Topics

SNA mods

 

To check whether you need this mod, connect the two SMAs by a short cable and select SNA mode
and my reference 20MHz. The reading must show -20dbm +-1db. If it shows anything like -17 or
-14 then this mod is needed for better performance.

I found the signal generator output level 10db too high. This will cause saturation of the detector stage.
Three ways of fixing this:

1. use a 10db commercial attenuator pad at the output (top)

2. Farhan's method - change R20 to 2.2K. I would suggest a 4.7K multiturn and set it so that display shows -20dbm.

3. My method R24/30 = 68 Ohms and R27 = 150 - tested on two units.

Have fun!

--
Raj, vu2zap
Bengaluru, South India.

Gerry Kavanagh
 

Thanks Raj.

 

Hi Raj,

My Antuino shows -17dbm with a cable between RF out and RF in. If I insert a variable anttenuator inline set to 8db it reads -20dbm, 10db of attenuation makes it read -21dbm. So, what value on R20 would compensate for this or what do I do to fix this?

Joel
N6ALT

 

Joel,

One of the test boards is exactly same as yours. The IF amp gain varies by a few dB but if you change that
then it affects SWR so better increase R20 like Farhans suggestion and see if that suits or you can adjust
the value to suit. Refernce is -20 dBm.

The SNA a function is the most funtastic part.

Try to see the performance of a LC filter and see what you can learn. A little pocket analyzer.

Raj

At 26-09-19, you wrote:
Hi Raj,

My Antuino shows -17dbm with a cable between RF out and RF in. If I insert a variable anttenuator inline set to 8db it reads -20dbm, 10db of attenuation makes it read -21dbm. So, what value on R20 would compensate for this or what do I do to fix this?

Joel
N6ALT

 

Okay, I removed the 470 ohm resistor soldered to R20 pads and glued a 15 turn 4.7K pot to the board and soldered the leads to the R20 pads, next adjusted for -20dbm with the straight through cable attached. It's -20dbm with the cable and -79dbm without. One thing I noticed is, the line at 20dbm is slightly below the 20dbm mark, if adjusted exactly on the 20dbm line it reads -19dbm. Probably not relevant but I thought I would report it anyway.

Joel
N6ALT

 

I recommend you measure the value and put in a fixed SMD which will behave better at higher frequencies.

At 27-09-19, you wrote:
Okay, I removed the 470 ohm resistor soldered to R20 pads and glued a 15 turn 4.7K pot to the board and soldered the leads to the R20 pads, next adjusted for -20dbm with the straight through cable attached. It's -20dbm with the cable and -79dbm without. One thing I noticed is, the line at 20dbm is slightly below the 20dbm mark, if adjusted exactly on the 20dbm line it reads -19dbm. Probably not relevant but I thought I would report it anyway.

Joel
N6ALT

 

Raj,

I agree, I will do that. Thank you for the help.

Joel
N6ALT

On Sep 26, 2019, at 8:20 PM, Raj vu2zap <@Raj> wrote:

I recommend you measure the value and put in a fixed SMD which will behave better at higher frequencies.

At 27-09-19, you wrote:
Okay, I removed the 470 ohm resistor soldered to R20 pads and glued a 15 turn 4.7K pot to the board and soldered the leads to the R20 pads, next adjusted for -20dbm with the straight through cable attached. It's -20dbm with the cable and -79dbm without. One thing I noticed is, the line at 20dbm is slightly below the 20dbm mark, if adjusted exactly on the 20dbm line it reads -19dbm. Probably not relevant but I thought I would report it anyway.

Joel
N6ALT


 

Raj and group,

The value of R20 on my Antuino ended up being 1.1K, my resistor kit didn't have that value so I stacked two 2.2K 1206 style resistors for a value of 1.1K, now it reads exactly -20dbm with a straight through cable. Thanks for the help.

Joel
N6ALT

Jim WA3APC
 

Is the original R20 a size 1206 style?

 

Yes it is.

Joel 
N6ALT

On Oct 5, 2019, at 4:29 PM, Jim Cook <Jscook@...> wrote:

Is the original R20 a size 1206 style?

Ashhar Farhan
 

You can easily solder a regular through hole resistor too.


On Sun 6 Oct, 2019, 7:23 AM Joel Caulkins/N6ALT, <caulktel@...> wrote:
Yes it is.

Joel 
N6ALT

On Oct 5, 2019, at 4:29 PM, Jim Cook <Jscook@...> wrote:

Is the original R20 a size 1206 style?

Jim WA3APC
 

I'm seeing a consistent -16dbm  so maybe will give it a go. Thanks

Jim

Ashhar Farhan
 

It should drop to -20 dbm as the diode mixer overloads beyond that level, no ha done but you need to recalibrate your eyes. Better change the resistor and give grey cells a rest.

73, f

On Sun 6 Oct, 2019, 8:25 AM Jim WA3APC, <Jscook@...> wrote:
I'm seeing a consistent -16dbm  so maybe will give it a go. Thanks

Jim

Damian 243
 

1.1k brought it to -20db here, did the same installation as Joel.

Gavin Jacobs
 

I started with -16dBm. I put in R20 = 1.2k and now it shows -19 sometimes -20.

For anyone else attempting this repair:
- the 6 screws holding the main PCB to the chassis are different lengths, the 4 longer ones go around the LCD display
- there are plastic spacers between the LCD PCB and the main PCB, be careful they don't slide out and get lost

For HFSIGNALS:
Why isn't this function tested during QC?

Jake

Ashhar Farhan
 

Jake,
for different purposes the signal level out of the output needs to be different. the 'reference' loss will depend upon a number of factors including your own pigtail cables. it comes calibrated with the resistors soldered in.
- f

On Wed, Oct 16, 2019 at 11:38 PM Gavin Jacobs <apriljunkbox@...> wrote:
I started with -16dBm. I put in R20 = 1.2k and now it shows -19 sometimes -20.

For anyone else attempting this repair:
- the 6 screws holding the main PCB to the chassis are different lengths, the 4 longer ones go around the LCD display
- there are plastic spacers between the LCD PCB and the main PCB, be careful they don't slide out and get lost

For HFSIGNALS:
Why isn't this function tested during QC?

Jake

Daniel Walter
 

FYI for FDIM version here. Only had 5 screws and all were short length (6mm) with no spacers. I swapped out 4 slightly longer (10mm screws from my junk box) for the display area and made spacers from PCB material. I then drilled them and glued them in place to hold the display in without distorting the circuit boards. I did this as part of my h/w upgrade of parts to make it hardware Antuino version 2.1.

Also, when upgrading the firmware, I had to tell Arduino IDE 1.8.10 that board was Arduino Duemilnove, not a straight Nano board. All good now. 

Checking SNA, mine shows -19dBm after h/w upgrade and no change to R20 from original. 

Dan   NM3A

Michael Shreeve
 

I did a more stupid KISS method and took it apart. Wouldn't recommend that as it has some larger and smaller screws and , though not that bad, the above methods would be better. That said, mine, which just arrived, has all the new circuit board mods, and has , I think , ver 2.1 software which apparently is the latest. So, I'm good.

zr6dx@...
 

On checking the SNA by shorting the output and input, I found my Antuino to give -16dBm.  I did the same as Joel  with two stacked 2K2 resistors and now it reads -19dBm.  Graph is perfectly aligned with the -20dBm line.

73
Noel
ZR6DX