From: Bill Somerville <g4wjs@cl...> - 2016-01-24 11:23:15
>> Backing off on the receiver's RF gain is caused by two possible reasons:
>>
>> 1. Noise level in receiver's AF output is too high.
>> In urban noise environment full size vertical or dipole antenna have
>> bad SNR.
>> There are two possible solutions:
>> - compact near field active omnidirectional whip antenna, like PA0RDT
>> mini-whip, it has much better SNR in comparison to full size vertical
>> - directional receiving antenna.
>>
>> 2. With AGC turned OFF there are multiple intermodulation products in
>> AF output caused by mixing of the strong
>> signals.
>> The best solution is to use high dynamic range receiver: SDR receiver
>> with no analog AF output have not got this issue.
>>
>> Backing off on the receiver's RF gain is compromise 'must-to-do'
>> solution that deals with the results of using the poor antenna system
>> or analog receiver.
>>
>Hi Igor,
>
>I do not agree with your analysis for the following reasons.
Hi Bill,
It were my statements. I am absolutely agree with everything you said but.. you
have just missed my point.
Now I have to bring some analysis.
1. HF receiver has approximately -135dBm noise floor in 2500 Hz bandwidth.
We keep AGC OFF as per our advice given to WSJT-X users.
Let's assume there is no noise coming from antenna.
With -80dBm signal level we will get signal distortion in the AF line output of
the most HF analog receivers,
hence dynamic range of analog receiver is less than mod(-135-80) = 55dB.
As you said, signal level in AF output does not exceed 60dB SNR. Reason for it
is saturation in AF stages
of analog receiver.
Now, let's assume we have two JT signals with level -75dBm each. For ADC 90dB
dynamic range we still
have (90-mod(-135-75)) = 30dB headroom, but due to the high level of
intermodulation products
in the receiver AF output we must decrease RF gain to bring signals back to the
receiver dynamic range.
We reduced RF gain and IMD disappeared, but.. doing so we also made weak
signals to disappear,
still having 30dB headroom in ADC.
Now let's assume user have not got AF line output and have to use phone output
from receiver.
There is additional AF amplifier used, that with AGC OFF decreases receiver's
dynamic range.
When you turn AGC on, you making the same thing with weak signals as does RF
gain knob,
while AGC is dynamic control for RF/IF gain, rather than static one.
2. In urban enviroment we getting approximately -80dBm noise level on 40m band
and -100dBm noise level
on 20m band from quarter wave vertical antenna. In terms of receiver dynamic
range for AGC OFF it means we have
no dynamic range headroom on 40m band and have only 20dB headroom on 20m band.
We adjust noise level in WSJT-X to 30dB and still have 60 dB ADC headroom, but
it is almost useless now on 40m band
as weak signals now as hidden under the noise and receiver intermodulation
products produced by strong signals.
There is little difference now for 40m band if we use RF gain or AGC ON, anyway
we have already lost weak signals.
As I said in my previous message, this scenario partially can be handled with
low noise antenna - the best solution
is directional antenna.
3. Let's evaluate SDR with digital AF output. For 16bit audio digital stream
dynamic range is approximately 90dB in comparison to 55dB for analog receiver.
-135dBm/2500Hz noise floor.
We keep AGC OFF as per our advice given to WSJT-X users.
Let's assume there is no noise coming from antenna. Hence this SDR will be
handling well the same dynamic range as ADC one, letting us
to put (-135-(-90)= -45dBm signals through ADC with no distortion. In this case
if WSJT-X slider adjusted to 0dB we can decode weak signals more than 80dB
below the -45dBm level.
Now let's assume that -80dBm noise level coming from antenna and we adjusted
WSJT-X level slider to 30dB.
We have 60dB headroom in ADC and (90-mod(-135-80))=35dB headroom in SDR
receiver for strong signals.
Short summary: analog receiver is current bottleneck, and playing with AGC or
RF gain is just compromise solution
that will take effect on symptoms, rather than work with reasons.
73 Igor UA3DJY
------------------------------------------------------------------------------
Site24x7 APM Insight: Get Deep Visibility into Application Performance
APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month
Monitor end-to-end web transactions and take corrective actions now
Troubleshoot faster and improve end-user experience. Signup Now!
http://pubads.g.doubleclick.net/gampad/clk?id=267308311&iu=/4140
_______________________________________________
wsjt-devel mailing list
wsjt-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/wsjt-devel
