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Hello Earl. Thank you for clarifying that! I agree with your observation that above 10M, RF gain is far less important! In fact not really needed much. For FM mode... zero.
I asked if you were using "electricity" in a lighthearted way 100%. I monitor VHF/UHF quite a lot and I too NEVER need to touch the RF gain! For low band SWL, 120 meters - 13 meters I do use it as needed depending on the time of year and conditions/mode. S/N on the SWL bands is
often a struggle, and fiddling with receive antenna's, RF gain, is key especially on 120m-41m. Yeah, there is life outside the "honey baked ham bands".
J
On Wed, Jan 8, 2025 at 03:13 AM, Joseph K7CBR wrote:
Are you still using electricity?
On Wednesday, January 8, 2025 at 05:19:40 PM EST, N3 EG <n3eg@...> wrote:
The RF Gain control has always been the most useless thing to put on a radio.? It's like a high/low switch on a vacuum cleaner, which should be called "Clean/Less Clean."? In the case of HF radios, it should be called the Ignore Weak Signal Stations And Screw Up Your S-Meter Reading Control.?
?
I've never found a good reason to use it in at least 52 years of HF listening, and it's totally useless on 10 meters and up.
|
A while back a ham published an article discussing what he called the "free 3 dB". I can't find the article, but it essentially discusses when using the RF gain control is advantageous. Bill N0CU
Hello Earl. Thank you for clarifying that! I agree with your observation that above 10M, RF gain is far less important! In fact not really needed much. For FM mode... zero.
I asked if you were using "electricity" in a lighthearted way 100%. I monitor VHF/UHF quite a lot and I too NEVER need to touch the RF gain! For low band SWL, 120 meters - 13 meters I do use it as needed depending on the time of year and conditions/mode. S/N on the SWL bands is
often a struggle, and fiddling with receive antenna's, RF gain, is key especially on 120m-41m. Yeah, there is life outside the "honey baked ham bands".
J
On Wed, Jan 8, 2025 at 03:13 AM, Joseph K7CBR wrote:
Are you still using electricity?
On Wednesday, January 8, 2025 at 05:19:40 PM EST, N3 EG <n3eg@...> wrote:
The RF Gain control has always been the most useless thing to put on a radio.? It's like a high/low switch on a vacuum cleaner, which should be called "Clean/Less Clean."? In the case of HF radios, it should be called the Ignore Weak Signal Stations And Screw Up Your S-Meter Reading Control.?
?
I've never found a good reason to use it in at least 52 years of HF listening, and it's totally useless on 10 meters and up.
|
Bob Heil (SK) did a Youtube video about when you might get better copy in noisy conditions by lowering the RF gain and making some other adjustments.? On Youtube, look for Ham Nation episode 130.
Art Delibert, KB3FJO
toggle quoted message
Show quoted text
A while back a ham published an article discussing what he called the "free 3 dB". I can't find the article, but it essentially discusses when using the RF gain control is advantageous.
Bill N0CU
Hello Earl. Thank you for clarifying that! I agree with your observation that above 10M, RF gain is far less important! In fact not really needed much.
For FM mode... zero.
I asked if you were using "electricity" in a lighthearted way 100%. I monitor VHF/UHF quite a lot and I too NEVER need to touch the RF gain!
For low band SWL, 120 meters - 13 meters I do use it as needed depending on the time of year and conditions/mode. S/N on the SWL bands is
often a struggle, and fiddling with receive antenna's, RF gain, is key especially on 120m-41m. Yeah, there is life outside the "honey baked ham bands".
J
On Wed, Jan 8, 2025 at 03:13 AM, Joseph K7CBR wrote:
Are you still using electricity?
On Wednesday, January 8, 2025 at 05:19:40 PM EST, N3 EG <n3eg@...> wrote:
The RF Gain control has always been the most useless thing to put on a radio.? It's like a high/low switch on a vacuum cleaner, which should be called "Clean/Less Clean."? In the case of HF radios, it should be called the Ignore Weak Signal Stations And
Screw Up Your S-Meter Reading Control.?
?
I've never found a good reason to use it in at least 52 years of HF listening, and it's totally useless on 10 meters and up.
|
The meat of this is that internally generated distortion products increase or decrease in amplitude faster than the desired signal. The figure quoted in the thread linked below is 3 dB for every 1 dB of signal strength: On Thu, 9 Jan 2025 22:53:26 +0000 "Art Delibert via groups.io" <radio75a3@...> wrote: Bob Heil (SK) did a Youtube video about when you might get better copy in noisy conditions by lowering the RF gain and making some other adjustments. On Youtube, look for Ham Nation episode 130.
Art Delibert, KB3FJO -- 73 -Jim NU0C
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AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about. Most receivers do not cope with this very well. I have a real problem with listener fatigue, and its one reason I do not listen to HF SSB or CW with an "open" receiver, unless in an active conversation. I use squelch, or syllabic squelch, and attenuation, a lot! I travel in the SWL circles, some of us use small loop antenna's, and other terminated listening antenna's for the low bands. This puts the noise level of those bands closer to the noise floor of the receiver which is more desirable, allowing less AGC action.
On Thursday, January 9, 2025 at 08:45:59 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
The meat of this is that internally generated distortion products increase or decrease in amplitude faster than the desired signal. The figure quoted in the thread linked below is 3 dB for every 1 dB of signal strength:
On Thu, 9 Jan 2025 22:53:26 +0000
"Art Delibert via groups.io" <radio75a3@...> wrote:
> Bob Heil (SK) did a Youtube video about when you might get better copy in noisy conditions by lowering the RF gain and making some other adjustments.? On Youtube, look for Ham Nation episode 130.
>
> Art Delibert, KB3FJO
--
73
-Jim
NU0C
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That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total. On Fri, 10 Jan 2025 12:05:42 +0000 (UTC) "Joseph K7CBR via groups.io" <k7cbr@...> wrote: AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about -- 73 -Jim NU0C
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have you been checked for rabies? lol* read my whole note from 7:07AM ET. AGC/RF gain was my topic. I shall leave Drake IMD issues to manufacturers and other ops to try and conquer. Im running a 52 year old set of boxes. Run them as clean as I can (xmit/rcv). full stop...
*asking this, could get me banned for life... hope not yet its A-OK...
Joseph
On Friday, January 10, 2025 at 07:53:48 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total.
On Fri, 10 Jan 2025 12:05:42 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
>? AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about
--
73
-Jim
NU0C
|
On 160+ 80m,? reducing the RF gain control works superb.? That and 6 db of attenuation.? ? ?When u reduce the RF gain control, (Into a DL) notice the S? meter comes up......normal.?
What's happening is the agc threshold increases.? ?When you have a S5 to S8 noise level, it's pointless to run the RF gain at max.? Instead, increase the Audio gain a bunch, then reduce the RF gain control....till the S- meter has swung up to the level of the noise, and just above the noise level.? ? Then you will notice the noise drops off like a rock.? Signals and band noise that is below the agc threshold will drop off at a faster rate.?
The above method has been around for eons.? On 40m it will also work, but that depends on band noise.? ? On 30-10m, run the RF gain at max.?
With RF gain at max, agc threshold is extremely low...... and any signal / and / or band noise above the threshold will? be compressed.? IE: per drake, 60 db > S9? signal? will increase the audio output by only 3db...... compared to A S1 signal.? ?That's the whole idea behind agc.?
On the lower bands, with lotsa band noise, you don't require the agc threshold down at S1.? ?With RF gain at max, the S5 to S8 band noise? will be loud in the audio, which you will readily hear when no signals present, between words or just tuning around.?
AGC attack distortion is much reduced when the RG gain control is reduced.? ?On the R4C, attack distortion is not too bad.? ?On my Yaesu? FT-1000D, attack distortion is a built in design flaw since day 1.....and the only fix is to reduce the RF gain a bit.? ?AGC delay plays a role in this too.? With the agc delay set too fast, it just compounds the problem.
What's really needed on RX is...'hang type AGC'.? ? ? As is..... we have.... attack-hold release.? On ham gear we have fast attack, zero hold....and adjustable release.? ?W8ZR and arrl was gung ho abt? hang type agc eons ago.? With fast attack, and a 120 msec hang time....and an adjustable release, ssb audio will sound a lot better.? And if lotsa noise on the lower bands, RF gain is reduced, till the noise is below the threshold.?
Jim? ?VE7RF
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And you are missing my point. Reducing the RF gain makes internally generated IMD products decrease faster than the desired signal. Which is entirely on topic for the usage of the RF Gain control. ACG attack distortion and IMD are certainly related but AGC is not the sole cause of internally generated IMD. On Fri, 10 Jan 2025 15:47:44 +0000 (UTC) "Joseph K7CBR via groups.io" <k7cbr@...> wrote: have you been checked for rabies? lol* read my whole note from 7:07AM ET. AGC/RF gain was my topic. I shall leave Drake IMD issues to manufacturers and other ops to try and conquer. Im running a 52 year old set of boxes. Run them as clean as I can (xmit/rcv). full stop...
*asking this, could get me banned for life... hope not yet its A-OK...
Joseph
On Friday, January 10, 2025 at 07:53:48 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total.
On Fri, 10 Jan 2025 12:05:42 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
? AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about -- 73 -Jim NU0C
|
OK. Just so we are on the same page, I need you to show me how you are measuring all this so I can too can see the empirical evidence and be convinced that this indeed is as critical as you say it is in the 2.1 to 2.8khz pass-band of my device? I can't weigh in on this until you explain and show me how you measured all this.
Joseph
On Friday, January 10, 2025 at 05:38:21 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
And you are missing my point. Reducing the RF gain makes internally generated IMD products decrease faster than the desired signal. Which is entirely on topic for the usage of the RF Gain control. ACG attack distortion and IMD are certainly related but AGC is not the sole cause of internally generated IMD.
On Fri, 10 Jan 2025 15:47:44 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
>? have you been checked for rabies? lol* read my whole note from 7:07AM ET. AGC/RF gain was my topic. I shall leave Drake IMD issues to manufacturers and other ops to try and conquer. Im running a 52 year old set of boxes. Run them as clean as I can (xmit/rcv). full stop...
>
> *asking this, could get me banned for life... hope not yet its A-OK...
> Joseph
>
>? ? On Friday, January 10, 2025 at 07:53:48 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:?
>?
>?
> That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total.
>
> On Fri, 10 Jan 2025 12:05:42 +0000 (UTC)
> "Joseph K7CBR via groups.io" <k7cbr@...> wrote:
>
> >? AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about?
>
>
--
73
-Jim
NU0C
|
There is a demonstration given by a fellow in the link I posted in message /g/DRAKE-RADIO/message/81833 . However it is common knowledge that IMD products change at a faster rate than the fundamentals. Given that there is no such thing as a perfect amplifier your radio has some degree of internally generated distortion products. On Fri, 10 Jan 2025 23:08:06 +0000 (UTC) "Joseph K7CBR via groups.io" <k7cbr@...> wrote: OK. Just so we are on the same page, I need you to show me how you are measuring all this so I can too can see the empirical evidence and be convinced that this indeed is as critical as you say it is in the 2.1 to 2.8khz pass-band of my device? I can't weigh in on this until you explain and show me how you measured all this.
Joseph
On Friday, January 10, 2025 at 05:38:21 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
And you are missing my point. Reducing the RF gain makes internally generated IMD products decrease faster than the desired signal. Which is entirely on topic for the usage of the RF Gain control. ACG attack distortion and IMD are certainly related but AGC is not the sole cause of internally generated IMD.
On Fri, 10 Jan 2025 15:47:44 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
? have you been checked for rabies? lol* read my whole note from 7:07AM ET. AGC/RF gain was my topic. I shall leave Drake IMD issues to manufacturers and other ops to try and conquer. Im running a 52 year old set of boxes. Run them as clean as I can (xmit/rcv). full stop...
*asking this, could get me banned for life... hope not yet its A-OK...
Joseph
? ? On Friday, January 10, 2025 at 07:53:48 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:?
?
?
That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total.
On Fri, 10 Jan 2025 12:05:42 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
? AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about?
-- 73 -Jim NU0C
|
No, that demo you linked would NOT account for observations on my own hardware at a 250hz, 500hz, or even 2.8khz bandwidth as far as "internally generated noise". That has to do with showing the mess in crowded band conditions. I do know that radio hardware have internal distortion, this is not a secret.
The demo you linked to, shows a span of 10MHz, 1MHz/div. Your link does not account for internal IMD in the passband of my receiver at the settings I asked about. He demonstrated IMD of his transmitter. The receiver mixing scheme and inter-stage bias class (through to AF) does factor into this, yet I wanted you to explain clearly and coherently what you mean, and how this is significant enough to be a consideration to our audible perception? In the link video, if I park my R4C (it wont park at 1GHz), and have my 600Hz roofing filter in line, cascading through my 250Hz filter, listening to CW, I wont hear those other signals.
So please explain so we can all get onto the same page. Explain and show how you are measuring this so I can do the same. Thanks
Joseph
On Friday, January 10, 2025 at 08:19:05 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
There is a demonstration given by a fellow in the link I posted in message /g/DRAKE-RADIO/message/81833 . However it is common knowledge that IMD products change at a faster rate than the fundamentals. Given that there is no such thing as a perfect amplifier your radio has some degree of internally generated distortion products.
On Fri, 10 Jan 2025 23:08:06 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
>? OK. Just so we are on the same page, I need you to show me how you are measuring all this so I can too can see the empirical evidence and be convinced that this indeed is as critical as you say it is in the 2.1 to 2.8khz pass-band of my device? I can't weigh in on this until you explain and show me how you measured all this.
> Joseph
>
>? ? On Friday, January 10, 2025 at 05:38:21 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:?
>?
>?
> And you are missing my point. Reducing the RF gain makes internally generated IMD products decrease faster than the desired signal. Which is entirely on topic for the usage of the RF Gain control. ACG attack distortion and IMD are certainly related but AGC is not the sole cause of internally generated IMD.
>
> On Fri, 10 Jan 2025 15:47:44 +0000 (UTC)
> "Joseph K7CBR via groups.io" <k7cbr@...> wrote:
>
> >? have you been checked for rabies? lol* read my whole note from 7:07AM ET. AGC/RF gain was my topic. I shall leave Drake IMD issues to manufacturers and other ops to try and conquer. Im running a 52 year old set of boxes. Run them as clean as I can (xmit/rcv). full stop...
> >
> > *asking this, could get me banned for life... hope not yet its A-OK...
> > Joseph
> >
> >? ? On Friday, January 10, 2025 at 07:53:48 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:?
> >?
> >?
> > That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total.
> >
> > On Fri, 10 Jan 2025 12:05:42 +0000 (UTC)
> > "Joseph K7CBR via groups.io" <k7cbr@...> wrote:
> >?
> > >? AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about??
> >
> >?
>
>
>
--
73
-Jim
NU0C
|
Now you are just being argumentative. It sounds like you are saying that intermodulation distortion behaves differently in your RX. Fine. I will leave it up to you to educate yourself with technical journals and Google searches. Then you can measure your own RX with a low distortion signal generator and a capable spectrum analyzer. Have fun. On Sat, 11 Jan 2025 13:29:54 +0000 (UTC) "Joseph K7CBR via groups.io" <k7cbr@...> wrote: No, that demo you linked would NOT account for observations on my own hardware at a 250hz, 500hz, or even 2.8khz bandwidth as far as "internally generated noise". That has to do with showing the mess in crowded band conditions. I do know that radio hardware have internal distortion, this is not a secret.
The demo you linked to, shows a span of 10MHz, 1MHz/div. Your link does not account for internal IMD in the passband of my receiver at the settings I asked about. He demonstrated IMD of his transmitter. The receiver mixing scheme and inter-stage bias class (through to AF) does factor into this, yet I wanted you to explain clearly and coherently what you mean, and how this is significant enough to be a consideration to our audible perception? In the link video, if I park my R4C (it wont park at 1GHz), and have my 600Hz roofing filter in line, cascading through my 250Hz filter, listening to CW, I wont hear those other signals.
So please explain so we can all get onto the same page. Explain and show how you are measuring this so I can do the same. Thanks
Joseph
On Friday, January 10, 2025 at 08:19:05 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
There is a demonstration given by a fellow in the link I posted in message /g/DRAKE-RADIO/message/81833 .
However it is common knowledge that IMD products change at a faster rate than the fundamentals. Given that there is no such thing as a perfect amplifier your radio has some degree of internally generated distortion products.
On Fri, 10 Jan 2025 23:08:06 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
? OK. Just so we are on the same page, I need you to show me how you are measuring all this so I can too can see the empirical evidence and be convinced that this indeed is as critical as you say it is in the 2.1 to 2.8khz pass-band of my device? I can't weigh in on this until you explain and show me how you measured all this.
Joseph
? ? On Friday, January 10, 2025 at 05:38:21 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:?
?
?
And you are missing my point. Reducing the RF gain makes internally generated IMD products decrease faster than the desired signal. Which is entirely on topic for the usage of the RF Gain control. ACG attack distortion and IMD are certainly related but AGC is not the sole cause of internally generated IMD.
On Fri, 10 Jan 2025 15:47:44 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
? have you been checked for rabies? lol* read my whole note from 7:07AM ET. AGC/RF gain was my topic. I shall leave Drake IMD issues to manufacturers and other ops to try and conquer. Im running a 52 year old set of boxes. Run them as clean as I can (xmit/rcv). full stop...
*asking this, could get me banned for life... hope not yet its A-OK...
Joseph
? ? On Friday, January 10, 2025 at 07:53:48 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:?
?
?
That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total.
On Fri, 10 Jan 2025 12:05:42 +0000 (UTC)
"Joseph K7CBR via groups.io" <k7cbr@...> wrote:
?
? AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about?? ?
-- 73 -Jim NU0C
|
I was looking forward to an explanation. We ask a simple question, "does the path have a heart?" Sometimes the path does not.
On Saturday, January 11, 2025 at 09:25:42 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:
Now you are just being argumentative. It sounds like you are saying that intermodulation distortion behaves differently in your RX. Fine. I will leave it up to you to educate yourself with technical journals and Google searches. Then you can measure your own RX with a low distortion signal generator and a capable spectrum analyzer. Have fun.
On Sat, 11 Jan 2025 13:29:54 +0000 (UTC) "Joseph K7CBR via groups.io" <k7cbr@...> wrote: >? No, that demo you linked would NOT account for observations on my own hardware at a 250hz, 500hz, or even 2.8khz bandwidth as far as "internally generated noise". That has to do with showing the mess in crowded band conditions. I do know that radio hardware have internal distortion, this is not a secret. > The demo you linked to, shows a span of 10MHz, 1MHz/div. Your link does not account for internal IMD in the passband of my receiver at the settings I asked about. He demonstrated IMD of his transmitter. The receiver mixing scheme and inter-stage bias class (through to AF) does factor into this, yet I wanted you to explain clearly and coherently what you mean, and how this is significant enough to be a consideration to our audible perception? In the link video, if I park my R4C (it wont park at 1GHz), and have my 600Hz roofing filter in line, cascading through my 250Hz filter, listening to CW, I wont hear those other signals. > So please explain so we can all get onto the same page. Explain and show how you are measuring this so I can do the same. Thanks > > Joseph > >? ? On Friday, January 10, 2025 at 08:19:05 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:? >? >? > There is a demonstration given by a fellow in the link I posted in message /g/DRAKE-RADIO/message/81833 . > > However it is common knowledge that IMD products change at a faster rate than the fundamentals. Given that there is no such thing as a perfect amplifier your radio has some degree of internally generated distortion products. > > On Fri, 10 Jan 2025 23:08:06 +0000 (UTC) > "Joseph K7CBR via groups.io" <k7cbr@...> wrote: > > >? OK. Just so we are on the same page, I need you to show me how you are measuring all this so I can too can see the empirical evidence and be convinced that this indeed is as critical as you say it is in the 2.1 to 2.8khz pass-band of my device? I can't weigh in on this until you explain and show me how you measured all this. > > Joseph > > > >? ? On Friday, January 10, 2025 at 05:38:21 PM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:? > >? > >? > > And you are missing my point. Reducing the RF gain makes internally generated IMD products decrease faster than the desired signal. Which is entirely on topic for the usage of the RF Gain control. ACG attack distortion and IMD are certainly related but AGC is not the sole cause of internally generated IMD. > > > > On Fri, 10 Jan 2025 15:47:44 +0000 (UTC) > > "Joseph K7CBR via groups.io" <k7cbr@...> wrote: > >? > > >? have you been checked for rabies? lol* read my whole note from 7:07AM ET. AGC/RF gain was my topic. I shall leave Drake IMD issues to manufacturers and other ops to try and conquer. Im running a 52 year old set of boxes. Run them as clean as I can (xmit/rcv). full stop... > > > > > > *asking this, could get me banned for life... hope not yet its A-OK... > > > Joseph > > > > > >? ? On Friday, January 10, 2025 at 07:53:48 AM EST, Jim Shorney via groups.io <jimnu0c@...> wrote:? > > >? > > >? > > > That is one aspect of the issue. Every amplifier has IMD whether gain controlled or not. It is a question of how much. It may be very little or it may be a lot but each stage contributes to the total. > > > > > > On Fri, 10 Jan 2025 12:05:42 +0000 (UTC) > > > "Joseph K7CBR via groups.io" <k7cbr@...> wrote: > > >?? > > > >? AGC attack distortion is the actual problem. Decreasing RF gain on low bands where the noise level remains high is what we are talking about??? > > > > > >?? > > > > > >? > > > -- 73 -Jim NU0C
|
Jim, IMD? (3rd order intercept is calculated)? is easily measured on any RX.? On the Drake tube gear, like R4B / R4C, it was never outstanding to begin with.? The problem is the 2nd mixer, it doesn't have any dynamic range to begin with.? Rob Sherwood's 'fix' for that was the various roofing xtal filters....which work superb.? The roofing filters? will eliminate or partially eliminate? one of the 2 x offending? signals.....and bye bye IMD resulting from them.?
?
?On my yaesu FT-1000D? ?and also my FT-1000MP-MKV, after doing the W8JI noise blanker mod ( blanker is not totally shut off when switched off),? I can't find any IMD.? ?I need 2 x signals, both 50 db > S9, and 2khz apart,? to produce an IMD product equal to the noise floor....and that's on the bench.? ?That's just not happening with any ant connected.? My noise floor is S2? to S8, depending on the band.? ?Ok, now any IMD is well below my noise floor...so it's a non issue, since I can't hear it.??
IF you reduce the RF gain a bunch..... the agc threshold INCREASES ........ and any IMD from 2 x loud, closely spaced signals will be BELOW the agc threshold.? ?So yeah, if u had zero band noise, and 2 x super loud, closely spaced signals on RX,? and could actually? hear IMD, it will VANISH? if the RF gain is turned down.? ? It will also vanish if you insert some attenuation in the RX path.? ?The IMD will drop off at triple the value of the inserted attenuation.??
Jim? VE7RF
|
Jim accurately describes the manner in which IMD is typically tested and reported ¡ª i.e., when two close-in signals produce an IMD product that exceeds the noise floor.? But it seems to me that the IMD inherent in just about any amplifier will actually raise
the noise floor, because any noise within the passband is causing some low-level IMD products that also appear within the passband.??
So the noise floor might be said to consist of two components, which we'll call Original Noise and IMD Noise.? When we reduce the RF gain, we're reducing the Signal and the Original Noise by the same amount, but reducing the IMD Noise faster, because of the
1:3 ratio between the Original Noise and the IMD Noise.
If we're considering just the noise floor of the receiver circuitry, this effect would be pretty small.? But if we consider band noise as a component of the Original Noise,? and consider how the IMD products of that band noise contribute to what we're calling
IMD Noise, the effect may not be so small, particularly on the low bands where most of us experience a lot of noise.? In this case, turning down the rf gain could make a noticeable contribution to the S/N ratio.? I think this is why Bob Heil's video is able
to show a marked increase in readability of an 80-meter QSO by turning down the rf gain.
Thoughts welcome.
Art Delibert, KB3FJO
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Jim, IMD? (3rd order intercept is calculated)? is easily measured on any RX.? On the Drake tube gear, like R4B / R4C, it was never outstanding to begin with.? The problem is the 2nd mixer, it doesn't have any dynamic range to begin with.? Rob Sherwood's
'fix' for that was the various roofing xtal filters....which work superb.? The roofing filters? will eliminate or partially eliminate? one of the 2 x offending? signals.....and bye bye IMD resulting from them.?
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?On my yaesu FT-1000D? ?and also my FT-1000MP-MKV, after doing the W8JI noise blanker mod ( blanker is not totally shut off when switched off),? I can't find any IMD.? ?I need 2 x signals, both 50 db > S9, and 2khz apart,? to produce an IMD product equal
to the noise floor....and that's on the bench.? ?That's just not happening with any ant connected.? My noise floor is S2? to S8, depending on the band.? ?Ok, now any IMD is well below my noise floor...so it's a non issue, since I can't hear it.??
IF you reduce the RF gain a bunch..... the agc threshold INCREASES ........ and any IMD from 2 x loud, closely spaced signals will be BELOW the agc threshold.? ?So yeah, if u had zero band noise, and 2 x super loud, closely spaced signals on RX,? and could
actually? hear IMD, it will VANISH? if the RF gain is turned down.? ? It will also vanish if you insert some attenuation in the RX path.? ?The IMD will drop off at triple the value of the inserted attenuation.??
Jim? VE7RF
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I provided you with an eloquent explanation from W8II and you did not accept it. So I leave it up to you to find your own as you will likely not accept mine either. 73 AR SK On Sat, 11 Jan 2025 16:55:25 +0000 (UTC) "Joseph K7CBR via groups.io" <k7cbr@...> wrote: I was looking forward to an explanation. -- 73 -Jim NU0C
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FWIW, when I was experimenting with RF stages in an old receiver I added a manual gain control to the first RF only. Simply a bypassed variable cathode resistor. I found it necessary when there were many strong signals but only for some tubes. Not necessary for a 6BA6 on AVC but when I tried some very high gain pentodes and a cascode I found it was necessary. This was a war time Super-Pro which had been worked over before I got it. Eventually I restored the original circuits except for adding a VR tube to the local oscillator. Unfortunately I do not still have it. I think part of the reason for needing the gain reduction was the linearity of the following RF but I did not have the instruments to measure it at the time.
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On 1/11/2025 12:58 PM, Jim Shorney via groups.io wrote: I provided you with an eloquent explanation from W8II and you did not accept it. So I leave it up to you to find your own as you will likely not accept mine either. 73 AR SK
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Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
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On Sat, Jan 11, 2025 at 07:28 PM, Art Delibert wrote:
>Jim accurately describes the manner in which IMD is typically tested and reported ¡ª i.e., when two close-in signals produce an IMD product that exceeds the noise floor.? But it seems to me that the IMD inherent in just >about any amplifier will actually raise the noise floor, because any noise within the passband is causing some low-level IMD products that also appear within the passband.??
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>So the noise floor might be said to consist of two components, which we'll call Original Noise and IMD Noise.? When we reduce the RF gain, we're reducing the Signal and the Original Noise by the same amount, but >reducing the IMD Noise faster, because of the 1:3 ratio between the Original Noise and the IMD Noise.
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>If we're considering just the noise floor of the receiver circuitry, this effect would be pretty small.? But if we consider band noise as a component of the Original Noise,? and consider how the IMD products of that band >noise contribute to what we're calling IMD Noise, the effect may not be so small, particularly on the low bands where most of us experience a lot of noise.? In this case, turning down the rf gain could make a noticeable >contribution to the S/N ratio.? I think this is why Bob Heil's video is able to show a marked increase in readability of an 80-meter QSO by turning down the rf gain.
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>Thoughts welcome.
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>Art Delibert, KB3FJO
I find it real hard to? believe that band noise alone will? create enough IMD to? degrade anything.? ?With band noise like S4 to S8....any imd would be through the floor.? ?You wouldn't even be able to measure it, if say using 2 x test tones, and each? S8.? When turning down the RF gain, the agc threshold increases.? Assume a? S7 to S8 noise level on 75m.? ?And the SSB stations you are talking to are all S9 + 10 db.? ?Reduce the RF gain till the S meter swings up to? S9.? ?The agc threshold has risen greatly,? and any signal <S9? will drop off rapidly.? ?Ok, the S7-S8 band noise just vanishes.? ?Why, because the noise is now below the agc threshold.? And none of this has anything to do with IMD.? ?Heil didn't invent anything new.? ? This technique about reducing the RF gain control a bunch, on the lower bands, when loads of band noise is present.... is old news.? Your only other option would be to run the RF gain at MAX.....then introduce enough RX padding,? such that the S7-S8 band noise is inaudible.? IE: add say aprx? 30-40 db of padding, such that the band noise is below the S2 agc threshold.? ?Your S9 + 10 db SSB signals will drop by 30-40db.
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Jim Shorney