Topband: ADC Overload

Tue Oct 13 16:26:04 EDT 2015

I though your original post was excellent, Jim, and right on the money.

On Tue, Oct 13, 2015 at 1:29 PM, Jim Garland <4cx250b at miamioh.edu> wrote:

> Having said that, here's the fly in the ointment. There are several
> assumptions buried in the Central Limit Theorem, so its validity isn't as
> universal as I was suggesting. One assumption is that all these signals
> have to have about the same amplitude. If one signal is vastly stronger
> than the others, as Tom W8JI finds in his contest station, then that will
> dominate the input of the SDR and you won't get this cancellation effect.
>

I showed
a simulation of 
this last weekend at DCC -- when the other signal drops to ~10dB below the
first, on first glance the data seen by the ADC looks a lot like just the
one, stronger signal.  This video can be seen here:

https://www.youtube.com/watch?v=CgYtpyNp7hg

Forward in to minute #9 for the start of the discussion or minute #13 to
see the simulation.

A single strong signal that reaches the overload point of the receiver will
have serious detrimental effects.  These signals will be in the S9+70-80dB
range.  Two signals at this level will also have a serious detrimental
effect requiring them to be 6dB lower than a single signal that would cause
issues.  The answer here is generally to filter signals like this.
Receiver preselectors in the front of the radio are designed to knock
signals like this down 20-30dB.  I think it should be rare to have a
co-located signal that would be in the S9+90dB range that would not be
helped by a preselector.  But, when you have multiple operators on the same
band, you lose the benefit of a preselector and have to rely, instead, on
station design, stubs, etc. to increase antenna isolation.  W2VJN's book is
an excellent reference on dealing with interference locally.

> Similarly, if you have a really strong AM broadcast station near your QTH,
> then that could still overload the ADC in your SDR transceiver. Keep in
> mind that the Central Limit Theorem is a complex mathematical statement
> whose validity is only as good as the validity of its underlying
> assumptions. To me, the CLT provides a ballpark guess as to how an SDR will
> perform, but it shouldn't be taken too literally. The real world has a
> habit of not conforming to mathematical theorems!
>

Agreed.  AM broadcast can be a especially difficult to deal with because
of the amplitudes involved.  A HPF below 160m is a good solution.  Our
radios all have this, but the filter is removed if you tune below 1.8MHz
(because your intention appears to be to look at the broadcast band rather
than reject it).  I don't believe a broadcast filter is generally used in
a superhet because they require bandpass filters and the bandpass filters
necessarily filter the broadcast band too.

>
> As I've thought more about this issue, I think a key advantage of an SDR
> actually comes, not just from the cancellation effect, but from the fact
> that an SDR has no front-end RF amplifier or subsequent IF amplifiers.
>

The presence of a front-end amplifier does vary by design -- some designs
have an amplifier in front of the data converter.  The typical NF of a data
converter in this frequency range is 20-30dB, but this can be affected by a
transformer on the front of the ADC or an amplifier.  Because the NF
requirement for many HF bands is below that of the converter, many designs
can run without the amplifier which can have benefits as amplifiers can
introduce issues such as distortion and loss of dynamic range.  We use the
AD9467 and generally run without an amplifier, but the amplifier is
required on the higher bands.  Other common designs use the LTC2208/9
family and the recommendation on that ADC is to use an amplifier and so
they typically have one.

Not having mixers and amplifiers further down the chain (IF) is a clear win
for direct sampling SDRs.

> Basically the HF spectrum goes directly into the SDR's analog-to-digital
> converter. I believe (hope someone can verify this) that modern, fast ADCs
> can handle a volt or more at their inputs without overloading, which gives
> them a tremendous advantage over superhet radios which use high gain RF and
> IF amplfiers.
>

This is all correct.  The part we use has 2-2.5V peak-to-peak max input
(selectable).  The LTC2209 operates in either 1.5V or 2.25V peak-to-peak
modes.

> Signals in the millivolt range that fall outside the IF passband in
> conventional superhets won't capture the receiver's AGC and can therefore
> overload or dessense the front end. That won't be a problem with SDR radios
> that can handle a volt or more without overloading their ADC.
>

Correct.  There is no RF AGC in a typical "good" direct sampling design.
The only AGC that exists is to map the total dynamic range (in excess of
100dB) into the available "comfortable" dynamic range of your ear (the ear
has a total dynamic range of something around 130dB, but for comfortable
listening we assume something in the order of 40dB.  It is not comfortable
to strain to hear a weak signal at the bottom volume level only to have a
strong signal blow your earphones off).  This AGC is a final step in the
signal chain after demodulation and should not be confused with RF AGC
which tries to compensate for a lack of instantaneous dynamic range in a
radio by shifting a smaller dynamic range up and down a ladder.

Steve

Stephen Hicks, N5AC
VP Engineering
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