Topics

Installing BCI and Kit-Projects AGC

Joel Trenalone
 

Hello All,
I have been working on a UBITX V5 and am installing both a BCI filter and the Kit-Projects AGC. According to what I have read regarding the BCI FILTER and the Kit-Projects AGC, both are installed by cutting in the RF receive path between K3 & K1. 

The way I read the schematic, the receive path travels from the antenna through K3 to K1 and on to the receiver. All the instructions (Kit-Projects and UBITX.net) show the AGC installed immediately after K3, then install the BCI then onto K1 (in that order). 

My my question is for the AGC to work properly, shouldn’t it be installed after the BCI filter (So the AM broadcast is filtered out)?

Please excuse my ignorance, but before I cut the trace and install these two modifications, I want to make sure they are in the right order,

Thanks in advance,
Joel KD6AGW :)

Jerry Gaffke
 

The AGC board operates on two different signals in the radio.

AGC attenuation level is determined by measuring the audio signal level
after all of the filters that pick out only your signal of interest.
The AGC board is physically placed in the incoming RF path (not in the audio section)
so it can best attenuate the very low level signal coming in from the antenna.  

You might be right that the BCI filter would be better placed in front of the AGC board,
this could help if incoming AM broadcast band signals are 10 times stronger than all the shortwave signals.
But if the AGC board's RF attenuator is sufficiently linear, it won't make any difference.
I don't see any advantage in having the BCI filter after the AGC attenuator.

Jerry, KE7ER


On Sun, Jun 9, 2019 at 09:52 PM, Joel Trenalone wrote:
Hello All,
I have been working on a UBITX V5 and am installing both a BCI filter and the Kit-Projects AGC. According to what I have read regarding the BCI FILTER and the Kit-Projects AGC, both are installed by cutting in the RF receive path between K3 & K1. 

The way I read the schematic, the receive path travels from the antenna through K3 to K1 and on to the receiver. All the instructions (Kit-Projects and UBITX.net) show the AGC installed immediately after K3, then install the BCI then onto K1 (in that order). 

My my question is for the AGC to work properly, shouldn’t it be installed after the BCI filter (So the AM broadcast is filtered out)?

Please excuse my ignorance, but before I cut the trace and install these two modifications, I want to make sure they are in the right order,

Thanks in advance,
Joel KD6AGW :)

Curt
 

Joel

no such thing as ignorance here - hopefully we are all still learning! 

this is what I see:

The AGC operates on downstream signals, after the IF filtering.  The sensing part of the AGC does NOT see the AM (medium wave) signals. 

The AGC does its attenuation with MOSFET devices.  My experience with devices is that their linearity does vary with bias - I am thinking they can be less linear when given an in-between bias.  So I think your logic is best - to put the filter first so that the AGC attenuator does not experience large out-of-band energy. 

Of course - if your ubitx is not operated anywhere near a many-kW AM station -- then it may not matter at all. 

73 Curt

PS - Mr Magoo here (me) with poor vision is straightening out the signal-strength meter wiring connected to a VK3YE circuit.  I like this AGC, and the meter is now working decently.  Instead of Peter's configuration in parallel with the LED, I have it in series via a shunt resistor.  It better represents signal strength than the parallel implementation.  You guys can snicker at this 'minority' AGC circuit but it works nicely.  Best wishes with whatever circuitry you all are using - this is a cool rig! 

KE2GKB
 

Hey guys! 
So the agc bias is actually determined by what it in the audio passband not the RF passband. It works as a variable attenuator on the RX path based off the demodulated audio out as bias.

In short it honestly should not matter if it's before or after the BCI! 

Personally I would put the BCI before it no need to risk greater distortion on receive due to overload on any of the parts but hey. 

73

On Mon, Jun 10, 2019, 8:29 AM Curt via Groups.Io <wb8yyy=yahoo.com@groups.io> wrote:
Joel

no such thing as ignorance here - hopefully we are all still learning! 

this is what I see:

The AGC operates on downstream signals, after the IF filtering.  The sensing part of the AGC does NOT see the AM (medium wave) signals. 

The AGC does its attenuation with MOSFET devices.  My experience with devices is that their linearity does vary with bias - I am thinking they can be less linear when given an in-between bias.  So I think your logic is best - to put the filter first so that the AGC attenuator does not experience large out-of-band energy. 

Of course - if your ubitx is not operated anywhere near a many-kW AM station -- then it may not matter at all. 

73 Curt

PS - Mr Magoo here (me) with poor vision is straightening out the signal-strength meter wiring connected to a VK3YE circuit.  I like this AGC, and the meter is now working decently.  Instead of Peter's configuration in parallel with the LED, I have it in series via a shunt resistor.  It better represents signal strength than the parallel implementation.  You guys can snicker at this 'minority' AGC circuit but it works nicely.  Best wishes with whatever circuitry you all are using - this is a cool rig! 


--
Tim Keller - KE2GKB
https://shop.kit-projects.com

Joel Trenalone
 

Thanks so much for all the information :)

Now I understand how it works. I’m inclined to install the BCI before the AGC, except that the Kit-Projects board is designed so nice as to fit like a puzzle piece so nicely as the instructions show. There is really no other spot that works as good. 


If i if I can figure out another point to intercept the receive rf for the BCI that puts it before the AGC I would do it, but I’m not seeing any good places. 

Ill think about this bit..........

KE2GKB
 

Which BCI are you using? I can take a look my self! I sadly know the layout acceptable well for now :)

-- Tim Keller - KE2GKB https://shop.kit-projects.com

Joel Trenalone
 

I have ordered the qrpme kit here (haven’t received it yet):
http://qrpme.com/?p=product&id=BCI

And by the way the AM interference is pretty strong, enough that I can’t really use the radio until I get it filtered. 

Curt
 

Joel

obviously test the filter to see how well it works before working hard to integrate it.  and take advantage of natural integration of the AGC board.  I am confident you would find a way somehow.  also okay to experiment to 'prove' I am wrong with my recommendation. 

Tim

Its the intermodulation of huge signals that reach the MOSFETs up front in the AGC that would concern me.  When they get a strong bias to be completely on or off they may have okay performance, but a bias voltage in between often makes it more susceptible.  yes the mosfets behave like a resistor, but there is also a transistor junction there that can operate like a mixer - not the behavior we want.  I imagine in the vast majority of cases the ubitx operator may not need a BCI filter - but in Joel's case he clearly thinks he needs it. 

Experiment on.  Results are generally 'numeric' and actual operation will tell the truth. 

Curt

KE2GKB
 

Curt we are on the same page! I actually have a rather loud SWL station near me at 7.300 that loves to blow out my front end. My comments were more on the unless it gets through into the audio output the agc will not react specifically to the loud adjacent station in the IF passband!

-- Tim Keller - KE2GKB https://shop.kit-projects.com

Jack, W8TEE
 

If the AM is domestic, the BCI filter I used works well. Here is the schematic:

Inline image
and the response plot:

Inline image

Insertion loss is small and I used axial inductors purchased from Tayda Electronics. I think the total cost was under $1 for a QRP version. For a 100W model, I used toroids and silver mica caps; details at A Cheap and Easy BCI Filter, CQ Magazine, August, 2016.


Jack, W8TEE

On Monday, June 10, 2019, 9:08:30 PM EDT, Joel Trenalone via Groups.Io <jtrenalone@...> wrote:


I have ordered the qrpme kit here (haven’t received it yet):
http://qrpme.com/?p=product&id=BCI

And by the way the AM interference is pretty strong, enough that I can’t really use the radio until I get it filtered. 

Joel Trenalone
 

Curt,
I think I will take your advice and install the Kit-Projects AGC in the location it is supposed to go. Then I’ll try the BCI after the AGC and see how it works. The local broadcast only interferes when the antenna is hooked up, so maybe it will be fine. But if not I can see if there is another location ahead of the AGC to put it. 

Jack,
thanks for the schematic. If the filter I ordered is not enough I will try yours. Hopefully ill I’ll get the filter this week and try it out. 

I do have fun experimenting, but I just don’t want to break anything :)



 

Joel Trenalone
 

Ok, so I added the BCI today and it works great. All AM is gone, and I was able to make my first contact. 

Now onto  to the AGC to verify that it will work OK before the BCI. 

One question. All the guidelines on installing the BCI include grounding the filter on both ends to the ground at C216 & C210. I only grounded on one side, because it’s all ground right?  I’m not sure why it really needs to be grounded in two places. Just wondering, and correct me if I’m wrong :)

 

Hi Tim,

I'm sure Joel, the OP won't mind me hijacking his thread, he is my brother in law:-)  I installed the AGC board today and noticed that the volume is very low now, in fact I have to crank it all the way up to hear much at all. It is installed on a V3 board. I didn't use a switch for AGC speed, I just jumped it to Fast position. I also installed the V lead to the high side of the volume control. Other than that I can't think of anything else that could be causing this. Any ideas?

Joel
N6ALT

KE2GKB
 

HI Joel,
Can you send us some pictures of your installation? Low volume can be caused by a few things, Most notably a bad RX path or A bad RX gain pot setting.
Tim Keller


On Tue, Dec 10, 2019 at 10:10 AM Joel Caulkins/N6ALT <caulktel@...> wrote:
Hi Tim,

I'm sure Joel, the OP won't mind me hijacking his thread, he is my brother in law:-)  I installed the AGC board today and noticed that the volume is very low now, in fact I have to crank it all the way up to hear much at all. It is installed on a V3 board. I didn't use a switch for AGC speed, I just jumped it to Fast position. I also installed the V lead to the high side of the volume control. Other than that I can't think of anything else that could be causing this. Any ideas?

Joel
N6ALT


--
Tim Keller - KE2GKB
https://shop.kit-projects.com

 

Tim,

Thanks for the reply, here is a picture of the install.

Joel
N6ALT

 

Here is the pic as a attachment.

Joel
N6ALT

KE2GKB
 

From the quality of the picture all looks right,
Lets check a few things! Did you cut the RX trace that is now underneath the AGC board?


On Tue, Dec 10, 2019 at 11:51 AM Joel Caulkins/N6ALT <caulktel@...> wrote:
Here is the pic as a attachment.

Joel
N6ALT


--
Tim Keller - KE2GKB
https://shop.kit-projects.com

 

Yes I did, with a exacto knife. i made sure there was no continuity between the the two traces after the cut. Strange thing is, I can short between the RF in and the RF out and there is absolutely no difference in volume. You would think if the AGC board was dragging the level down that would eliminate the board.

Joel

KE2GKB
 

Just jumping RF In/Out wont remove the agc from the circuit, It has a shunt element that would still be active :) IN order to remove the shunt element as well you would need to put the switch pads into the OFF position which will remove any bias from the Shunt FET making it high impedance/inactive.


If doing that does not make a difference I will have to ask for a better quality picture of the board so we can triple check solder work, David does an awesome job but sometimes a mistake slips through. And sometimes the 7002's break during soldering/handling we have had a non 0 amount of these in the past.

Let me know how that goes!
Tim Keller


On Tue, Dec 10, 2019 at 12:14 PM Joel Caulkins/N6ALT <caulktel@...> wrote:
Yes I did, with a exacto knife. i made sure there was no continuity between the the two traces after the cut. Strange thing is, I can short between the RF in and the RF out and there is absolutely no difference in volume. You would think if the AGC board was dragging the level down that would eliminate the board.

Joel


--
Tim Keller - KE2GKB
https://shop.kit-projects.com

 

Tim,

It doesn't make any difference where the switch selection is, the audio sounds the same. Attached is the best picture I can take.

Joel