Topics

limited dynamic range of the BITX40

Henning Weddig
 

All,

in preparation of a talk I will give on the coming Saturday (September 9) at the "UKW-Tagung" in Weinheim / Germany about the BITX40 and ideas of my improved "deluxe" version (still to be proven by a real build) and listening to Youtube videos I have analyzed the RF and audio part of the BITX using LTSPICE.

I always was wondering why the low RF gain of the circuitry gives a "hearable" audio otuput. The secret are the two audio stages: the audio preamp (Q16)  giving about 46 dB gain when using a 10 k pot as a load, as simulated with LTSPICE XVII. Also the LM386 vill give 46 dB gain (accroding the the datasheet) if pin 1 and 8 are shorted by a 10 µF cap.

LTSPICE simulation tells me a beginning distortion if the input signal is above 5 mVpp. The simulation result complies with observations on Youtube videos giving a distorted signal if a "loud" station comes in.

Whatever Audio AGC one uses, it will not change the situation, as the audio AGC will react AFTER the audio preamp.

Assuming a maximum outptu signal of the bidiamps of 0 dBm, larger output signals are possiible but then the non operationg amp (BE-diode of the "off" transistor may/will  start to conduct) will limit the output signal, and assuming a mimimum input signal of -107 dBm (1µV) a dynamic range of 75 dB is possible, due to the audio preamp this range will be only 43.5 dB now!! We are loosing 31.5 dB of dynamic range!

To overcome this situation: a lower gain (20 dB?) but high output signal capable audio preamp, audio AGC, audio PA.

The total gain of the  RF stages including mixer losses is only 21.5 dB, so the "minimum" audio signal is in the 10... µV range!

Henning Weddig

DK5LV

Jerry Gaffke
 

Or keep the high gain audio pre-amp, use that PIN diode AGC at RF?


On Mon, Sep 4, 2017 at 01:17 am, Henning Weddig wrote:
To overcome this situation: a lower gain (20 dB?) but high output signal capable audio preamp, audio AGC, audio PA.

Henning Weddig
 

of course a RF attenautor (e.g. PIN diode in Pi configuration or even better using two diodes anti-series in the series lag and the other two in the parallel leg, see AVAGO app notes) will help.

BUT: please put this circuitry AFTER the xtal filter!!  It is of no good use to put it in front of the RF input. Not onnly because the xtal filter will give a  extra time delay which may and will cause instabilities in the closed control loop!!   

Henning


Am 04.09.2017 um 15:21 schrieb Jerry Gaffke via Groups.Io:

Or keep the high gain audio pre-amp, use that PIN diode AGC at RF?

On Mon, Sep 4, 2017 at 01:17 am, Henning Weddig wrote:
To overcome this situation: a lower gain (20 dB?) but high output signal capable audio preamp, audio AGC, audio PA.

Jerry Gaffke
 

I like the idea of a PIN attenuator between Q3 and C33.
Won't disturb the BPF.
The 4dB pad at R37,38,39 should help clean up impedance variations from the PIN attenuator into the demodulator.
Perhaps a single PIN diode is sufficient if Q3 can drive the varying load presented by the PIN without too much trouble.

The audio derived AGC control signal to the PIN attenuator should attack quickly, but hang around for milliseconds.
Delays through a 3khz crystal filter should be on the order of a few hundred microseconds.
I'd be surprised if that crystal filter delay made the AGC unstable when the attenuator is placed in front of the filter.
But then I'm often surprised.

Henning's insight that the audio pre-amp is severely limiting the receiver's dynamic range 
is well worth pursuing.  

Here's an Avago app note on PIN diode attenuators:  http://www.hp.woodshot.com/hprfhelp/4_downld/lit/diodelit/an1048.pdf
And one from Skyworks:  http://www.skyworksinc.com/uploads/documents/200313B.pdf
An interesting part:  http://www.nxp.com/docs/en/data-sheet/BAP64Q.pdf

Jerry, KE7ER
 


On Mon, Sep 4, 2017 at 06:45 am, Henning Weddig wrote:

of course a RF attenautor (e.g. PIN diode in Pi configuration or even better using two diodes anti-series in the series lag and the other two in the parallel leg, see AVAGO app notes) will help.

BUT: please put this circuitry AFTER the xtal filter!!  It is of no good use to put it in front of the RF input. Not onnly because the xtal filter will give a  extra time delay which may and will cause instabilities in the closed control loop!!   

 

Ashhar Farhan
 

The bitx was sketched on a flight. I didnt know muchP about dynamic range. In retrospect, the rf amp severely compromises the dynamic range. Someone should measure the iip3 with and without the rf amp.
I thought of redoing the bitx design for Bitx40 but in the end, decided to stick to the original for it to be called a bitx.
It is a sum of many tradeoffs and lazy choices (like the LM386).
- f

On 4 Sep 2017 10:11 pm, "Jerry Gaffke via Groups.Io" <jgaffke=yahoo.com@groups.io> wrote:
I like the idea of a PIN attenuator between Q3 and C33.
Won't disturb the BPF.
The 4dB pad at R37,38,39 should help clean up impedance variations from the PIN attenuator into the demodulator.
Perhaps a single PIN diode is sufficient if Q3 can drive the varying load presented by the PIN without too much trouble.

The audio derived AGC control signal to the PIN attenuator should attack quickly, but hang around for milliseconds.
Delays through a 3khz crystal filter should be on the order of a few hundred microseconds.
I'd be surprised if that crystal filter delay made the AGC unstable when the attenuator is placed in front of the filter.
But then I'm often surprised.

Henning's insight that the audio pre-amp is severely limiting the receiver's dynamic range 
is well worth pursuing.  

Here's an Avago app note on PIN diode attenuators:  http://www.hp.woodshot.com/hprfhelp/4_downld/lit/diodelit/an1048.pdf
And one from Skyworks:  http://www.skyworksinc.com/uploads/documents/200313B.pdf
An interesting part:  http://www.nxp.com/docs/en/data-sheet/BAP64Q.pdf

Jerry, KE7ER
 

On Mon, Sep 4, 2017 at 06:45 am, Henning Weddig wrote:

of course a RF attenautor (e.g. PIN diode in Pi configuration or even better using two diodes anti-series in the series lag and the other two in the parallel leg, see AVAGO app notes) will help.

BUT: please put this circuitry AFTER the xtal filter!!  It is of no good use to put it in front of the RF input. Not onnly because the xtal filter will give a  extra time delay which may and will cause instabilities in the closed control loop!!   

 

Dexter N Muir
 

Profuse thanks, Farhan! Despite those trade-offs, perhaps even because of them, the BITX concept, and the BITX40 kit, have been a 'shot in the arm' for Ham Radio! For myself, getting old and with limited finance (retiree), it has been a transition from VHF 'myopia' and an eye-opening education. The on-line community's discovery of the various 'shortcomings', and of the means of overcoming them, has been and remains a delight to this 'old fart'.
73
Dex, ZL2DEX

Jerry Gaffke
 

Reducing that front end RF gain could well help.
But Henning is suggesting that there's more than 15 or 20 dB at stake here:
  >  a dynamic range of 75 dB is possible, due to the audio preamp this range will be only 43.5 dB now!! We are loosing 31.5 dB of dynamic range!

I'm by no means an RF kind of guy.  
But I like his argument, and I like the idea of a PIN diode attenuator for AGC between the crystal filter and modulator.
That NXP BAP64Q quad PIN diode device is $0.25 in quantity, $0.50 for a single unit from Mouser.
With that AGC in place perhaps we can leave the first RF amp as is, so the rx still works with a poor antenna. 

Jerry


On Mon, Sep 4, 2017 at 01:12 pm, Ashhar Farhan wrote:
The bitx was sketched on a flight. I didnt know muchP about dynamic range. In retrospect, the rf amp severely compromises the dynamic range. Someone should measure the iip3 with and without the rf amp.
I thought of redoing the bitx design for Bitx40 but in the end, decided to stick to the original for it to be called a bitx.
It is a sum of many tradeoffs and lazy choices (like the LM386).
- f

Ashhar Farhan
 

yeah. you could even try a 1N4007, at audio frequencies, it will do just as well. These rectifiers have PIN structure too. They work very well as attenuators through the HF range.

- f

On Tue, Sep 5, 2017 at 4:05 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:
Reducing that front end RF gain could well help.
But Henning is suggesting that there's more than 15 or 20 dB at stake here:
  >  a dynamic range of 75 dB is possible, due to the audio preamp this range will be only 43.5 dB now!! We are loosing 31.5 dB of dynamic range!

I'm by no means an RF kind of guy.  
But I like his argument, and I like the idea of a PIN diode attenuator for AGC between the crystal filter and modulator.
That NXP BAP64Q quad PIN diode device is $0.25 in quantity, $0.50 for a single unit from Mouser.
With that AGC in place perhaps we can leave the first RF amp as is, so the rx still works with a poor antenna. 

Jerry


On Mon, Sep 4, 2017 at 01:12 pm, Ashhar Farhan wrote:
The bitx was sketched on a flight. I didnt know muchP about dynamic range. In retrospect, the rf amp severely compromises the dynamic range. Someone should measure the iip3 with and without the rf amp.
I thought of redoing the bitx design for Bitx40 but in the end, decided to stick to the original for it to be called a bitx.
It is a sum of many tradeoffs and lazy choices (like the LM386).
- f


Weddig, Henning-Christof <Henning-Christof.Weddig@...>
 

Concerning control loop instabilities:

my post was just a general warning to possible instabilities, as i know from experience when working in my old QRL about 30 years ago on such problems within a commercial main maritime receiver (Hagenuk RX 1001M 10 kHz to 30 MHz).
In contrast to its predecessor (RX1001) the actuator was placed in the module in front of the filterbank (xtal filters of various bandwidth, minimum 6 pole filters).
In this configuration the AGC control loop got unstable if narrow bandwidth filters were used. The cure was a so called lead lag filter in the AGC control loop, eliminating the extra negative phase shift by partly introducing a positive phase shift, but also affecting the high frequency ripple of the filtered AGC voltage.

I am not an expert in control theory but just wanted to show that to construct a fast attack acting and slow decay (with a hang time in between of severals 100 ms not only some milliseconds) is not an easy task!!
In addition this type of control loop is non-linear or should I say "dB-linear"?
Henning
DK5LV



Am 04.09.2017 um 18:40 schrieb Jerry Gaffke via Groups.Io:

I like the idea of a PIN attenuator between Q3 and C33.
Won't disturb the BPF.
The 4dB pad at R37,38,39 should help clean up impedance variations from the PIN attenuator into the demodulator.
Perhaps a single PIN diode is sufficient if Q3 can drive the varying load presented by the PIN without too much trouble.
The audio derived AGC control signal to the PIN attenuator should attack quickly, but hang around for milliseconds.
Delays through a 3khz crystal filter should be on the order of a few hundred microseconds.
I'd be surprised if that crystal filter delay made the AGC unstable when the attenuator is placed in front of the filter.
But then I'm often surprised.
Henning's insight that the audio pre-amp is severely limiting the receiver's dynamic range
is well worth pursuing.
Here's an Avago app note on PIN diode attenuators: http://www.hp.woodshot.com/hprfhelp/4_downld/lit/diodelit/an1048.pdf
And one from Skyworks: http://www.skyworksinc.com/uploads/documents/200313B.pdf
An interesting part: http://www.nxp.com/docs/en/data-sheet/BAP64Q.pdf
Jerry, KE7ER
On Mon, Sep 4, 2017 at 06:45 am, Henning Weddig wrote:
of course a RF attenautor (e.g. PIN diode in Pi configuration or
even better using two diodes anti-series in the series lag and the
other two in the parallel leg, see AVAGO app notes) will help.
BUT: please put this circuitry AFTER the xtal filter!!  It is of no
good use to put it in front of the RF input. Not onnly because the
xtal filter will give a  extra time delay which may and will cause
instabilities in the closed control loop!!

Jerry Gaffke
 

I'd like to do a uBitx like rig without relays as they seem large and unreliable, use analog switches and PIN diodes instead, perhaps 1n4007's for the TR switch.   Lots of folks claiming success with the 1n4007 in that position, including "ElectronicAntenna Switching" by Wes Hayward in QEX, May 1995.  But worth looking at some of the rants here:
    http://forums.qrz.com/index.php?threads/looking-for-a-pin-diode.147454/page-2

Specifically this one:

"Since the 1N4007 has been made by probably fifty different companies over the years, and is offered in epoxy, glass, ceramic packages with square, round, and hexagonal die, and with die geometry from less than .040 square to larger than .090 round (and everything between), and may be fabricated by single diffusion, double diffusion, epitaxial, suprataxial or just about any other method a manufacturer chooses, nobody having a 1N4007 really has any idea what he has.The 1N4007 is also available as a die alone, without any package, in several geometries and diffusions.Since its Cj and trr are unspecified, they vary a lot from fabrication to fabrication. The really good 1N4007s made 30 years ago by Motorola in Scottsdale are long gone and now they're all made offshore, to absolutely no standards.This is why I wouldn't generalize about using a 1N4007 as a PIN substitute: Some might work great this way, others absolutely won't."

 
That guy strikes me as a bit cranky and opinionated, but he may have earned the right.  Apparently worked at Unitrode, and contributed to their "The PIN Diode Circuit Designer's Handbook".  I'd pick a few different 1n4007's from big name manufacturers on Mouser, stick with whatever brand works best.
As you push toward 30mhz the capacitance across a 1n4007 gets significant, whereas that $0.25 NXP part is good for several GHz and is designed for low distortion and a wide adjustment range as an attenuator.

If you look back to my post of a week or so ago, you will see that I didn't really know what a PIN diode was.  
So take my opinions with a large grain of salt.

Jerry, KE7ER
 


On Tue, Sep 5, 2017 at 12:54 am, Ashhar Farhan wrote:
yeah. you could even try a 1N4007, at audio frequencies, it will do just as well. These rectifiers have PIN structure too. They work very well as attenuators through the HF range.
 

Vince Vielhaber
 

In the thread you reference (below) here's a link to the pin diode
handbook they talked about. The link in the article was no good.

https://www.ieee.li/pdf/essay/pin_diode_handbook.pdf

Vince.

I'd like to do a uBitx like rig without relays as they seem large and
unreliable, use analog switches and PIN diodes instead, perhaps 1n4007's
for the TR switch.   Lots of folks claiming success with the 1n4007 in
that position, including " ElectronicAntenna Switching" by Wes
Hayward in QEX, May 1995.  But worth looking at some of the rants here:
 
  http://forums.qrz.com/index.php?threads/looking-for-a-pin-diode.147454/page-2
(
http://forums.qrz.com/index.php?threads/looking-for-a-pin-diode.147454/page-2
)

Specifically this one:

"Since the 1N4007 has been made by probably fifty different companies over
the years, and is offered in epoxy, glass, ceramic packages with square,
round, and hexagonal die, and with die geometry from less than .040 square
to larger than .090 round (and everything between), and may be fabricated
by single diffusion, double diffusion, epitaxial, suprataxial or just
about any other method a manufacturer chooses, nobody having a 1N4007
really has any idea what he has.The 1N4007 is also available as a die
alone, without any package, in several geometries and diffusions.Since its
Cj and trr are unspecified, they vary a lot from fabrication to
fabrication. The really good 1N4007s made 30 years ago by Motorola in
Scottsdale are long gone and now they're all made offshore, to absolutely
no standards.This is why I wouldn't generalize about using a 1N4007 as a
PIN substitute: Some might work great this way, others absolutely won't."

 
That guy strikes me as a bit cranky and opinionated, but he may have
earned the right.  Apparently worked at Unitrode, and contributed to
their "The PIN Diode Circuit Designer's Handbook".  I'd pick a few
different 1n4007's from big name manufacturers on Mouser, stick with
whatever brand works best.
As you push toward 30mhz the capacitance across a 1n4007 gets significant,
whereas that $0.25 NXP part is good for several GHz and is designed for
low distortion and a wide adjustment range as an attenuator.

If you look back to my post of a week or so ago, you will see that I
didn't really know what a PIN diode was.  
So take my opinions with a large grain of salt.

Jerry, KE7ER
 

On Tue, Sep 5, 2017 at 12:54 am, Ashhar Farhan wrote:


yeah. you could even try a 1N4007, at audio frequencies, it will do just
as well. These rectifiers have PIN structure too. They work very well as
attenuators through the HF range.
 

Jerry Gaffke
 


Here's a nice publicly accessible webpage on using 1n4007's in a TR switch, 
apparently good for up to 100W.

http://wa5bdu.blogspot.com/2017/03/pin-diode-tr-switch.html

The tough part is that it needs a high voltage supply, higher than the peak RF.
But note that the peak RF voltage is half of the peak-to-peak RF voltage.
We can easily create a DC voltage that is equal to the peak-to-peak RF voltage by hanging a couple diodes and a cap
off the transmit RF, stealing a wee bit of RF power.  The RF envelope should ramp up slowly enough when you hit PTT
that this DC voltage can keep ahead of the RF peak voltage.  (If it ramps up too quickly, you splatter into adjacent channels.)

A PIN TR switch would be fast and quiet, much better than relays for CW ops.
That HV DC generator from RF trick is still half baked.
Have never seen mention of it.
Weird, as it seems obvious enough.
Why would it not work?

Jerry, KE7ER

Jerry Gaffke
 

I'm assuming the HV DC generator is stealing RF from before the transmit LPF,
and that any distortion created by clipping the peaks would be easily filtered harmonics of the transmit frequency.


On Tue, Sep 5, 2017 at 12:15 pm, Jerry Gaffke wrote:
A PIN TR switch would be fast and quiet, much better than relays for CW ops.
That HV DC generator from RF trick is still half baked.
Have never seen mention of it.
Weird, as it seems obvious enough.
Why would it not work?

William Kimber
 

Hi,

Maybe have a look at the datasheet of the obsolete for years Plessy
sl621 AGC chip. I have seen a discrete transistor version published
somewhere, however it is not simple using about a dozen transistors!!

Cheers,
Will

On 09/05/2017 11:27 PM, Weddig, Henning-Christof wrote:
Concerning control loop  instabilities:

my post was just a general warning to possible instabilities, as i know
from experience when working in my old QRL about 30 years ago on such
problems within a commercial main maritime receiver (Hagenuk RX 1001M 10
kHz to 30 MHz).
In contrast to its predecessor (RX1001) the actuator was placed in the
module in front of the filterbank (xtal filters of various bandwidth,
minimum 6 pole filters).
In this configuration the AGC control loop got unstable if narrow
bandwidth filters were used. The cure was a so called lead lag filter in
the AGC control loop, eliminating the extra negative phase shift by
partly introducing a positive phase shift, but also affecting the high
frequency ripple of the filtered AGC voltage.

I am not an expert in control theory but just wanted to show that to
construct a fast attack acting and slow decay (with a hang time in
between of severals 100 ms not only some milliseconds) is not an easy
task!!
In addition this type of control loop is non-linear or should I say
"dB-linear"?
Henning

 

Take a look at AN6123.

Raj

At 06/09/2017, you wrote:
Hi,

Maybe have a look at the datasheet of the obsolete for years Plessy
sl621 AGC chip. I have seen a discrete transistor version published
somewhere, however it is not simple using about a dozen transistors!!

Cheers,
Will

Smitty, KR6ZY
 

Forgive my ignorance here. What is the dynamic range we are losing? I can already hear signals as low as my RF noise floor, and signals that are so loud I have to throw my headphones off quickly. I don't know what that is in db, but unless increasing the dynamic range is going to lower my noise floor, I don't see the benefit. 

Is the whole point to lower the in-circuit noise floor for people who have a better RF noise floor than I do? Or is there something I'm missing?

Thanks for helping me understand. 

-Mark



On Sep 4, 2017, at 3:35 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

Reducing that front end RF gain could well help.
But Henning is suggesting that there's more than 15 or 20 dB at stake here:
  >  a dynamic range of 75 dB is possible, due to the audio preamp this range will be only 43.5 dB now!! We are loosing 31.5 dB of dynamic range!

I'm by no means an RF kind of guy.  
But I like his argument, and I like the idea of a PIN diode attenuator for AGC between the crystal filter and modulator.
That NXP BAP64Q quad PIN diode device is $0.25 in quantity, $0.50 for a single unit from Mouser.
With that AGC in place perhaps we can leave the first RF amp as is, so the rx still works with a poor antenna. 

Jerry


On Mon, Sep 4, 2017 at 01:12 pm, Ashhar Farhan wrote:
The bitx was sketched on a flight. I didnt know muchP about dynamic range. In retrospect, the rf amp severely compromises the dynamic range. Someone should measure the iip3 with and without the rf amp.
I thought of redoing the bitx design for Bitx40 but in the end, decided to stick to the original for it to be called a bitx.
It is a sum of many tradeoffs and lazy choices (like the LM386).
- f

Jerry Gaffke
 

Many report that the Bitx40 receiver does better on weak signals than their commercial gear.
The issue is with strong signals.
If you never have trouble with distortion on strong signals then this is not an issue for you.

The rig is kept simple so no AGC, if a strong local signal suddenly pops in you will have to quickly turn down the volume.
But the audio pre-amp at Q16 will start distorting on moderately strong signals and the volume control is after Q16.
You can turn the audio volume control all the way down, but Q16 will still be distorting. 

We could add an RF gain control, perhaps just a 1k pot where the signal from the antenna goes into the receiver.
That would allow you to reduce the incomming signal if it was loud enough to cause distortion at Q16.

Better would be to implement some kind of Automatic Gain Control (AGC) circuit that reduces the gain at some point prior to Q16
when a strong signal is received.  No need to reach for the rf gain or the volume control.  Or tear off the headphones.

There are several AGC circuits floating around here, but most of them reduce the gain after Q16.
So they don't solve the problem of Q16 distorting on strong signals.
However, most folks here find those AGC circuits to be plenty good enough.
And if you want to work some guy across town running a kilowatt, you could just ask him to reduce power.



On Tue, Sep 5, 2017 at 11:51 pm, Smitty, KR6ZY wrote:
Forgive my ignorance here. What is the dynamic range we are losing? I can already hear signals as low as my RF noise floor, and signals that are so loud I have to throw my headphones off quickly. I don't know what that is in db, but unless increasing the dynamic range is going to lower my noise floor, I don't see the benefit. 
 
Is the whole point to lower the in-circuit noise floor for people who have a better RF noise floor than I do? Or is there something I'm missing?
 

Dexter N Muir
 

... "ask him to reduce power?" I have encountered the mentality (in a completely unrelated context)  that would retort "I've got the power, I've worked hard to get it, I'm gonna USE it!" :)
73.

Smitty, KR6ZY
 

Ahh! Thank you! That was an excellent explanation. 

How many others use their BitX with headphones instead of speakers? I've found the gain on the LM386 to be so high, even after a few mods, that I barely go past 10% on the volume control. 

The mods I've made are: I've removed the cap on the 386 that increases the gain, and I've added 100ohm series resistors between the output and the headphones (one each for left and right on the 'phones, so in parallel they're roughly the 47ohm called for) for the 1k (actually 3k to each left and right, in parallel roughly the 1k called for) from the microcontroller for side tone injection. 

Both of these do lower the gain on the 386 stage, but it's still really loud. Because of this, there's very little head room for turning down strong received signals. I could just pre-attenuate the volume pot by putting a 10k, or maybe even a 47k, on the high side. But having All Teh Gain!! Followed by attenuation seems like a way to increase the noise floor. 

Would dropping the fixed gain at or before Q16 be feasible? Would it kill two birds with one stone? It wouldn't get me back 21db, and lacking an AGC I'll still be throwing the cans off my head. But I'll have an easier time of adjusting the volume myself, and hopefully will have a little less noise in the receiver. 

-Mark


On Sep 6, 2017, at 12:42 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

Many report that the Bitx40 receiver does better on weak signals than their commercial gear.
The issue is with strong signals.
If you never have trouble with distortion on strong signals then this is not an issue for you.

The rig is kept simple so no AGC, if a strong local signal suddenly pops in you will have to quickly turn down the volume.
But the audio pre-amp at Q16 will start distorting on moderately strong signals and the volume control is after Q16.
You can turn the audio volume control all the way down, but Q16 will still be distorting. 

We could add an RF gain control, perhaps just a 1k pot where the signal from the antenna goes into the receiver.
That would allow you to reduce the incomming signal if it was loud enough to cause distortion at Q16.

Better would be to implement some kind of Automatic Gain Control (AGC) circuit that reduces the gain at some point prior to Q16
when a strong signal is received.  No need to reach for the rf gain or the volume control.  Or tear off the headphones.

There are several AGC circuits floating around here, but most of them reduce the gain after Q16.
So they don't solve the problem of Q16 distorting on strong signals.
However, most folks here find those AGC circuits to be plenty good enough.
And if you want to work some guy across town running a kilowatt, you could just ask him to reduce power.



On Tue, Sep 5, 2017 at 11:51 pm, Smitty, KR6ZY wrote:
Forgive my ignorance here. What is the dynamic range we are losing? I can already hear signals as low as my RF noise floor, and signals that are so loud I have to throw my headphones off quickly. I don't know what that is in db, but unless increasing the dynamic range is going to lower my noise floor, I don't see the benefit. 
 
Is the whole point to lower the in-circuit noise floor for people who have a better RF noise floor than I do? Or is there something I'm missing?
 

Smitty, KR6ZY
 

As for "received better than commercial gear," I definitely agree that the BitX has become my rig of choice right now. 

Having said that, I think the Magic of the BitX is its lack of AGC and high RF gain, and a wider IF. It sounds like I'm able to hear the RF spectrum directly as if my brain had a 6th sense, instead of going through this thing that's converting it for me. I can hear background noise as only a bit of noise not slammed in my face, I can hear the difference between weak and strong signals as an actual difference in volume. It's the first time I've ever heard that. 

Since listening to the BitX, when ever I use my FT-920, I've taken to turning down the RF gain to the point where the ACG isn't engaging on weak to moderate signals and just let it engage only on strong signals, and I can kinda recreate that BitX feel. I lose the S-meter, but that's ok.

I imagine this is what the BitX is like with the AGC mods you all are talking about. Maybe I should do those too. :-)

-Mark

On Sep 6, 2017, at 12:42 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

Many report that the Bitx40 receiver does better on weak signals than their commercial gear.
The issue is with strong signals.
If you never have trouble with distortion on strong signals then this is not an issue for you.

The rig is kept simple so no AGC, if a strong local signal suddenly pops in you will have to quickly turn down the volume.
But the audio pre-amp at Q16 will start distorting on moderately strong signals and the volume control is after Q16.
You can turn the audio volume control all the way down, but Q16 will still be distorting. 

We could add an RF gain control, perhaps just a 1k pot where the signal from the antenna goes into the receiver.
That would allow you to reduce the incomming signal if it was loud enough to cause distortion at Q16.

Better would be to implement some kind of Automatic Gain Control (AGC) circuit that reduces the gain at some point prior to Q16
when a strong signal is received.  No need to reach for the rf gain or the volume control.  Or tear off the headphones.

There are several AGC circuits floating around here, but most of them reduce the gain after Q16.
So they don't solve the problem of Q16 distorting on strong signals.
However, most folks here find those AGC circuits to be plenty good enough.
And if you want to work some guy across town running a kilowatt, you could just ask him to reduce power.



On Tue, Sep 5, 2017 at 11:51 pm, Smitty, KR6ZY wrote:
Forgive my ignorance here. What is the dynamic range we are losing? I can already hear signals as low as my RF noise floor, and signals that are so loud I have to throw my headphones off quickly. I don't know what that is in db, but unless increasing the dynamic range is going to lower my noise floor, I don't see the benefit. 
 
Is the whole point to lower the in-circuit noise floor for people who have a better RF noise floor than I do? Or is there something I'm missing?