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Jack, W8TEE

Dear OM vu3yda

Type this into a search engine:? ?ubitx bci filter

I see several useful hits. Ubitx.net shows designs using toroidal. I see a YouTube video describing a filter not needing toroidal, if you need that approach.?

This filter may need to go into receive path only, parts may not handle high power well or the loss could be too high for transmit.?

These large signals are overloading the receiver, while at the same time you may be able to tune in ssb and cw. Idea is to reject these lower frequency signals.?

Curt wb8yyy?

VU3YDA,

I found this filter from kits-projects to work well and is designed for the uBiTx, all versions.? It is installed in the receive only part of the signal path.


Easy to add and can be used in addition to the AGC.? Like the BCI, this is also installed in the receive-only part of the signal path.? There is room for both.


Very reasonable costs and easy to install.? There are 3 toroids to wind, so not too difficult.
73
Evan
AC9TU

Well his shipping from us may cost more than the parts, and likely he can build without a circuit board. I sense this is the only piece he needs to have a useful rig, hence I wanted to provide a variety of designs to fit whatever parts can be found.?

73 curt

This may sound odd to some, but this is an experience I had.

After correcting some common mode current problems, RF locking up the raduino, my next issue was exactly this: broadcast across all bands.? Before I finished building my BCI filter, someone (outside of this group) suggested I try just reflashing the firmware.? This was quick and painless and did the trick.? I am no longer getting broadcast across all bands and I still have not completed my BCI filter.

The thought process on this was that the RF may have caused some corruption in the NVRAM of the arduino.? Whether that was the case or not, it did resolve my problem.? Which makes a lot of sense to me (a computer guy) because I was having this broadcast issue in two locations 200 miles apart, so I did not understand.? The real test will be this coming week when I go the cabin and try again.

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Mark, kb9woo
Milwaukee, WI uBITX v6

Mark,

A high pass filter before the optional AGC is recommended. I have? great success with the filter published in older ARRL handbooks since it permits 160m to fully function. I recommend installing it before the AGC since I have found distortions from strong AM station otherwise. (I have a full power AM station less than 2 miles away) Without the filter, broadcast stations overdrive the wide open input of the radio.?

John

Second the motion for AGC! I was operating Field Day on CW, and a local came on and
nearly blew my ears off. Ouch! And to add insult to injury, he went away before I
could QSO him :).

- Jerry

On Thu, Jul 8, 2021 at 09:43 AM, John Cunliffe W7ZQ wrote:

A high pass filter before the optional AGC is recommended. I have? great success with the filter published in older ARRL handbooks since it permits 160m to fully function. I recommend installing it before the AGC since I have found distortions from strong AM station otherwise. (I have a full power AM station less than 2 miles away) Without the filter, broadcast stations overdrive the wide open input of the radio.?

John,

Would you please help me understand why the AGC should be after the BCI?? At least for the kit-projects AGC and BCI it seems to me that it would not really matter.? The AGC takes its level from the audio and then attenuates the RF coming from the antenna.? The fact that the broadcast signal is removed after the AGC should not really matter.? The attenuation is done based on the audio part of the signal that does get through both.

Putting the BCI after the AGC is the way that kit-projects designed the two add ons.

73
Evan
AC9TU

I am definitely not discounting the BCI nor the AGC, I will eventually do both.? I was merely sharing an experience I had.? I didn't have broadcast issues and then I did, in two different locations quite a few miles apart. It made sense at my home QTH, but didn't make sense at my remote location. After reflashing, I no longer have the issue at home. I still have the BCI kit, but I'm less frantically installing it.

Evan,

I installed the filter originally after the AGC. I found I had strange distortions on the bands that I could not explain since the BCB filter I installed looked good on a sweep and had an attenuation >80db with one of the notches tuned to the frequency of the local transmitter(WHAM). I proceeded to probe the line with my spectrum analyzer and discovered that those products were caused in the AGC when my AGC pot (4.7Mohm) that I installed in the off position was in a certain range.I installed it to make the AGC adjustable something that i.m.o. really helps the receive . I think that the distortions are produced in one of the FETs and stem from the very strong signal of WHAM radio. Installing the filter before the AGC solved the problem. It was easy to do using very thin coaxial cable. The problem may or may not show up with the fixed step AGC or with a lower AM station input level.

John
Btw, the Station is so strong here we could hear it in our land line telephone? before we got rid of it and went to cell phone and it is audible in the background of most commercial audio amplifiers.I god rid of my audio system that used cables to the speakers and installed a blue tooth audio system. That solved that problem.

John,

Thank you for the explanation.? It does make sense when you look at that strong of a signal.? I suspect the AM signal is being rectified by the audio detector and modulating the attenuation.? One simple test is to see if the distortion goes away when the AGC is turned off.

Again, thank you for taking the time to explain.

73
Evan
AC9TU

Hi Mark,

If I understand your situation, this is the second one that I have read about on this board.? That one also had a high RF situation so high that it took out Q90.? It seems these are susceptible to high RF fields.? A good reason to purchase the metal case and then install the protections that have been identified in multiple threads.? Mostly it is diode protection on the receive input and ferrites on the lines into and out of the uBiTx.

73
Evan
AC9TU

Something from long ago … the first time I installed my shack at this QTH in about 1990, I put up a sky wave loop for 80M. Surprise, the 50kW AM BCB station a couple of miles down the road — on 1300 kHz — wiped me out on ALL HF bands. I calculated what their third harmonic was at my QTH and checked their legal limit. They were legal.

So I went with a vertical antenna instead, rather than put a high-pass BCB filter on the loop.

30 years later, we have improved receiver technology and I have a whole lot more test gear. I might try the loop again. With a BCB notch filter on 1300 this time.

73
Jim N6OTQ


Sent from my quenched-gap spark transmitter.

OM Abhrajit

Wonderful news that alignment addressed it. People here will debate agc forever, it works fine without it, but I added a simple LED and photoresistor agc circuit that helps when tuning across the band. Enjoy the nice transceiver.?

Curt

Question: AGC or AVC?
AGC is automatic GAIN control - that cuts the gain of the incoming signal to knock back overly STRONG signals.

AVC is automatic VOLUME control - that cuts the gain (usually of the audio stage/s) to knock back overly LOUD signals.

From reading, it would seem to ARRL circuitry is RF-sensing AGC - that will cut back gain (below 160m?) by detecting overly strong RF (there), where kit-parts is AF-detecting AVC of overly loud audio, though cutting RF gain based on that. That allows overly strong RF right through wide-open receiver until Audio is detected, allowing overload of the previous stages. Both will have effect, but ARRL's is kinder to the RF stages and could be more generally applicable, allowing weak signal with strong audio (RFI and static) full sensitivity / gain. I don't have access to either hardware, software or design/s to test: someone with such access (or able to afford it :) ) might verify or debunk as appropriate.
73 all de ZL2DEX, OTC (50yr), NZCE.

Re-read question. High-pass filter is the BCI cure. AGC or AVC is kinder on the ear.
73!

Maybe sometimes there is a nuance between agc and avc from the tube aka valve days. Its rarely as easy as it sounds. In a modern receiver like the ubitx, gain is minimized until after the xtal filter. Then there is a opportunity to provide gain control using a complex circuit, or merely forgo it completely and let the operator use the volume control. It works decently. The bidirectional amplifier makes implementing IF agc a bit more complex, but it could be done. Most ubitx solutions major on implementing agc from the audio stage, whether the actual attenuation occurs at audio or near the front of the receiver. I find that a partial agc helps when tuning across a band. Respecting we have many languages on the planet, the debate among us English speakers whether to call this audio agc or avc seems one of style or dialect. Maybe like elevator or lift. Enjoy tickling ions with your silicon or thermions.

Curt wb8yyy?