF gain control will flatten the signal and reduce the signal-to-noise relationship.  The higher the signal-to-noise (S/N ratio) the easier it is to copy the signal.  Reducing the RF gain will lower the noise faster than the signal and increase the S/N ratio, making the signal stand out.  I use my ATTenuator most of the time, as it does the same thing.

As for the "S"meter, that is usually tied to the AGC and changing the RF gain, affects the AGC, the result is an apparently reversed reading.  Try setting the RF gain to the point just before the "S"meter starts to move.

Hope this helps....

...bill  nr4c
Logged
AD4U
Member

Posts: 2164




Ignore
« Reply #2 on: December 21, 2011, 05:46:44 AM »

The lower bands (160, 80, 40) are typically noiser than the higher bands.  There are several reasons why.  Sometimes some of this interferrence can be eliminated or reduced AT THE SOURCE if it is local.  If is MUCH better to find and eliminate the noise than to try to reduce it in a receiver.  If it is static crashes and general "low band atmospheric noise", you just have to learn to live with it.  Sometimes a good DSP can reduce it.  Some antennas are more prone to pick up noise than others.  What kind of antenna are you using?

Most HAMs run the RF gain control fully "open" most of the time, which is usually fully CW.  There are times and band conditions when the RF gain can be "backed off" a bit to reduce certain types of interferrence.  For example - if the desired signal is stronger than the band noise, backing the RF gain control off until the band noise is reduced (or eliminated) can make it easier and more pleasant to copy the desired signal.  However if the band noise is stronger than the desired signal, the results are not nearly as effective.

The attenuator and the RF gain control (same thing) reduces receiver sensitivity on ALL signals (as you posted).  Sometimes enabling the attenuator or backing off the RF gain control can reduce adjacent strong signals (near but not on the desired frequency) which can overload the receiver and MAY make it easier to copy the desired signal.  In a properly designed receiver, usually this is not necessary.  Unfortunately many of the low - middle priced HAM receivers (and I put the Yaesu 857D in this catagory) cannot handle REALLY strong signals adjacent to the desired signal.

I have never heard of the S meter "dropping" when the RF gain is turned fully "open" (fully CW).  It usually works the other way.  Are you sure?

Dick  AD4U
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #3 on: December 21, 2011, 06:15:38 AM »

I have never heard of the S meter "dropping" when the RF gain is turned fully "open" (fully CW).  It usually works the other way.  Are you sure?

That's exactly how it works on my FT-857D, my Kenwood TS-440S and several other radios that I've used.

I think of the meter needle as a "threshold" in this situation... i.e. the radio's gain has been reduced to the point that a received station needs to be stronger than a "full gain S9" to move the meter.

I'm not sure that the behavior is that planned, because like NR4C says, the "RF Gain" is tied to the AGC.  I think these radios just supply a fixed AGC voltage to the variable gain amp and that AGC control voltage is also used to run the S-meter Smiley
« Last Edit: December 21, 2011, 06:22:29 AM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W0TLP
Member

Posts: 83




Ignore
« Reply #4 on: December 21, 2011, 06:31:46 AM »

Thanks, guys. This helps.

I suspect I'm doing it right -- adjust things until they sound good without destroying my ears or sanity.

As for excessive noise, I'm in the process of determining what may be RFI and what may be band noise. I'm pretty certain some of it is RFI and will work to eliminate it. The power company should be cooperating soon.  Undecided

I have never heard of the S meter "dropping" when the RF gain is turned fully "open" (fully CW).  It usually works the other way.  Are you sure?

Yep. When I turn the RF gain to the point that I hear nothing the needle points fully past +60 dB. When turned all the way in the other direction -- when I get the most RF gain and the most noise -- the needle drops to zero.

As for the "S"meter, that is usually tied to the AGC and changing the RF gain, affects the AGC, the result is an apparently reversed reading.  Try setting the RF gain to the point just before the "S"meter starts to move.

I think that makes sense.  Grin. Either way, I'll try it.
Logged
VA7CPC
Member

Posts: 2386




Ignore
« Reply #5 on: December 21, 2011, 06:48:28 AM »

Fundamental misunderstanding:

. . . The S-meter does not measure signal strength.  It measures the AGC voltage.

The IF and RF amplifiers are "gain-controlled" amps, and the AGC voltage is their controlling voltage.  As AGC voltage rises, the RF / IF gain decreases.

The manual "RF Gain" control actually controls the _minimum value_ (floor) of the AGC voltage.   As you decrease "RF Gain", the floor AGC increases.  The "S-meter" increases, whether or not there's any signal.  [You can check this by disconnecting the antenna.]

If you turn off the automatic AGC (possible on most rigs), you can get full control of the RF/IF Gain manually.  The "S-meter" goes up as you decrease the gain, whether there's any signal or not.

Most of us have accidentally set the RF Gain to 0 (or close to it), and looked at an S-meter pinned at S9+20 dB, and a silent radio, and wondered:

. . ."What's wrong with this picture?"

You're not alone.

           Charles
Logged
AD4U
Member

Posts: 2164




Ignore
« Reply #6 on: December 21, 2011, 06:58:34 AM »

N3OX - you are right.  Perhaps I mis-understood the termanology or what he meant when he said the S meter dropped when the RF gain control was advanced from the fully CCW position to the fully CW position.  

On most rigs the S meter fully "peg" on the right side S9+60 of the meter face when the RF gain control is fully CCW.  As the RF gain control is advanced toward fully CW, the S meter will move toward the left side S0 of the meter.

I stand corrected on what I mis-understood that he was trying to convey.

Dick  AD4U

Logged
W8JX
Member

Posts: 5790




Ignore
« Reply #7 on: December 21, 2011, 07:02:06 AM »

Fundamental misunderstanding:

. . . The S-meter does not measure signal strength.  It measures the AGC voltage.

The IF and RF amplifiers are "gain-controlled" amps, and the AGC voltage is their controlling voltage.  As AGC voltage rises, the RF / IF gain decreases.

The manual "RF Gain" control actually controls the _minimum value_ (floor) of the AGC voltage.   As you decrease "RF Gain", the floor AGC increases.  The "S-meter" increases, whether or not there's any signal.  [You can check this by disconnecting the antenna.]

If you turn off the automatic AGC (possible on most rigs), you can get full control of the RF/IF Gain manually.  The "S-meter" goes up as you decrease the gain, whether there's any signal or not.

Most of us have accidentally set the RF Gain to 0 (or close to it), and looked at an S-meter pinned at S9+20 dB, and a silent radio, and wondered:

. . ."What's wrong with this picture?"

You're not alone.

           Charles

Nice explanation. Smiley
Logged

--------------------------------------
All posted wireless using Win 8.1 RT, a Android tablet using 4G/LTE/WiFi or Sprint Note 3.
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #8 on: December 21, 2011, 12:42:36 PM »

S meters are one of the least standard things in receivers, both for calibration and operation.

The S meter does not always measure or read in relationship to AGC voltage. Sometimes it does, sometimes it does not.

Some radios show an increase in S meter reading when the RF gain is reduced, some radios do not.

It really depends on how the designer did the system, but I have radios as old as from the 1950's and 60's that do not increase S reading as RF gain is reduced. As a matter of fact, many radios call the IF gain control RF gain, which makes it more confusing.

Here is some stuff on noise:

http://www.w8ji.com/noise.htm

73 Tom
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!