Elecraft

K3 dbV in 2.38

Classic

List

Threaded

26 messages Options

Alan Bloom

Re: K3 dbV in 2.38

> Into a linear load, and I presume we have one here[A], there is no
> difference between dB relative to a voltage, using the 20 log10,
> voltage, formula and dB relative to the power corresponding to that
> voltage using the 10 log10. power, formula.

That's only true if the source and load impedance are equal. For
example, if an amplifier has a high-impedance input and a near-zero-ohm
output, then the power gain (in dB) is almost infinite even if the
voltage gain (in dB) is small.

That's probably why Elecraft chose the dBV notation to mean "deciBels of
voltage gain" even though it might be confused with the official
meaning, "deciBels relative to 1.0 volt".

Al N1AL

On Wed, 2008-09-17 at 00:03, David Woolley (E.L) wrote:

> Lyle Johnson wrote:
>
> >
> > We have a total of three characters available, and we wanted to express
> > that this was a dB value, and it was voltage based. So we used dBV.
>
> Into a linear load, and I presume we have one here[A], there is no
> difference between dB relative to a voltage, using the 20 log10,
> voltage, formula and dB relative to the power corresponding to that
> voltage using the 10 log10. power, formula. The difference between 10
> and 20 is to account for the need to square the voltage to get the power.
>
> In this case the units should be bare dB. You will still have problems
> because of the large number of people who don't know the definition of a
> dB and are only used to seeing dB's relative to some defined standard.
>
>
> [A] strictly speaking the load at any one stage could be non-linear, as
> long as the total system, through to the air outside the speaker
> believes in a linear way.

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

alsopb

Re: K3 dbV in 2.38

In reply to this post by Don Wilhelm-4

Apparently I need some help in trying to use this dBV capability with on the air signals.

For example, using it as an AGC off "relative" S-meter with digital readout to run some antenna pattern checks for skewness etc.

I've used it just fine in "lab type" measurements with a constant signal source.

First question. What is meant by stable value of AFV?

Here's what I see for a bunch of AFV edited values one after another with no change in K3 settings.
1579,1688,1598,1624,1696,1613, 2168,1758,1425.
The range is 6db.

Is the reference value of AFV used in the dbV calculation the last one taken at the instant one leaves the menu entry? Yes, it doesn't really matter if that values is used forever.

Suppose one then takes a series of dbV measurements, is the reference AFV ever changed during the set of readings?

The integration times available are 500 and 1000 ms. It would seem that longer integration times are required for off air type measurements.

Is there any integration time in the measured voltage used in the dBV calculation? Or is it nearly an instantaneous value? For off the air type measurements, it would seem that a suitably long value is needed.

73 de Brian/K3KO

KK7P

Re: K3 dbV in 2.38

> I've used it just fine in "lab type" measurements with a constant signal
> source.

Which is precisely what it was designed for.

> Is the reference value of AFV used in the dbV calculation the last one taken
> at the instant one leaves the menu entry?

It is the last one. It might be the last one DISPLAYED, or if it has
started another measurement interval, then it will be the next one.

> Suppose one then takes a series of dbV measurements, is the reference AFV
> ever changed during the set of readings?

No, the reference is only updated if you go back to AFV.

> The integration times available are 500 and 1000 ms.

Correct. These are the choices.

> Is there any integration time in the measured voltage used in the dBV
> calculation?

500 or 1000 ms, as noted above.

73,

Lyle KK7P

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

Eric Swartz - WA6HHQ

Re: K3 dbV in 2.38

Administrator

In reply to this post by alsopb

Hi Brian,

The dBV and AFV functions were designed primarily for use with steady
state lab signals to test receiver sensitivity etc. We added these two
displays to speed our final test of the K3 in production, and as an aid
to do this in the field with simple tools like our XG2 test oscillator.
They were not intended for dynamic signal, off the air measurements.

73, Eric WA6HHQ
---

_..._

alsopb wrote:

> Apparently I need some help in trying to use this dBV capability with on the
> air signals.
>
> For example, using it as an AGC off "relative" S-meter with digital readout
> to run some antenna pattern checks for skewness etc.
>
> I've used it just fine in "lab type" measurements with a constant signal
> source.
>
> First question. What is meant by stable value of AFV?
>
> Here's what I see for a bunch of AFV edited values one after another with no
> change in K3 settings.
> 1579,1688,1598,1624,1696,1613, 2168,1758,1425.
> The range is 6db.
>
> Is the reference value of AFV used in the dbV calculation the last one taken
> at the instant one leaves the menu entry? Yes, it doesn't really matter if
> that values is used forever.
>
> Suppose one then takes a series of dbV measurements, is the reference AFV
> ever changed during the set of readings?
>
> The integration times available are 500 and 1000 ms. It would seem that
> longer integration times are required for off air type measurements.
>
> Is there any integration time in the measured voltage used in the dBV
> calculation? Or is it nearly an instantaneous value? For off the air type
> measurements, it would seem that a suitably long value is needed.
>
> 73 de Brian/K3KO
>

Jack Smith-6

Re: K3 dbV in 2.38

I've independently measured the linear range of the DBV function as just
over 60 dB, which is quite impressive. You can see the plot at
http://www.cliftonlaboratories.com/elecraft_k3_noise_blanker_and_crystal_dsp_filtering.htm.

That page also provides measured filter response curves using the K3's
DBV function.

DBV is a very useful tool and a tip of the hat to the lads at Elecraft
for including it. (lasses as well as lads, if appropriate.)

Jack K8ZOA
www.cliftonlaboratories.com

Eric Swartz WA6HHQ - Elecraft wrote:

> Hi Brian,
>
> The dBV and AFV functions were designed primarily for use with steady
> state lab signals to test receiver sensitivity etc. We added these two
> displays to speed our final test of the K3 in production, and as an
> aid to do this in the field with simple tools like our XG2 test
> oscillator. They were not intended for dynamic signal, off the air
> measurements.
>
> 73, Eric WA6HHQ
> ---
>
> _..._
>
>
>
> alsopb wrote:
>> Apparently I need some help in trying to use this dBV capability with
>> on the
>> air signals.
>>
>> For example, using it as an AGC off "relative" S-meter with digital
>> readout
>> to run some antenna pattern checks for skewness etc.
>>
>> I've used it just fine in "lab type" measurements with a constant signal
>> source.
>>
>> First question. What is meant by stable value of AFV?
>>
>> Here's what I see for a bunch of AFV edited values one after another
>> with no
>> change in K3 settings.
>> 1579,1688,1598,1624,1696,1613, 2168,1758,1425.
>> The range is 6db.
>> Is the reference value of AFV used in the dbV calculation the last
>> one taken
>> at the instant one leaves the menu entry? Yes, it doesn't really
>> matter if
>> that values is used forever.
>>
>> Suppose one then takes a series of dbV measurements, is the
>> reference AFV
>> ever changed during the set of readings?
>>
>> The integration times available are 500 and 1000 ms. It would seem that
>> longer integration times are required for off air type measurements.
>>
>> Is there any integration time in the measured voltage used in the dBV
>> calculation? Or is it nearly an instantaneous value? For off the
>> air type
>> measurements, it would seem that a suitably long value is needed.
>>
>> 73 de Brian/K3KO
>>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>

Jim Brown-10

Re: K3 dbV in 2.38

In reply to this post by alsopb

On Thu, 18 Sep 2008 05:36:40 -0700 (PDT), alsopb wrote:

>First question. What is meant by stable value of AFV?

>Here's what I see for a bunch of AFV edited values one after another with no
>change in K3 settings.
>1579,1688,1598,1624,1696,1613, 2168,1758,1425.
>The range is 6db.

Sure. It's audio, and you're looking at the reading of a voltmeter looking at
that audio. ANY voltmeter will have a varying reading on program audio (that
is, music, someone talking, even band noise and QRN), because audio voltage
varies with time over a range of 40-60 dB. Many audio voltmeters, including
the one in the K3 that generates the AFV and dBV readings include an
integration circuit that averages the voltage over a selected time period (1
second or one half second in the K3). This integration drastically reduces
the variation as someone talks or static crashes crash. That's why your
readings vary by only 6 dB rather than 30-50 dB. As Eric and Lyle have
observed, these voltmeters are designed for use with test signals that are
constant (like a signal genertor).

As noted, it's quite nice to have this function. The only thing wrong is
improperly calling it dBV. dBV MEANS an AC voltage reading where 0dBV = 1
volt RMS. dBm means 1 milliwatt in whatever impedance is being used. If you
don't believe that, look at the manual for any HP product that generates or
measures voltage. Voltages expressed in dB with respect to some reference are
widely used in audio. Often, some reference voltage other than the standard
(for example, the output of a generator, or the peak value of a system's
response) can be selected as a reference, and all subsequent readings
referenced to that voltage. In that scenario, the notation dBr is often used.

I suggest that the log voltmeter (that is, reading in dB) begin with one of
the international references, either 0.78V (0dBu) or 1 V (0dBV), and then
change to dBr when some other reference has been selected. Now, you have an
actual calibrated audio voltmeter, rather than only a relative voltage
indicator.

73,

Jim Brown K9YC
Audio Systems Group, Inc.

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com