RF Gain vs Attenuation

[Brett Bell]

Good evening :) 

So I have bought myself a little Xiegu X6100 and it is quite a neat little HF rig.  It has many quirks and is due a few firmware updates to fix some things but I really enjoy using it so it may become an everyday carry (like a mobile phone). 

The purpose of this post is to highlight how sensitive the rig is and to get your advice/thoughts on the use of RF gain vs Attenuators. 

My Xiegu is connected to an EFHW with 49:1 balun placed in a loop on the ground fashion and it is bringing in a ton of signal so the use of RG gain has been essential. 

Today, I have started using the Attenuator. Now this is a thing I thought I would use for needle bending signals but that is not quite the case is it, I've been learning it is also a good way of managing noise levels and I am learning how big signals/high noise levels can have an effect on the receiver.

I sought some advice from the t'internet and found this (see the end of this post).  

The attenuation on the xiegu is fixed (dunno about the 991a I also have). but thought i'd share and I would love to hear your thoughts and advice on the use of these controls. 

73

Brett

Using Attenuator vs using RF Gain Pot; Differences ?

 by Ed Senio » Sun, 01 Jun 2003 01:55:48

Hi Bob -

In the typical HF receiver, RF gain is simply manual control
of the AGC (Automatic Gain Control) circuitry... nothing more,
and nothing less.  This is why when you turn down the RF gain,
the S-meter moves up as though a strong signal was being received.
This also means that using MGC does nothing to protect the receiver
from strong signals that the AGC wouldn't do as well or better.

Unless you just enjoy diddling the RF Gain knob, about the only
reason to do so is if you don't want AGC action bringing up the
background noise between elements of your desired signal.
And "Slow" AGC should do about as well in those cases.

In the typical HF receiver, the preamp and first mixer are not
included in the AGC loop.  So reducing gain via MGC/AGC
will not protect those components from strong-signal overload.
That is what the front end attenuator is for.

My FT-1000D has a more versatile front-end attenuator setup.
It offers preamp in, preamp bypassed, 6, 12, and 18 dB attenuation
positions.  If I increase attenuation by 6 dB, and the noise level
indicated by the S-meter goes down by MORE than 6 dB, this
means that the noise level is dominated by intermodulation products.
That means that the attenuator is needed, and that using it will
IMPROVE weak-signal SNR.

But that is the sophisticated approach.  Yaesu's recommended
short-cut is just to bypass preamp and/or increase attenuation
until the background noise level does not exceed S 1 on the meter.
Normally, the need for this only occurs on 7 MHz and below.

The bottom line:  I suggest you run AGC all the time.  And if
the background noise (to include background signal clutter) is
so high that it reads S1 or S2 with the 20 dB attenuator ON, then
leave the attenuator ON.  But if the attenuator drops your noise
and clutter to S0, then do not use the attenuator.

Enjoy your listening,

73, Ed W6LOL

[ALan G3UQW]

Hi Brett

Interesting, I have been watching the 6100 as I already have the G90, which I have found to be an impressive little rig, but the 4" display would be nice to have...

Assuming it is similar, I wouldn't agree with Ed's comments - with the G90 it went through several revisions of the firmware before it got the AGC sorted out - but the Xiegu engineering team did respond quickly to the feedback! (There is a lot of debate in the G90 IO.Group which you might find of interest).

The G90 is basically a direct (zero) conversion receiver followed by a DSP, and I believe that the dynamic range of that configuration is not great, so  the AGC/RF gain/attenuator/preamp controls are quite important. The general modus operandii on a busy band, e.g. 80m, seems to be to set the RF gain at about 1/3...

The main problem with the AGC was that after transmitting it cleared the received AGC level, so that it deafened one on key up until the AGC caught up. That aside I think it is amazing value for money - so will be interested to see how the 6100 works out.

ALan G3UQW

ps. I also bought the XPA125, but when it arrived the power lead was wired incorrectly - the polarity was reversed!

Bad sign, but having correctd that, it blew the final on a matched load within 10 seconds of operation, so I returned it (no quibble).

[Brett Bell]

Hi Brett

Thanks for coming back to me Alan.  Are you the RADARC Discord server?  I hope to post some pics/video there for anyone who is interested. So please check it out when I do.

Interestingly a new firmware update has appeared to enable the WIFI and Bluetooth operation that was claimed in the sales brochure.  I wonder what fixes it includes.  I am loving the fact that the little rig has some built-in encoders for RTTY, Morse and CW via its app feature so it is quite an exciting device - i can hookup a keyboard and mouse apparently!

I appreciated the tips / advice, there is a x6100 iogroups reflector so I have joined that and there's quite a busy presence there so this bodes well. I've even tried the tips on my IC-705 and that has transformed 80 meters for me so I am made up so will do the same for the Xiegu.

FT8 works well and gets out around Europe (40m, 80 at night) on 5 watts.  Yet to try phone mode (I need to get the PSU cable sorted to get my 10w), so hopefully, the AGC will behave itself when I do.

cheers

Brett

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