Topics

My son's V6


Bob Lunsford
 

I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

This may be a consideration for a V7 in the future.

My replacement V6 has not arrived yet because I mistook the Oct 27 delivery date for Oct 21... I had three eye surgeries on the left eye and five on the right so I sometimes make such mistakes. Where I live, at the edge of the Appalachians, there is no problem a with overloading my front end, no one around me uses more than 100W.

My son said he plans to dedicate the V6 for when he goes to the mountains, where there are no high powered stations nearby.

Bob — KK5R


Curt
 

Bob

find out if the overload is from AM broadcasts - yes below 1.7 MHz.  as there isn't any high pass filtering in this rig - others have this issue and it can be addressed by a simple filter.  this topic has been frequently posted here, and there are example designs here and on the web. 

73 curt


Jack, W8TEE
 

Bob:

I had BCI problems with my Forty-9er and build a cheap filter for it...cost was $0.87! Works well.



Inline image

Inline image
As I recall, I bought the molded inductors from taydaelectronics.com.

Jack, W8TEE

On Friday, October 23, 2020, 3:22:39 PM EDT, Bob Lunsford via groups.io <nocrud222@...> wrote:


I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

This may be a consideration for a V7 in the future.

My replacement V6 has not arrived yet because I mistook the Oct 27 delivery date for Oct 21... I had three eye surgeries on the left eye and five on the right so I sometimes make such mistakes. Where I live, at the edge of the Appalachians, there is no problem a with overloading my front end, no one around me uses more than 100W.

My son said he plans to dedicate the V6 for when he goes to the mountains, where there are no high powered stations nearby.

Bob — KK5R

--
Jack, W8TEE


Evan Hand
 

Bob,
Curt is correct, there have been numerous requests for broadcast am filtering.  At least one solution that I know of (I have not needed any as I do not have the issue at my QTH) is 
https://ubitx.net/fix-bci-filter/

There are also kit and modules available:
http://www.w0eb.com/
https://www.amazon.com/Flamingo-AM-Broadcast-Bandstop-Applications/dp/B079CMB44V

And I am sure there are others that I do not know of.
73
Evan
AC9TU


Evan Hand
 

One point I missed, though it is in the ubitx.net link, is to install the filter in the line between K1 and K3.  That trace is only used to receive.


Jerry Gaffke
 

AGC circuits for the uBitx that only reduce the audio gain in the vicinity of 
the volume control at RV4 are fairly common, but won't work for strong signals.
The audio pre-amp at Q70 can easily be overloaded by strong signals.
An extremely strong signal might also overload the two IF amps.

The trace between K3-pin11 and K1-pin12 is receiver RF only, directly from the antenna.
That is where you would want an RX RF gain pot.
On a simple and understandable rig such as the uBitx, I'd just put down
a three pin cable header on V7 for an optional RF gain pot and leave it at that.
Must cut a trace between two of those pins to make it usable.

Reducing the gain at the first IF amp (Q10) has also proven effective.

If your son is having trouble with strong out-of-band signals,
one solution would be plug-in band-specific bandpass filters to replace
the 30mhz low-pass-filter at L1,L2,L3,L4.
This filter is used for both RX and TX.
Perhaps V7 could have pins around this LPF to make a plug in filter easier.

For V7, I'd give priority to test facilities such as a diode RF probe.
A Nano pin to drive the mike input with 1khz audio. 
Instructions for using a second Raduino as an RF generator. 
A few resistors on the back for use as attenuators.
A 5W 50 ohm dummy load. 
Include instructions using the above to completely check out the rig with just a DVM.
Except for the spare Raduino (borrowed from a friend, perhaps) that comes to 
less than a dollar in additional parts.  Makes trouble shooting far less complicated.
And does not complicate the radio itself.

The primary reason to buy a uBitx is because it is simple and understandable.
Hopefully it will remain that way.

Jerry, KE7ER


On Fri, Oct 23, 2020 at 12:22 PM, Bob Lunsford wrote:
I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.
 
 


MadRadioModder
 

Automatic VOLUME Control (AVC) vs. true Automatic Gain Control (AGC).  There are some great AGC circuits that have been discussed in this forum.  Search.

 

From: BITX20@groups.io <BITX20@groups.io> On Behalf Of Jerry Gaffke via groups.io
Sent: Friday, October 23, 2020 2:59 PM
To: BITX20@groups.io
Subject: Re: [BITX20] My son's V6

 

AGC circuits for the uBitx that only reduce the audio gain in the vicinity of 
the volume control at RV4 are fairly common, but won't work for strong signals.
The audio pre-amp at Q70 can easily be overloaded by strong signals.
An extremely strong signal might also overload the two IF amps.

The trace between K3-pin11 and K1-pin12 is receiver RF only, directly from the antenna.
That is where you would want an RX RF gain pot.
On a simple and understandable rig such as the uBitx, I'd just put down
a three pin cable header on V7 for an optional RF gain pot and leave it at that.
Must cut a trace between two of those pins to make it usable.

Reducing the gain at the first IF amp (Q10) has also proven effective.

If your son is having trouble with strong out-of-band signals,
one solution would be plug-in band-specific bandpass filters to replace
the 30mhz low-pass-filter at L1,L2,L3,L4.
This filter is used for both RX and TX.
Perhaps V7 could have pins around this LPF to make a plug in filter easier.

For V7, I'd give priority to test facilities such as a diode RF probe.
A Nano pin to drive the mike input with 1khz audio. 
Instructions for using a second Raduino as an RF generator. 
A few resistors on the back for use as attenuators.
A 5W 50 ohm dummy load. 
Include instructions using the above to completely check out the rig with just a DVM.
Except for the spare Raduino (borrowed from a friend, perhaps) that comes to 
less than a dollar in additional parts.  Makes trouble shooting far less complicated.
And does not complicate the radio itself.

The primary reason to buy a uBitx is because it is simple and understandable.
Hopefully it will remain that way.

Jerry, KE7ER


On Fri, Oct 23, 2020 at 12:22 PM, Bob Lunsford wrote:

I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

 

 


Virus-free. www.avg.com

--

…_. _._


Arv Evans
 

Way back in the dark ages (before Jack Purdum and myself were born) there 
were some high-end receivers that used Automatic Attenuation Control (AGC) 
applied to the 1st RF amplifier.  All the gain stages were low-noise design and 
ran at fixed gain.

The mechanism rectified voltage from the last IF stage and used that to change 
bias on the 1st RF amplifier, making it a variable gain stage.  This broadened the 
passband of the RF input tuned circuit as it attenuated incoming RF signals.




Arv
_._



On Fri, Oct 23, 2020 at 2:53 PM MadRadioModder <madradiomodder@...> wrote:

Automatic VOLUME Control (AVC) vs. true Automatic Gain Control (AGC).  There are some great AGC circuits that have been discussed in this forum.  Search.

 

From: BITX20@groups.io <BITX20@groups.io> On Behalf Of Jerry Gaffke via groups.io
Sent: Friday, October 23, 2020 2:59 PM
To: BITX20@groups.io
Subject: Re: [BITX20] My son's V6

 

AGC circuits for the uBitx that only reduce the audio gain in the vicinity of 
the volume control at RV4 are fairly common, but won't work for strong signals.
The audio pre-amp at Q70 can easily be overloaded by strong signals.
An extremely strong signal might also overload the two IF amps.

The trace between K3-pin11 and K1-pin12 is receiver RF only, directly from the antenna.
That is where you would want an RX RF gain pot.
On a simple and understandable rig such as the uBitx, I'd just put down
a three pin cable header on V7 for an optional RF gain pot and leave it at that.
Must cut a trace between two of those pins to make it usable.

Reducing the gain at the first IF amp (Q10) has also proven effective.

If your son is having trouble with strong out-of-band signals,
one solution would be plug-in band-specific bandpass filters to replace
the 30mhz low-pass-filter at L1,L2,L3,L4.
This filter is used for both RX and TX.
Perhaps V7 could have pins around this LPF to make a plug in filter easier.

For V7, I'd give priority to test facilities such as a diode RF probe.
A Nano pin to drive the mike input with 1khz audio. 
Instructions for using a second Raduino as an RF generator. 
A few resistors on the back for use as attenuators.
A 5W 50 ohm dummy load. 
Include instructions using the above to completely check out the rig with just a DVM.
Except for the spare Raduino (borrowed from a friend, perhaps) that comes to 
less than a dollar in additional parts.  Makes trouble shooting far less complicated.
And does not complicate the radio itself.

The primary reason to buy a uBitx is because it is simple and understandable.
Hopefully it will remain that way.

Jerry, KE7ER


On Fri, Oct 23, 2020 at 12:22 PM, Bob Lunsford wrote:

I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

 

 


Virus-free. www.avg.com

--

…_. _._


Jerry Gaffke
 

A totally audio AGC system is simple and useful, it keeps you from going deaf.
Especially when wearing headphones.
Gives you time to turn that new RF gain pot down.
This could be even implemented after the headphone jack, outside the rig.
Perhaps as simple as back-to-back diodes, a bunch of diodes in series for more volume.

AGC vs AVC vs AAC:
Some try to draw distinctions (such as inside a particular company), some (such as I) don't.
Historically AGC and AVC are synonymous, I've never run across AAC.

It all boils down to having a detector somewhere in the chain of amps that detects
signal strength, and this is used to control the gain of one of the amplifier stages.
The detector must be after the most selective element of the radio, in our case 
that is the crystal filter (unless you have added an audio filter for CW use).
The amplifier whose gain is controlled by this detected signal is almost always
prior to the detector, could be an RF,IF, or AF stage.
.
Jerry, KE7ER


On Fri, Oct 23, 2020 at 12:59 PM, Jerry Gaffke wrote:
AGC circuits for the uBitx that only reduce the audio gain in the vicinity of 
the volume control at RV4 are fairly common, but won't work for strong signals.
The audio pre-amp at Q70 can easily be overloaded by strong signals.
An extremely strong signal might also overload the two IF amps.


Shirley Dulcey KE1L
 

Most modern HF receivers don't have any amplification stages before the first mixer, except for a preamplifier that can optionally be switched in. The preamp is rarely necessary except on 15 meters and up, and in a noisy urban environment you'll never need it on HF at all unless you are using a high loss receiving antenna. If it exists, it's either a fixed-gain amplifier or has a couple of gain settings.

Preserving high dynamic range in an amplifier, especially a solid state amplifier, with variable gain is difficult. As a result, AGC is typically applied only after the first filter.

On Fri, Oct 23, 2020 at 7:15 PM Arv Evans <arvid.evans@...> wrote:
Way back in the dark ages (before Jack Purdum and myself were born) there 
were some high-end receivers that used Automatic Attenuation Control (AGC) 
applied to the 1st RF amplifier.  All the gain stages were low-noise design and 
ran at fixed gain.

The mechanism rectified voltage from the last IF stage and used that to change 
bias on the 1st RF amplifier, making it a variable gain stage.  This broadened the 
passband of the RF input tuned circuit as it attenuated incoming RF signals.




Arv
_._



On Fri, Oct 23, 2020 at 2:53 PM MadRadioModder <madradiomodder@...> wrote:

Automatic VOLUME Control (AVC) vs. true Automatic Gain Control (AGC).  There are some great AGC circuits that have been discussed in this forum.  Search.

 

From: BITX20@groups.io <BITX20@groups.io> On Behalf Of Jerry Gaffke via groups.io
Sent: Friday, October 23, 2020 2:59 PM
To: BITX20@groups.io
Subject: Re: [BITX20] My son's V6

 

AGC circuits for the uBitx that only reduce the audio gain in the vicinity of 
the volume control at RV4 are fairly common, but won't work for strong signals.
The audio pre-amp at Q70 can easily be overloaded by strong signals.
An extremely strong signal might also overload the two IF amps.

The trace between K3-pin11 and K1-pin12 is receiver RF only, directly from the antenna.
That is where you would want an RX RF gain pot.
On a simple and understandable rig such as the uBitx, I'd just put down
a three pin cable header on V7 for an optional RF gain pot and leave it at that.
Must cut a trace between two of those pins to make it usable.

Reducing the gain at the first IF amp (Q10) has also proven effective.

If your son is having trouble with strong out-of-band signals,
one solution would be plug-in band-specific bandpass filters to replace
the 30mhz low-pass-filter at L1,L2,L3,L4.
This filter is used for both RX and TX.
Perhaps V7 could have pins around this LPF to make a plug in filter easier.

For V7, I'd give priority to test facilities such as a diode RF probe.
A Nano pin to drive the mike input with 1khz audio. 
Instructions for using a second Raduino as an RF generator. 
A few resistors on the back for use as attenuators.
A 5W 50 ohm dummy load. 
Include instructions using the above to completely check out the rig with just a DVM.
Except for the spare Raduino (borrowed from a friend, perhaps) that comes to 
less than a dollar in additional parts.  Makes trouble shooting far less complicated.
And does not complicate the radio itself.

The primary reason to buy a uBitx is because it is simple and understandable.
Hopefully it will remain that way.

Jerry, KE7ER


On Fri, Oct 23, 2020 at 12:22 PM, Bob Lunsford wrote:

I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

 

 


Virus-free. www.avg.com

--

…_. _._


Jerry Gaffke
 

To be clear, even I would not call back-to-back diodes an "AGC system".
They just clip, they don't adjust gain.
But they can definitely save your eardrums, give you time to reach for a knob.

Shirley wrote:
#  Preserving high dynamic range in an amplifier, especially a solid state amplifier,
#  with variable gain is difficult. As a result, AGC is typically applied only after the first filter.

Interesting.
So it's easier to implement an amplifier with an extra 50dB of dynamic range
than it is to implement one with 50dB of variable gain?
Only after the first filter, so the above does not apply to a narrow band amplifier?
Is the uBitx's 45mhz filter sufficient?

I think we can all agree that the AGC detector must be implemented after the filter,
but I had thought the variable gain stage could be before the filter.
The delay going through the filter could well make this less desirable than having
the detector and variable gain all wrapped up in the same amplifier stage.
Otherwise you might hear a pop when a strong signal comes on before the AGC can react.

Jerry, KE7ER 



On Fri, Oct 23, 2020 at 05:45 PM, Jerry Gaffke wrote:
A totally audio AGC system is simple and useful, it keeps you from going deaf.
Especially when wearing headphones.
Gives you time to turn that new RF gain pot down.
This could be even implemented after the headphone jack, outside the rig.
Perhaps as simple as back-to-back diodes, a bunch of diodes in series for more volume.


Evan Hand
 

I know of 3 different methods that AGC is implemented in the uBITX series of kits:
1 - Audio gain control
2 - First IF gain control
3 - RF gain control

All have had good reports of meeting the purchaser's needs (though some have had issues to get it working).

I own two of these:
1 - A First IF gain control module from AmateurRadioKits
2 - An RF reduction gain control from Kit Projects

I have only installed the AmateurRadioKits AGC so far.  It came with a Nextion case that I was using when upgrading the Nextion Display.  I have not used the uBITX much since as other interests have gotten in the way (I like the building part, not much on the operating).

My point is that there are kits available for whichever flavor of AGC you want.  They all seem about the same in price.  Only the Kit Projects module requires cutting a trace, however, all require soldering onto the existing circuits of the radio.  I have read of challenges installing all of them (even the audio-based mods).

The Kit Projects is available for $15 (it includes a fast, slow, off switch).
The AmateeurRadioKits is available for $29 (it includes an S meter).

So none are really that expensive.

My 2 cents
73
Evan
AC9TU


Bob Lunsford
 

Thanks to all for the excellent ideas. I am forwarding all the messages to my son for his edification. I am also learning a lot by them. Thanks much to all and I'm sure I'm speaking for my son, also. He's been a ham since the mid-70's and got his start with a Heath HW-8, lives and breathes CW and used Russ Farnsworth's "By the Word Method" to learn CW. Passed the CW exam after three weeks listening to the tapes. He could talk to me and listen to CW in the background and he'd suddenly say, "I have to answer this guy, he just turned it back over to me."

Thanks again to everyone.

Bob — KK5R

On Friday, October 23, 2020, 3:42:49 PM EDT, Jack, W8TEE via groups.io <jjpurdum@...> wrote:


Bob:

I had BCI problems with my Forty-9er and build a cheap filter for it...cost was $0.87! Works well.



Inline image

Inline image
As I recall, I bought the molded inductors from taydaelectronics.com.

Jack, W8TEE

On Friday, October 23, 2020, 3:22:39 PM EDT, Bob Lunsford via groups.io <nocrud222@...> wrote:


I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

This may be a consideration for a V7 in the future.

My replacement V6 has not arrived yet because I mistook the Oct 27 delivery date for Oct 21... I had three eye surgeries on the left eye and five on the right so I sometimes make such mistakes. Where I live, at the edge of the Appalachians, there is no problem a with overloading my front end, no one around me uses more than 100W.

My son said he plans to dedicate the V6 for when he goes to the mountains, where there are no high powered stations nearby.

Bob — KK5R

--
Jack, W8TEE


MadRadioModder
 

This is a tricky thing, because if you add a bandpass filter as the first element in your receiver chain… as many receivers do, then the its usual to follow that up with an variable gain amplifier that serves two purposes… one to recover the signal that was lost by inserting the bandpass filter in the chain, and two, as the variable AGC gate.  Then comes the first mixer.

 

From: BITX20@groups.io <BITX20@groups.io> On Behalf Of Shirley Dulcey KE1L
Sent: Friday, October 23, 2020 7:51 PM
To: BITX20@groups.io
Subject: Re: [BITX20] My son's V6

 

Most modern HF receivers don't have any amplification stages before the first mixer, except for a preamplifier that can optionally be switched in. The preamp is rarely necessary except on 15 meters and up, and in a noisy urban environment you'll never need it on HF at all unless you are using a high loss receiving antenna. If it exists, it's either a fixed-gain amplifier or has a couple of gain settings.

 

Preserving high dynamic range in an amplifier, especially a solid state amplifier, with variable gain is difficult. As a result, AGC is typically applied only after the first filter.

 

On Fri, Oct 23, 2020 at 7:15 PM Arv Evans <arvid.evans@...> wrote:

Way back in the dark ages (before Jack Purdum and myself were born) there 

were some high-end receivers that used Automatic Attenuation Control (AGC) 

applied to the 1st RF amplifier.  All the gain stages were low-noise design and 

ran at fixed gain.

 

The mechanism rectified voltage from the last IF stage and used that to change 

bias on the 1st RF amplifier, making it a variable gain stage.  This broadened the 

passband of the RF input tuned circuit as it attenuated incoming RF signals.

 

 

 

 

Arv

_._

 

 

 

On Fri, Oct 23, 2020 at 2:53 PM MadRadioModder <madradiomodder@...> wrote:

Automatic VOLUME Control (AVC) vs. true Automatic Gain Control (AGC).  There are some great AGC circuits that have been discussed in this forum.  Search.

 

From: BITX20@groups.io <BITX20@groups.io> On Behalf Of Jerry Gaffke via groups.io
Sent: Friday, October 23, 2020 2:59 PM
To: BITX20@groups.io
Subject: Re: [BITX20] My son's V6

 

AGC circuits for the uBitx that only reduce the audio gain in the vicinity of 
the volume control at RV4 are fairly common, but won't work for strong signals.
The audio pre-amp at Q70 can easily be overloaded by strong signals.
An extremely strong signal might also overload the two IF amps.

The trace between K3-pin11 and K1-pin12 is receiver RF only, directly from the antenna.
That is where you would want an RX RF gain pot.
On a simple and understandable rig such as the uBitx, I'd just put down
a three pin cable header on V7 for an optional RF gain pot and leave it at that.
Must cut a trace between two of those pins to make it usable.

Reducing the gain at the first IF amp (Q10) has also proven effective.

If your son is having trouble with strong out-of-band signals,
one solution would be plug-in band-specific bandpass filters to replace
the 30mhz low-pass-filter at L1,L2,L3,L4.
This filter is used for both RX and TX.
Perhaps V7 could have pins around this LPF to make a plug in filter easier.

For V7, I'd give priority to test facilities such as a diode RF probe.
A Nano pin to drive the mike input with 1khz audio. 
Instructions for using a second Raduino as an RF generator. 
A few resistors on the back for use as attenuators.
A 5W 50 ohm dummy load. 
Include instructions using the above to completely check out the rig with just a DVM.
Except for the spare Raduino (borrowed from a friend, perhaps) that comes to 
less than a dollar in additional parts.  Makes trouble shooting far less complicated.
And does not complicate the radio itself.

The primary reason to buy a uBitx is because it is simple and understandable.
Hopefully it will remain that way.

Jerry, KE7ER


On Fri, Oct 23, 2020 at 12:22 PM, Bob Lunsford wrote:

I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

 

 


--

…_. _._


--

…_. _._


Bob Lunsford
 

Here's another thought I'd like to present to the group:

If you put an antenna tuner, auto or manual, at the radio's antenna jack, would this also attenuate any high power radio signal since the antenna tuner peaks the signal at the V6's transmit/receive signal frequency? How broad is a tuner when it is peaking the signal at that frequency? It is essentially a narrow, pass-band filter and would/should also cut loud signals above and below the operating frequency. Am I right? Would the attenuation be sufficient, when going against the receiver's inherent, overall gain, to stop the loud signal front end overload problem?

Bob — KK5R

On Saturday, October 24, 2020, 1:30:39 AM EDT, MadRadioModder <madradiomodder@...> wrote:


This is a tricky thing, because if you add a bandpass filter as the first element in your receiver chain… as many receivers do, then the its usual to follow that up with an variable gain amplifier that serves two purposes… one to recover the signal that was lost by inserting the bandpass filter in the chain, and two, as the variable AGC gate.  Then comes the first mixer.

 

From: BITX20@groups.io <BITX20@groups.io> On Behalf Of Shirley Dulcey KE1L
Sent: Friday, October 23, 2020 7:51 PM
To: BITX20@groups.io
Subject: Re: [BITX20] My son's V6

 

Most modern HF receivers don't have any amplification stages before the first mixer, except for a preamplifier that can optionally be switched in. The preamp is rarely necessary except on 15 meters and up, and in a noisy urban environment you'll never need it on HF at all unless you are using a high loss receiving antenna. If it exists, it's either a fixed-gain amplifier or has a couple of gain settings.

 

Preserving high dynamic range in an amplifier, especially a solid state amplifier, with variable gain is difficult. As a result, AGC is typically applied only after the first filter.

 

On Fri, Oct 23, 2020 at 7:15 PM Arv Evans <arvid.evans@...> wrote:

Way back in the dark ages (before Jack Purdum and myself were born) there 

were some high-end receivers that used Automatic Attenuation Control (AGC) 

applied to the 1st RF amplifier.  All the gain stages were low-noise design and 

ran at fixed gain.

 

The mechanism rectified voltage from the last IF stage and used that to change 

bias on the 1st RF amplifier, making it a variable gain stage.  This broadened the 

passband of the RF input tuned circuit as it attenuated incoming RF signals.

 

 

 

 

Arv

_._

 

 

 

On Fri, Oct 23, 2020 at 2:53 PM MadRadioModder <madradiomodder@...> wrote:

Automatic VOLUME Control (AVC) vs. true Automatic Gain Control (AGC).  There are some great AGC circuits that have been discussed in this forum.  Search.

 

From: BITX20@groups.io <BITX20@groups.io> On Behalf Of Jerry Gaffke via groups.io
Sent: Friday, October 23, 2020 2:59 PM
To: BITX20@groups.io
Subject: Re: [BITX20] My son's V6

 

AGC circuits for the uBitx that only reduce the audio gain in the vicinity of 
the volume control at RV4 are fairly common, but won't work for strong signals.
The audio pre-amp at Q70 can easily be overloaded by strong signals.
An extremely strong signal might also overload the two IF amps.

The trace between K3-pin11 and K1-pin12 is receiver RF only, directly from the antenna.
That is where you would want an RX RF gain pot.
On a simple and understandable rig such as the uBitx, I'd just put down
a three pin cable header on V7 for an optional RF gain pot and leave it at that.
Must cut a trace between two of those pins to make it usable.

Reducing the gain at the first IF amp (Q10) has also proven effective.

If your son is having trouble with strong out-of-band signals,
one solution would be plug-in band-specific bandpass filters to replace
the 30mhz low-pass-filter at L1,L2,L3,L4.
This filter is used for both RX and TX.
Perhaps V7 could have pins around this LPF to make a plug in filter easier.

For V7, I'd give priority to test facilities such as a diode RF probe.
A Nano pin to drive the mike input with 1khz audio. 
Instructions for using a second Raduino as an RF generator. 
A few resistors on the back for use as attenuators.
A 5W 50 ohm dummy load. 
Include instructions using the above to completely check out the rig with just a DVM.
Except for the spare Raduino (borrowed from a friend, perhaps) that comes to 
less than a dollar in additional parts.  Makes trouble shooting far less complicated.
And does not complicate the radio itself.

The primary reason to buy a uBitx is because it is simple and understandable.
Hopefully it will remain that way.

Jerry, KE7ER


On Fri, Oct 23, 2020 at 12:22 PM, Bob Lunsford wrote:

I informed the group that I sent my V6 to my son in Sacramento, CA. He told me that he likes the radio but where he lives, he needed a RF Gain contol because nearby high powered stations tended to overtax his V6 front end. (Perhaps AGC would help here). Considering how the radio is filled with bidirectional attributes where the same circuits are used for transmit and receive, putting in a 1K pot from signal path to ground would probably not work since it would also affect the transmit mode.

 

 


--

…_. _._


--

…_. _._


Curt
 

Bob

using an antenna tuner is a decent step to evaluate the overload issue - and at least one user reported success with this technique.  I would exercise the tuner to see if it can reject the overload.  if this is successful, the the challenge is tuning it for the band of interest - and seeing if it still has rejection. 

if a high pass filter is needed -- this video discusses solutions - and I see W8TEE has illustrated one as well.  I did not do a thorough search to see what solutions are around.

the matter of AGC is much more of a creature comfort - less critical.  I don't have monster sized antennas here - my ubitx does not overload from ham signals, and the VK3YE AGC does very well at suppressing the large ones.  I would not work too hard to implement AGC as its a small impact to the overall rig behavior. 

73 curt


Bob Lunsford
 

Using a tuner on an antenna that is designed/dedicated to a specific band or frequency would theoretically funnel the signal to that specific frequency but it would also, theoretically, suppress received signals up and down frequency from the tuner's settings. Since it is also a good approach to getting as much power as possible from a QRP radio to the antenna by allowing the transmitter's output stage to see a much better load and lessen the complications of a mismatch, it would/should be an automatic part of the system unless weight or system complexity are concerns. It is undoubtedly a prime candidate for experimentation to see if a tuner does indeed reduce the influence of a high powered transmitter either nearby in frequency or location.

Thanks, Curt, for your feedback on this. Sometimes a simple solution may be best even if not a complete solution. I agree that AGC may be nice but it is not always the best or necessary. For example, on my G90 if you turn off the AGC it reveals many weak stations and even if some others come booming in, it's a small price to pay to guarantee that all stations on the [net] frequency are heard. Unless it is truly excessive in volume, of course, and this is "in the ear of the hearer."

Bob — KK5R

On Saturday, October 24, 2020, 8:07:04 PM EDT, Curt via groups.io <wb8yyy@...> wrote:


Bob

using an antenna tuner is a decent step to evaluate the overload issue - and at least one user reported success with this technique.  I would exercise the tuner to see if it can reject the overload.  if this is successful, the the challenge is tuning it for the band of interest - and seeing if it still has rejection. 

if a high pass filter is needed -- this video discusses solutions - and I see W8TEE has illustrated one as well.  I did not do a thorough search to see what solutions are around.

the matter of AGC is much more of a creature comfort - less critical.  I don't have monster sized antennas here - my ubitx does not overload from ham signals, and the VK3YE AGC does very well at suppressing the large ones.  I would not work too hard to implement AGC as its a small impact to the overall rig behavior. 

73 curt


Jerry Gaffke
 

The primary selling point of the uBitx is its simplicity,
would be good to keep it that way.
The circuit improvements moving from V4 to V5 are a major win,
badly needed and no significant added complexity.

Though I would have kept the +12V for the IRF510's separate, much safer to 
debug all but the final amp of the transmitter with those IRF510's disabled.
And allows flexibility in setting the output power by adjusting the IRF510 supply voltage.
A simple way to adjust RF gain would be good.

Moving from the V5 16x2 LCD to the V6 TFT display adds lots of overhead
to the Nano's firmware.  I'd prefer the uBitx had kept the 16x2, at least as an option. 

An SWR meter and antenna tuner with a suitable dummy load for the uBitx 
might be a good accessory.  Audio and RF signal generators, a diode RF probe
(or AD8307), complete instructions to debug the rig, those would be most welcome.   
Perhaps make them all part of a single accessory?  An audio CW filter too?

Jerry, KE7ER


On Sat, Oct 24, 2020 at 05:49 PM, Bob Lunsford wrote:
Using a tuner on an antenna that is designed/dedicated to a specific band or frequency would theoretically funnel the signal to that specific frequency but it would also, theoretically, suppress received signals up and down frequency from the tuner's settings. Since it is also a good approach to getting as much power as possible from a QRP radio to the antenna by allowing the transmitter's output stage to see a much better load and lessen the complications of a mismatch, it would/should be an automatic part of the system unless weight or system complexity are concerns. It is undoubtedly a prime candidate for experimentation to see if a tuner does indeed reduce the influence of a high powered transmitter either nearby in frequency or location.
 
Thanks, Curt, for your feedback on this. Sometimes a simple solution may be best even if not a complete solution. I agree that AGC may be nice but it is not always the best or necessary. For example, on my G90 if you turn off the AGC it reveals many weak stations and even if some others come booming in, it's a small price to pay to guarantee that all stations on the [net] frequency are heard. Unless it is truly excessive in volume, of course, and this is "in the ear of the hearer."
 
Bob — KK5R