Topics

AGC circuit to try?

 

I had a Xiegu X1M a while ago (long since sold) and I used this circuit, which worked quite well. (See attached)  I don't see why it couldn't be added.  I ordered my uBITX some weeks ago and I'm still waiting.
I have a few blank SMD boards I'd be willing to mail out to any souls that would like to try this.  Be warned, the resistors are all 603 sized, so it is a small board!

Cheers,
Jim W0CHL

Philip
 

Don,t see how it would work. All you would get on the output is a DC level, related to the audio input.

Philip G7JUR

K9HZ <bill@...>
 

Yeah something is wrong with that circuit… there is no AC path for the audio… there is a DC low pass filter amplifier but that’s it?

 

 

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

Owner – Operator

Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com

Like us on Facebook! facebook icon

 

Moderator – North American QRO Yahoo Group.

 

email:  bill@...

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Jim Reagan
Sent: Friday, April 6, 2018 2:25 PM
To: BITX20@groups.io
Subject: [BITX20] AGC circuit to try?

 

I had a Xiegu X1M a while ago (long since sold) and I used this circuit, which worked quite well. (See attached)  I don't see why it couldn't be added.  I ordered my uBITX some weeks ago and I'm still waiting.
I have a few blank SMD boards I'd be willing to mail out to any souls that would like to try this.  Be warned, the resistors are all 603 sized, so it is a small board!

Cheers,
Jim W0CHL


Virus-free. www.avg.com

Paul Galburt - K2AYZ
 

This circuit is an audio envelope detector - it provides a dc voltage that corresponds to its input audio voltage. This is then applied as a gain control signal to a "voltage controlled amplifier" of VCA (the audio path) that is not shown. There are several solutions to implementing a good VCA, not all are trivial.

This sort of detection can be used to implement a "feed forward" AGC which effective "programs" the gain reduction based on the input level, or a "feed back" or closed loop AGC which acts to prevent the VCA output from exceeding some threshold. Each method has advantages and disadvantages. Which this particular usage is cannot be determined without seeing more of the circuit.

Hope this helps,

Paul K2AYZ

Paul Galburt - K2AYZ
 

BTW, is there a way to edit posts once submitted? I would like to make a minor change but don'ts see a tool for that.

Kees T
 

Any comments on using a H11F3M MOSFET Optocoupler as the AGC on the uBITX. One of their applications shows an "Audio AGC" circuit. Seems quite simple and easy to implement here.

73 Kees K5BCQ

ajparent1/KB1GMX
 

It is incomplete..

You have the audio detection to get an AGC control voltage but you don't show what your controlling.

What I find odd in general every one seems to be bent on leveling the audio volume in the audio circuit.
Why?  I understand protecting ones ears and all but what an audio compressor?  

The Bitx or uBitx has enough gain and handy places that RF gain control based on audio detection works
very well.  The easy way is replace R13 (ubitx) with a diode such as 1n400x (x=1 to 7) and controlling the
current through the diode to make it behave as a variable resistance at RF.  

The current would be about 4-6ma at max gain and decrease to zero (0) at minimum gain.  For that design
the AGC range is about 26 to 32db depening on band.  If you feel that is not enough agc range then add
the same mod at R35 then with both the agc range is near 60+ DB, generally enough.  AGC in this form
is less prone to overload distortion as you are lowering gain.  The control could be a pot between 8V
(or RX-V) and ground and a series 1K resistor to the he diode (x2 of your using both diodes).  That is a
manual gain control.   To make it automatic use a circuit like just published to detect the voltage at the
top of the audio gain pot and feed that voltage to the gain control diodes.  The circuit should be
arranged to put 4-8V out at NO Audio and decrease to zero volts with increasing audio.

Side note the 1n400x series with minor reservations makes a fine substitute for a PIN diode, the
prefered but more costly device for this function.  Beside being widely available and cheap
( I buy them in quantities of 100 for pennies each) makes it useful.  It also makes a good
20pf varicap and a 1A rectifier to 1000V (1n4007).

This was tested on the first bitx20 I built over a decade ago to test AGC and is used on several  older Tentec
radios and more than few of my own design.   That said its far from a new idea or design as its documented
in EMRFD and the older SSD (Solid State Design AARL press, out of print).   


Allison

Gordon Gibby
 

​Allison --- THanks VERY VERY much for that informative post about using a 1n400X diode as a rf gain control!!!!


Gordon




From: BITX20@groups.io <BITX20@groups.io> on behalf of ajparent1 <kb1gmx@...>
Sent: Saturday, April 7, 2018 3:15 PM
To: BITX20@groups.io
Subject: Re: [BITX20] AGC circuit to try?
 
It is incomplete..

You have the audio detection to get an AGC control voltage but you don't show what your controlling.

What I find odd in general every one seems to be bent on leveling the audio volume in the audio circuit.
Why?  I understand protecting ones ears and all but what an audio compressor?  

The Bitx or uBitx has enough gain and handy places that RF gain control based on audio detection works
very well.  The easy way is replace R13 (ubitx) with a diode such as 1n400x (x=1 to 7) and controlling the
current through the diode to make it behave as a variable resistance at RF.  

The current would be about 4-6ma at max gain and decrease to zero (0) at minimum gain.  For that design
the AGC range is about 26 to 32db depening on band.  If you feel that is not enough agc range then add
the same mod at R35 then with both the agc range is near 60+ DB, generally enough.  AGC in this form
is less prone to overload distortion as you are lowering gain.  The control could be a pot between 8V
(or RX-V) and ground and a series 1K resistor to the he diode (x2 of your using both diodes).  That is a
manual gain control.   To make it automatic use a circuit like just published to detect the voltage at the
top of the audio gain pot and feed that voltage to the gain control diodes.  The circuit should be
arranged to put 4-8V out at NO Audio and decrease to zero volts with increasing audio.

Side note the 1n400x series with minor reservations makes a fine substitute for a PIN diode, the
prefered but more costly device for this function.  Beside being widely available and cheap
( I buy them in quantities of 100 for pennies each) makes it useful.  It also makes a good
20pf varicap and a 1A rectifier to 1000V (1n4007).

This was tested on the first bitx20 I built over a decade ago to test AGC and is used on several  older Tentec
radios and more than few of my own design.   That said its far from a new idea or design as its documented
in EMRFD and the older SSD (Solid State Design AARL press, out of print).   


Allison

Jerry Gaffke
 

Allison said:
>  If you feel that is not enough agc range then add the same mod at R35

That should probably be R33, not R35

I have no idea what the capacitance of a slightly forward biased 1n400x is,
figure 6 here suggests it's north of 30pf:  https://www.vishay.com/docs/88503/1n4001.pdf
Might be marginal at 45mhz, can vary wildly with diode type and brand.

Consensus seems to be that a 1n4007 is preferred over other 1n400x flavors for use as a PIN.
Some experimentation may be required using diodes from different manufacturers:
    https://groups.io/g/BITX20/message/32099
That entire thread is worth looking over.

If you are paying $5 postage to ship in some 1n4007's, you might consider
also getting some BAP64-02's at $0.43 each single unit pricing, Mouser 771-BAP64-02-T/R.
Down to $0.10 or so if you buy a reel.
Fully spec'd for use as an RF PIN diode. 
We've discussed the BAP64Q array of four PIN diodes in the forum previously
for use as a (roughy) 50 ohm conrolled impedance bridge attenuator.
Easily searched for on the forum website.

Jerry, KE7ER


On Sat, Apr 7, 2018 at 12:15 pm, ajparent1 wrote:
You have the audio detection to get an AGC control voltage but you don't show what your controlling.

What I find odd in general every one seems to be bent on leveling the audio volume in the audio circuit.
Why?  I understand protecting ones ears and all but what an audio compressor?  

The Bitx or uBitx has enough gain and handy places that RF gain control based on audio detection works
very well.  The easy way is replace R13 (ubitx) with a diode such as 1n400x (x=1 to 7) and controlling the
current through the diode to make it behave as a variable resistance at RF.  

The current would be about 4-6ma at max gain and decrease to zero (0) at minimum gain.  For that design
the AGC range is about 26 to 32db depening on band.  If you feel that is not enough agc range then add
the same mod at R35 then with both the agc range is near 60+ DB, generally enough.  AGC in this form
is less prone to overload distortion as you are lowering gain.  The control could be a pot between 8V
(or RX-V) and ground and a series 1K resistor to the he diode (x2 of your using both diodes).  That is a
manual gain control.   To make it automatic use a circuit like just published to detect the voltage at the
top of the audio gain pot and feed that voltage to the gain control diodes.  The circuit should be
arranged to put 4-8V out at NO Audio and decrease to zero volts with increasing audio.

Side note the 1n400x series with minor reservations makes a fine substitute for a PIN diode, the
prefered but more costly device for this function.  Beside being widely available and cheap
( I buy them in quantities of 100 for pennies each) makes it useful.  It also makes a good
20pf varicap and a 1A rectifier to 1000V (1n4007).

This was tested on the first bitx20 I built over a decade ago to test AGC and is used on several  older Tentec
radios and more than few of my own design.   That said its far from a new idea or design as its documented
in EMRFD and the older SSD (Solid State Design AARL press, out of print).   

ajparent1/KB1GMX
 

Jerry,

Rare is an order for just one part here.  Typical order for my lab are 4-20 parts and quantity.
Stock devices like 1n4007 a hundred in a order is common, same for 2n2222A, 2n3906, 2n3904
as they are cheaper when purchased in volume.  I do have the BAP64 in various packages and
several others as well. Even some exotic Avago PIN parts.  I maintain a stock that allows me
to build then optimize.  I get annoyed when I don't have something unless exotic like 10ghz
gain blocks.

When I write for open audiences I tend to use parts I know they can get or easily and effortlessly 
substitute and not require shipping for 5 pieces. 

The junction capacitance is about 15-25PF at -4V depending on vendor. Vishay says 15pf and
Motorola says 20.  For the application that's small enough to not be a factor.  Reason is rare
will it be at zero bias as it is being use in the forward direction.  However, if bias is zero, the
capacitance measured for a few 1n4007s is around 25 pf and compared to the nominal
bypass for that stage it is larger than the emitter resistance used and far higher than the
gain set resistor that was there.  At 30mhz thats a capacitive reactance of around 200
ohms and the resistor was only 10, so gain reduction is still very significant.  

Regarding  "That should probably be R33, not R35"  depends on the version of the schematic
you have.   It is annoying but it is what it is.  For the latest dump of the manual 33 would be it.
When you follow a design for over 12 years to accumulate all the flavors and variants, big disks
are cheap so I keep information going back to the flood.  The Bitx folder is small only 43MB
and history is interesting.  

One of my radios has a twin-Tee attenuator in the front end as image reject DC receivers 
have few places at RF to put variable attenuation in.  It works very well in that place but might be
harder to insert into the (u)BITX board. 

It allows the BITX ( all variants) to have RF agc and on a trial version of a bitx20 about 10 years ago
I have used it in the TX path to allow ALC for power compression to allow increased talk power.
The agc signal in that case was rectified RF out.   IT has another useful characteristic in that any
time the SWR is unacceptable the RF at that point will climb and clamp the TX power.  Its not a 
true SWR protection but it helps.

Its a really neat design for those that like to mod.

FYI that opamp AGC mod was published by W4OP and also a Y0 a while ago as applicable to 
KNQ7A, KN10, and the X1M all of which use a MC1350 for the RX IF and require a 4 to 8V signal
for AGC where 4V is max gain and 8V is min (about -50DB) gain.  It would take a transistor
and a few resistors to invert the signal and apply it to the diodes.  An opamp can do that too
and a quad device would be convenient.  

Allison

Gordon Gibby
 

​Allison, what on earth do you do for a living, and for WHOM???

gordon



From: BITX20@groups.io <BITX20@groups.io> on behalf of ajparent1 <kb1gmx@...>
Sent: Saturday, April 7, 2018 5:29 PM
To: BITX20@groups.io
Subject: Re: [BITX20] AGC circuit to try?
 
Jerry,

Rare is an order for just one part here.  Typical order for my lab are 4-20 parts and quantity.
Stock devices like 1n4007 a hundred in a order is common, same for 2n2222A, 2n3906, 2n3904
as they are cheaper when purchased in volume.  I do have the BAP64 in various packages and
several others as well. Even some exotic Avago PIN parts.  I maintain a stock that allows me
to build then optimize.  I get annoyed when I don't have something unless exotic like 10ghz
gain blocks.

When I write for open audiences I tend to use parts I know they can get or easily and effortlessly 
substitute and not require shipping for 5 pieces. 

The junction capacitance is about 15-25PF at -4V depending on vendor. Vishay says 15pf and
Motorola says 20.  For the application that's small enough to not be a factor.  Reason is rare
will it be at zero bias as it is being use in the forward direction.  However, if bias is zero, the
capacitance measured for a few 1n4007s is around 25 pf and compared to the nominal
bypass for that stage it is larger than the emitter resistance used and far higher than the
gain set resistor that was there.  At 30mhz thats a capacitive reactance of around 200
ohms and the resistor was only 10, so gain reduction is still very significant.  

Regarding  "That should probably be R33, not R35"  depends on the version of the schematic
you have.   It is annoying but it is what it is.  For the latest dump of the manual 33 would be it.
When you follow a design for over 12 years to accumulate all the flavors and variants, big disks
are cheap so I keep information going back to the flood.  The Bitx folder is small only 43MB
and history is interesting.  

One of my radios has a twin-Tee attenuator in the front end as image reject DC receivers 
have few places at RF to put variable attenuation in.  It works very well in that place but might be
harder to insert into the (u)BITX board. 

It allows the BITX ( all variants) to have RF agc and on a trial version of a bitx20 about 10 years ago
I have used it in the TX path to allow ALC for power compression to allow increased talk power.
The agc signal in that case was rectified RF out.   IT has another useful characteristic in that any
time the SWR is unacceptable the RF at that point will climb and clamp the TX power.  Its not a 
true SWR protection but it helps.

Its a really neat design for those that like to mod.

FYI that opamp AGC mod was published by W4OP and also a Y0 a while ago as applicable to 
KNQ7A, KN10, and the X1M all of which use a MC1350 for the RX IF and require a 4 to 8V signal
for AGC where 4V is max gain and 8V is min (about -50DB) gain.  It would take a transistor
and a few resistors to invert the signal and apply it to the diodes.  An opamp can do that too
and a quad device would be convenient.  

Allison

ajparent1/KB1GMX
 

Retired engineer,  about 45 years of commercial antenna design, RF, Digital, Audio
add a decade or so more for my hobby work.

I have the mother of all junk boxes, from accumulating for that long.

Allison

Gordon Gibby
 

​how many cubic feet, perchance?

Do you use fishing tackle boxes to sort?



I have a bunch of junk, but not nearly the range of stuff you seem to have and I need to figure out how to allocate space in the "bonus room"


Gordon




From: BITX20@groups.io <BITX20@groups.io> on behalf of ajparent1 <kb1gmx@...>
Sent: Saturday, April 7, 2018 5:50 PM
To: BITX20@groups.io
Subject: Re: [BITX20] AGC circuit to try?
 
Retired engineer,  about 45 years of commercial antenna design, RF, Digital, Audio
add a decade or so more for my hobby work.

I have the mother of all junk boxes, from accumulating for that long.

Allison

Lee
 

I also have a X1M with the AGC board installed. I attached the install manual from the X1M group for those interested.

ajparent1/KB1GMX
 

How many cubic feet?  Best guess something on the order of 250-400 cubic feet.
Mind you not all of that is junk in the classic sense.  Much of that is what I'd call
stockpiles.

I use 50 drawer bins and some with larger drawers that are only 18 drawers in the same
overall size.  There are 8 of those for just routine stuff like screws, resistors, caps, diodes,
connectors, LEDs, common transistors, and so on.  ITs part of the non-junk part.  Tackle boxes
are not space efficient.   However there are maybe 12 of the variable pocket size clear
polyethylene boxes of varying heights for things like batteries, electrolytic caps, crystals, 
small relays and the like.

Junk aka random assemblies, chassis and boards, mechanical, cable and wire
are in large tubs maybe of the 30 gallon size.  FIgure 10 of those.  Plus another 15 
printer paper boxes (10 ream size) many are marked as to content.  

Its organized so the two car garage can hold two cars, and a workbench there
for messy activities, even after 35 years.  

The running joke with friends is, I likely have unobtainium.

Regarding the X1M info I have that and its good but cannot be used directly
with BITX as it was designed based on the MC1350 IF chip.  That would be
tough to shoe horn into the BITX design.  The basic idea is worth understanding
and with some changes could be applied would be very useful.

Allison 

Jerry Gaffke
 

That was a general "you", addressed to the now thousands of uBitx owners. 
I'm still concerned that a random 1n400* off an ebay site could do most anything at 45mhz
There are lots of different manufacturers using lots of different processes. 

So if ordering parts for this, I'd get a BAP64 or similar.
The BAP64-03 is in a bigger package than the -02, still plenty small.
If what's on hand are some one amp power rectifiers mostly used at 60hz,
by all means give them a try first.

Very encouraging that Allison usually finds these 1n400*'s to work well at RF. 

Jerry 


On Sat, Apr 7, 2018 at 02:29 pm, ajparent1 wrote:
Rare is an order for just one part here.  Typical order for my lab are 4-20 parts and quantity.
Stock devices like 1n4007 a hundred in a order is common, same for 2n2222A, 2n3906, 2n3904
as they are cheaper when purchased in volume.  I do have the BAP64 in various packages and
several others as well. Even some exotic Avago PIN parts.  I maintain a stock that allows me
to build then optimize.  I get annoyed when I don't have something unless exotic like 10ghz
gain blocks.
On Sat, Apr 7, 2018 at 01:32 pm, Jerry Gaffke wrote:

If you are paying $5 postage to ship in some 1n4007's, you might consider
also getting some BAP64-02's at $0.43 each single unit pricing, Mouser 771-BAP64-02-T/R.
Down to $0.10 or so if you buy a reel.
Fully spec'd for use as an RF PIN diode. 

Tim Gorman
 

Allison,

These amplifier circuits are very carefully designed to provide 50ohm
inputs and outputs so that proper terminations are provided to the
mixers in order to maximize rejection of unwanted carriers. The output
of the second amplifier is to provide the proper termination to the
crystal filter.

If you replace the emitter resistors with a diode then you have
effectively changed the input and output impedances of the amplifiers.
This would be especially bad for the termination into the crystal
filter.

Google "w7zoi" and "termination insensitive amplifiers" for a
discussion.

What you suggest would probably work but without testing it is
difficult to determine the actual result on the overall operation of
the receiver.

tim ab0wr



On Sat, 07 Apr 2018 12:15:15 -0700
"ajparent1" <kb1gmx@...> wrote:

It is incomplete..

You have the audio detection to get an AGC control voltage but you
don't show what your controlling.

What I find odd in general every one seems to be bent on leveling the
audio volume in the audio circuit. Why?  I understand protecting ones
ears and all but what an audio compressor?  

The Bitx or uBitx has enough gain and handy places that RF gain
control based on audio detection works very well.  The easy way is
replace R13 (ubitx) with a diode such as 1n400x (x=1 to 7) and
controlling the current through the diode to make it behave as a
variable resistance at RF.  

The current would be about 4-6ma at max gain and decrease to zero (0)
at minimum gain.  For that design the AGC range is about 26 to 32db
depening on band.  If you feel that is not enough agc range then add
the same mod at R35 then with both the agc range is near 60+ DB,
generally enough.  AGC in this form is less prone to overload
distortion as you are lowering gain.  The control could be a pot
between 8V (or RX-V) and ground and a series 1K resistor to the he
diode (x2 of your using both diodes).  That is a manual gain
control.   To make it automatic use a circuit like just published to
detect the voltage at the top of the audio gain pot and feed that
voltage to the gain control diodes.  The circuit should be arranged
to put 4-8V out at NO Audio and decrease to zero volts with
increasing audio.

Side note the 1n400x series with minor reservations makes a fine
substitute for a PIN diode, the prefered but more costly device for
this function.  Beside being widely available and cheap ( I buy them
in quantities of 100 for pennies each) makes it useful.  It also
makes a good 20pf varicap and a 1A rectifier to 1000V (1n4007).

This was tested on the first bitx20 I built over a decade ago to test
AGC and is used on several  older Tentec radios and more than few of
my own design.   That said its far from a new idea or design as its
documented in EMRFD and the older SSD (Solid State Design AARL press,
out of print).   

Allison

Gordon Gibby
 

Thanks!!!  Gives me lots of good ideas.


On Apr 7, 2018, at 19:29, ajparent1 <kb1gmx@...> wrote:

How many cubic feet?  Best guess something on the order of 250-400 cubic feet.
Mind you not all of that is junk in the classic sense.  Much of that is what I'd call
stockpiles.

I use 50 drawer bins and some with larger drawers that are only 18 drawers in the same
overall size.  There are 8 of those for just routine stuff like screws, resistors, caps, diodes,
connectors, LEDs, common transistors, and so on.  ITs part of the non-junk part.  Tackle boxes
are not space efficient.   However there are maybe 12 of the variable pocket size clear
polyethylene boxes of varying heights for things like batteries, electrolytic caps, crystals, 
small relays and the like.

Junk aka random assemblies, chassis and boards, mechanical, cable and wire
are in large tubs maybe of the 30 gallon size.  FIgure 10 of those.  Plus another 15 
printer paper boxes (10 ream size) many are marked as to content.  

Its organized so the two car garage can hold two cars, and a workbench there
for messy activities, even after 35 years.  

The running joke with friends is, I likely have unobtainium.

Regarding the X1M info I have that and its good but cannot be used directly
with BITX as it was designed based on the MC1350 IF chip.  That would be
tough to shoe horn into the BITX design.  The basic idea is worth understanding
and with some changes could be applied would be very useful.

Allison 

pierre@...
 

Dear All,

For those of you who are still wondering how does this X1M AGC circuit manages to control the gain of the RX (without any control element).

If you look at the circuit of the X1M transceiver you will see that it uses an MC1350p IF amp chip in its receiver, however the AGC pin on the MC1350p is not used for AGC action being held at a constant voltage in the standard X1M circuit.

You install the little X1M AGC circuit board in your X1M and carefully lift the AGC pin on the MC1350p and attach it to the output of the AGC circuit board, the input of the AGC circuit goes to the top of the volume control and you then get audio AGC for the MC1350p and your X1M.

The Bitx as built does not have an MC1350p, so no simple way of controlling the gain of the Bitx receiver using this X1M AGC board however as suggested by others you can use your imagination and add a controlling element(s) to your Bitx.

You could replace the RX 2nd IF amp in the Bitx with an MC1350p as used in the G6LBQ multi band Bitx circuit. You can google this circuit to see what it looks like.

Regards,

Peter VK1XP

On 08-04-2018 09:30, Jerry Gaffke via Groups.Io wrote:
That was a general "you", addressed to the now thousands of uBitx
owners.
I'm still concerned that a random 1n400* off an ebay site could do
most anything at 45mhz
There are lots of different manufacturers using lots of different
processes.
So if ordering parts for this, I'd get a BAP64 or similar.
The BAP64-03 is in a bigger package than the -02, still plenty small.
If what's on hand are some one amp power rectifiers mostly used at
60hz,
by all means give them a try first.
Very encouraging that Allison usually finds these 1n400*'s to work
well at RF.
Jerry
On Sat, Apr 7, 2018 at 02:29 pm, ajparent1 wrote:

Rare is an order for just one part here. Typical order for my lab
are 4-20 parts and quantity.
Stock devices like 1n4007 a hundred in a order is common, same for
2n2222A, 2n3906, 2n3904
as they are cheaper when purchased in volume. I do have the BAP64
in various packages and
several others as well. Even some exotic Avago PIN parts. I
maintain a stock that allows me
to build then optimize. I get annoyed when I don't have something
unless exotic like 10ghz
gain blocks.
On Sat, Apr 7, 2018 at 01:32 pm, Jerry Gaffke wrote:

If you are paying $5 postage to ship in some 1n4007's, you might
consider
also getting some BAP64-02's at $0.43 each single unit pricing,
Mouser 771-BAP64-02-T/R.
Down to $0.10 or so if you buy a reel.
Fully spec'd for use as an RF PIN diode.
Links:
------
[1] https://groups.io/g/BITX20/message/45998
[2] https://groups.io/mt/16905058/236039
[3] https://groups.io/g/BITX20/post
[4] https://groups.io/g/BITX20/editsub/236039
[5] https://groups.io/g/BITX20
[6] https://groups.io/static/tos
[7] https://groups.io/g/BITX20/leave/defanged

ajparent1/KB1GMX
 

Tim,

The TIA is why it does work well.  The input impedance does vary but not as much as you
might believe due to feedback  and parallel loading by many resistors for both TX and RX
on the first device.  The output impedance is stable due to the two emitter followers
(darlington pair) for isolation at the output replicating the driving impedance The
collector load resistor of the first device.

In practical use its not that much an issue even for the older bitx20 where the amp
was a single device.  Do the math and see why its not a big deal.  Hint a 3:1 change
is well within the performance range.  That and it work despite the fact that the AC
Beta for 2n3904s undergo a 10:1 change from 3-30mhz.  Plug that in too.

What happens to the filter if the impedance changes over a range?
What happens to the DBM with impedance changes in the 3:1 range, if the IF port is stable?
The 45mhz filter is 15khz wide if the impedance changes what is the impact?
What is the input impedance if there is no transistors at all?  

Hint: its been tried.  Doesn't catch fire.

What the TIA gets you in this design is a stable reproducible amplifier that can
tolerate component and device variation.  It gets around the fact that filters have
unstable impedance outside their bandpass which leads to instability and unexpected
gain and impedance changes.

Farhan did a very good job in the design.  It very tolerant of variation and is stable.

Use the spreadsheet to evaluate it.  As to google and Wes, he knows me.

I've only built from the ground up maybe a dozen SSB transceivers that I use
for bands 80 through 432 but there are more than a few experiments in the
pile to answer what happens if....?  I like those as They answer the questions 
even if I didn't think afterwards it was such a good idea.  Often it value is it
validates the math and topology that I may use for something else.


Allison