Topics

ND6T AGC Troubleshooting

Curt
 

I am beside myself that I cannot get this simple circuit doing something with the ubitx.  I am starting with the simple version with a single MOSFET, and I may add the series one later. 

I built one unit - nada.  So I build a second one, and left off the MOSFET.  I have its input attached to high side of the volume control pot (I think it is 10k in the Sunil case version), the ground attached to a connector in the closest corner of the board, and 5 volts from the raduino.  No matter how huge the signal -- I get no rectified voltage output.  What could I be doing wrong?  With hundreds of these boards in uBITX I am surprised by my result thus far. 

I am using a 2n2222A for the NPN, I have tried 3 different devices.  Yes the diodes each work on the DVM, and they are proper orientation. 

I also could not get the VK3YE AGC to work either.  The ubitx receives wonderfully and I am testing on 40m. 

73 Curt

PS - I do have a hybrid cascode but I am thinking to use it in a homebrew Rx -- not sure I wish to 'break' one of the bidirectional amplifiers here.  my goal is just to AGC the very largest signals. 

Albert Peter
 

Hi guys,

The new JackAl board for uBITX uses a hybrid JFET circuit for AGC control.  Have a look on   hamradiodesigns.com    and download the User Manual Section 2.  There is a complete AGC circuit in there which works very well.  You do have to break the IF chain in one spot, but nor other modifications and the uBITX reception is not changed.

 

Sent from Mail for Windows 10

 

From: Curt via Groups.Io
Sent: Thursday, January 10, 2019 9:54 PM
To: BITX20@groups.io
Subject: [BITX20] ND6T AGC Troubleshooting

 

I am beside myself that I cannot get this simple circuit doing something with the ubitx.  I am starting with the simple version with a single MOSFET, and I may add the series one later. 

I built one unit - nada.  So I build a second one, and left off the MOSFET.  I have its input attached to high side of the volume control pot (I think it is 10k in the Sunil case version), the ground attached to a connector in the closest corner of the board, and 5 volts from the raduino.  No matter how huge the signal -- I get no rectified voltage output.  What could I be doing wrong?  With hundreds of these boards in uBITX I am surprised by my result thus far. 

I am using a 2n2222A for the NPN, I have tried 3 different devices.  Yes the diodes each work on the DVM, and they are proper orientation. 

I also could not get the VK3YE AGC to work either.  The ubitx receives wonderfully and I am testing on 40m. 

73 Curt

PS - I do have a hybrid cascode but I am thinking to use it in a homebrew Rx -- not sure I wish to 'break' one of the bidirectional amplifiers here.  my goal is just to AGC the very largest signals. 

 

Don, ND6T
 

What is the voltage on the base of Q1? What signal source are you using when you see no voltage output? Are you measuring at the junction of C3, R3, the cathode of D2, and the gate of Q2 to ground? Did you measure the resistance from the other side of that pot to the point that you are using for ground? 73, Don

WA6PXC Rich <sigma@...>
 

Hi Curt,
I have tried a number of the AGC circuits floating around out there.
They all seem to work, but do not handle large (>1000 microvolts) signals.
Out of frustration, I have developed an AGC controller which controls the RF path
instead of the audio path. It begins to control the signal at about 30 uv and continues
to provide very good control up through about 7000 uv.
It essentially uses a BS170 FET in place of R12.
I have a neighbor 2 blocks away running 500 watts on 40 meters. His signal is not distorted
now...
If you are interested I can lead you through the process of building one...
Kind Regards..
Rich

-----Original Message-----
From: "Curt via Groups.Io"
Sent: Jan 10, 2019 6:54 PM
To: BITX20@groups.io
Subject: [BITX20] ND6T AGC Troubleshooting

I am beside myself that I cannot get this simple circuit doing something with the ubitx.  I am starting with the simple version with a single MOSFET, and I may add the series one later. 

I built one unit - nada.  So I build a second one, and left off the MOSFET.  I have its input attached to high side of the volume control pot (I think it is 10k in the Sunil case version), the ground attached to a connector in the closest corner of the board, and 5 volts from the raduino.  No matter how huge the signal -- I get no rectified voltage output.  What could I be doing wrong?  With hundreds of these boards in uBITX I am surprised by my result thus far. 

I am using a 2n2222A for the NPN, I have tried 3 different devices.  Yes the diodes each work on the DVM, and they are proper orientation. 

I also could not get the VK3YE AGC to work either.  The ubitx receives wonderfully and I am testing on 40m. 

73 Curt

PS - I do have a hybrid cascode but I am thinking to use it in a homebrew Rx -- not sure I wish to 'break' one of the bidirectional amplifiers here.  my goal is just to AGC the very largest signals. 

Curt
 

Don

reply gratefully appreciated. 

base is biased at 0.67 volts, and collector is at 5.07 volts as the raduino supply.  so this looks okay in terms of biasing the transistor. 

I have the rig plugged into a MA8040 vertical with a nice radial field here on the east coast.  I tune across 40m and look for huge signals.  A loud SSB, or much higher a shortwave broadcast has me cranking the volume down as low as it can go to stand it with headphones. 

resistance across the volume control pot is 9.9 k ohms, and I get same value when I touch the screw at the corner of the main board. 

At this point, in the circuit installed - I only have a 100k resistor and a 2.5 uF capacitor (non-polarized, I measured it to confirm) loading the output of the rectifier.  I built this board as a take 2, the other one is behaving similar with a MOSFET installed. 

the diodes each measure around 0.58 on diode setting of my DVM.  They seem to be diodes. 

Curt

Don, ND6T
 

I'm just sorry to hear that it isn't working for you. From your description it all seems correct. It would be nice if there was some substantial signal that you could tune to, something steady and strong. If you had a signal generator or some local signal source that you could tune into the audio pass-band then that would help. Barring that, a good AM broadcast signal, even WWV, would be nice. Use the carrier as the tone. I have usually obtained about 3 volts or more (at the point where you are testing) with a strong 1 mv signal. That mosfet needs a half volt to begin to conduct and if you aren't getting that there is a problem. A strong signal should be measurable at the high side of your volume control when using a DVM on the AC scale. If you can get that, then you should be able to walk it back up the AGC circuit and see what is going on. Yes, the Q1 bias is about right and the forward barrier voltage of your diodes are in the ball park. Maybe a short, open, or faulty component. -Don

Don, ND6T
 

Curt,
Just to make sure, I opened up my uBIITX and checked voltages. With a 50 uV (S9, -73 dBm) there was about 1 to 2 millivolts RMS on the top of my volume control. But with 1,000 uV (about S9+27, -46 dBm) there was a substantial .6 VRMS at the top of the volume control. That gave a nice 1.1 VDC at the AGC bias with it active. This is the dual mosfet model so it was giving me a pleasant output. That will hold up to nearly 4 volts before it runs out of control headroom and cannot attenuate any more at around S9+50! Your single mosfet should hold it up to S9+20 at which point that test point will read about 4 volts DC. Those measurements were taken with the signal about 1 KHz from the displayed frequency, just a constant CW tone. That allows the digital meter to track without resorting to peak holding.
-Don

Curt
 

Don

Thanks for wealth of info. Possibly with low solar flux it dawned on me that maybe signals are down a bit. I can see what my ft950 reads for largest signals. I have a fcc2 dds that has audio output, and a scope. So I will take some more data. This second unit I built very conservative,  I have measured to affirm its okay.  I seem to be confirming I don't have signals large enough to bias the diodes. Nor do I have plans to add a 40m yagi. The jackal agc does use rectifier diodes, with more audio stages. For now I am content with lcd display, so will continue with pursuing a stand alone agc.

Curt

DJ2AG
 

Hi Rich,

I‘m interested in your AGC controller. Is it possible to get the circuit diagram or some more infos?

kind regards 
Armin, DJ2AG

WA6PXC Rich <sigma@...>
 

Hi Armin,
I developed this AGC because all of the others act on the audio signal.
When the signal is really large, the audio is already distorted so we
just control a distorted signal. Please see the Attachment.

To get an idea of how well this type of AGC will work on your radio,
remove R-12 and replace it with the FET as shown in the schematic,
with just the 10K resistor going to the gate.
Using a 10K pot, connect it across 5v and connect the wiper to the Gate resistor.
As you turn the pot, it will vary the RF gain.

Have fun...
Rich

-----Original Message-----
From: DJ2AG
Sent: Jan 12, 2019 5:42 AM
To: BITX20@groups.io
Subject: Re: [BITX20] ND6T AGC Troubleshooting

Hi Rich,

I‘m interested in your AGC controller. Is it possible to get the circuit diagram or some more infos?

kind regards 
Armin, DJ2AG

Jerry Gaffke
 

Rich,

You are correct, it is better to have AGC reduce gain at RF or in one of the IF amps.
The audio pre-amp at Q70 of the uBitx (or Q16 of the Bitx40) is the weak link, 
restricting available dynamic range if AGC attenuation occurs after that audio pre-amp.

However, take another look at the ND6T AGC, it attenuates the RF as it comes in from the antenna.
Not unlike your design, but using FET's instead of op-amps.
Simple and effective.

Would be interesting to do an A/B comparison of your implementation vs the ND6T AGC.
Lots of variables here regarding dynamic range of the attenuation, attack and release times, distortion, ...

Jerry, KE7ER


On Sat, Jan 12, 2019 at 08:46 AM, WA6PXC Rich wrote:
Hi Armin,
I developed this AGC because all of the others act on the audio signal.
When the signal is really large, the audio is already distorted so we
just control a distorted signal. Please see the Attachment.
 
To get an idea of how well this type of AGC will work on your radio,
remove R-12 and replace it with the FET as shown in the schematic,
with just the 10K resistor going to the gate.
Using a 10K pot, connect it across 5v and connect the wiper to the Gate resistor.
As you turn the pot, it will vary the RF gain.
 
Have fun...
Rich

WA6PXC Rich <sigma@...>
 

Hi Jerry,
Thanks for your input. I agree, his circuit is elegant & simple .
I wanted to build an AGC and Squelch circuit on a separate board and just
remove one component & connect two wires to the BitX board (minimum mod to BitX board).
See attachment..
Rich

-----Original Message-----
From: "Jerry Gaffke via Groups.Io"
Sent: Jan 12, 2019 9:18 AM
To: BITX20@groups.io
Subject: Re: [BITX20] ND6T AGC Troubleshooting

Rich,

You are correct, it is better to have AGC reduce gain at RF or in one of the IF amps.
The audio pre-amp at Q70 of the uBitx (or Q16 of the Bitx40) is the weak link, 
restricting available dynamic range if AGC attenuation occurs after that audio pre-amp.

However, take another look at the ND6T AGC, it attenuates the RF as it comes in from the antenna.
Not unlike your design, but using FET's instead of op-amps.
Simple and effective.

Would be interesting to do an A/B comparison of your implementation vs the ND6T AGC.
Lots of variables here regarding dynamic range of the attenuation, attack and release times, distortion, ...

Jerry, KE7ER


On Sat, Jan 12, 2019 at 08:46 AM, WA6PXC Rich wrote:
Hi Armin,
I developed this AGC because all of the others act on the audio signal.
When the signal is really large, the audio is already distorted so we
just control a distorted signal. Please see the Attachment.
 
To get an idea of how well this type of AGC will work on your radio,
remove R-12 and replace it with the FET as shown in the schematic,
with just the 10K resistor going to the gate.
Using a 10K pot, connect it across 5v and connect the wiper to the Gate resistor.
As you turn the pot, it will vary the RF gain.
 
Have fun...
Rich

DJ2AG
 

Hi Rich,

thanks for the diagram. Will give it at try.

Armin, DJ2AG

Harvey
 

Rich, in your AGC circuit what do you set Q1 gate voltage to with no signal? It should be fully on with no audio signal and turn off as detected audio is applied.

Harvey, WA2AAE

WA6PXC Rich <sigma@...>
 


Hi Harvey,
I pulled the board and measured the gate voltage on the BS170.
With no RF signal it reads 3.2629v

-----Original Message-----
From: Harvey
Sent: Jan 16, 2019 11:18 AM
To: BITX20@groups.io
Subject: Re: [BITX20] ND6T AGC Troubleshooting

Rich, in your AGC circuit what do you set Q1 gate voltage to with no signal? It should be fully on with no audio signal and turn off as detected audio is applied.

Harvey, WA2AAE

Harvey
 

Thanks, now I know where to set the pot.


On 1/16/2019 7:31 PM, WA6PXC Rich wrote:

Hi Harvey,
I pulled the board and measured the gate voltage on the BS170.
With no RF signal it reads 3.2629v
-----Original Message-----
From: Harvey
Sent: Jan 16, 2019 11:18 AM
To: BITX20@groups.io
Subject: Re: [BITX20] ND6T AGC Troubleshooting

Rich, in your AGC circuit what do you set Q1 gate voltage to with no signal? It should be fully on with no audio signal and turn off as detected audio is applied.

Harvey, WA2AAE