Add S-meter to VK3YE Audio AGC

Curt WB8YYY has been pleased with the  somewhat unusual VK3YE AGC circuit, that uses a LDR and LED pair,  as it nicely removes the top of large signals.

VK3YE has suggested measuring current in parallel with the LED that drives the AGC action, but he found this gave little indication of relative signal strength.  In fact, it works much better measuring current in series with the LED.

Curt is using a small meter movement supplied by Sunil, about 250 uV peak current.  A shunt resistor across the meter is necessary since the LED current at peak is at least 20 mA.

The approximate value of the shunt resistor can be found using the formula Rsh = ( Im x Rm ) / Ish.  Rm was unknown but Curt was able to measure it with his DVM at around 500 ohms.  Inserting the two known values gives a shunt resistance value of 6.25 ohms.

Curt found a resistor of around 5 ohms and it working nicely.

He says “Its not a real S-meter response – let’s call it a signal strength meter.”  It can discern signals from approximately S5 to S9.  for signals that do not result in meter movement, the LED itself could be observed to sense signal strength – but the meter represents a nice touch.


Switching AGC delay settings with ND6T mod

Derek G4VWI has used a sub-miniature MIL spec rotary switch to switch between common, off/slow/fast as per  the ND6T wiring diagram for the Kit Projects board.

A screened pre-made wiring loom helps ensure the exposed sections are kept nice and short. Derek suggests choosing good quality connecting wire and pre-soldering the pads before tacking on the leads for best results.  He believes that this is by far the easiest AGC board design to fit.


Quick review of the Kit-Projects AGC board

Jim W0EB, TSW Project Coordinator,  bought two of these kits for his 2 uBITX Version 5 radios. First off, the directions for installing this board are brief, but they are easily followed and the boards are easy to install.

Not wanting to drill a hole for the included switch and run a bunch of wires to it, he just wired the common pad to the “fast” pad for always on, Fast AGC.  A “Via” hole was identified in the trace between R70 and “Vol HI” and Jim ran a wire from there to the “VOL” pad on the AGC board. This worked great.

Even though the “S-meter” output of this AGC system was designed to work with the CEC software, we found it worked with TSW’s BITeensio board as well.

The BITeensio uses the A19 analog input on the Teensy 3.6 for the S-meter. This little AGC system drives the S-meter routines just fine on the BITeensio.

A 50 microvolt (-73 dBm) signal was fed into the antenna connector and Jim adjusted the software’s S-meter routine’s division ratio so the touch screen’s display read S9 as it should with a 50µV input. The rest of the S units were so close to correct that no further tweaking was deemed necessary.

He also found that adjusting the on board RF gain control for max recieved signal was the best way to adjust the level control. As you turn the control counter clockwise, the gain increases and there is a point past which saturation occurs. This is obvious when listening to a weak signal and you can hear the gain drop past this point. Simply adjust the gain to that point and turn it back to where the signal is just peaking.  It is best to just leave it there if you want your S-meter to work right.

Once adjusted, this little AGC board keeps the RF input nicely within bounds on strong signals quite well.

Jim calls out ND6T and N8DAH and says “Well done guys, well done! The kit is certainly worth the price IMO”.


Kit Projects AGC board

David N8DAH has AGC boards for the µBITx available to order.

If you have any questions please e-mail either David N8DAH or Tim, KE2GKB at: info at kit-projects dot com.

The cost of the kit is US$15.  Pretty much all international shipping is US$ 14 or US$15 with the exception of Canada which is US$10.50.

Order and information on the kit can be found here:

ND6P Volume limited and distortion reducer

ND6P came up with this mod after testing his uBITX on the bench to determine how much signal it could handle before distorting at the speaker due to clipping of the audio signal.  What he found is that RF input signals greater than S9+5dB (-68 dBm) become distorted due to the audio output clipping on the negative side.

What the mod does is insert enough attenuation at the RF input to prevent the audio from clipping for signals stronger than S9+5dBm. Signals S9+5dBm and weaker are not attenuated. He’s tested with RF input as strong as S9+40dBm (-33 dBm) and gets a clean output.

He says he can now listen to a weak signal without having to reach for the volume control when a strong signal comes on. So effectively this is an automatic RF attenuation circuit to provide AGC functionality for the uBITX.

Points connected to the volume control are in parallel with existing wiring.

The pin diodes (MA27B) are available at RF Parts Co.


ND6T AGC design a winner – boards coming!

There has been a bit of BITX20 IO Groups list chatter about the winner in ND6T’s AGC circuit (with RF gain control).

Ion VA3NOI has two versions of an ND6T AGC circuit board designed.

  • A through-hole single-side version measuring 17X38 mm.
  • An SMT board measuring 18X31 mm, with SMT components that are 0805 in size.

The . 1 uF cap is tantalum or niobium polarized.

The design was done using Eagle 7.5 freeware. The zip file in the Gerbers Folders should be uploaded to the PCB manufacturing house.

The boards are roughly 1 square inch in surface. You can place 10 boards on a 100X100 mm panel and get 10 panels for $5 plus shipping.   For making into a kit, Ian recommends the SMT version as it will be more affordable and he expects it to have better performance on the upper bands.

He has ordered a through-hole version for experimenting with some values (AGC release time minimum resistor and coupling capacitors on RF line) and this is en route to Canada.

The PCBs have provision for adding a pot to adjust AGC release time and a switch to disable the AGC.   Ion has posted the files (schematic, board layout and Gerbers) here:

Constructors should feel free to use the files as they see fit.  He is also happy for anyone wanting to put kits together.

Kees K5BCQ is looking into whether it can be sold as a kit through his website.