[Elecraft] K3 pwr out on digimodes
hb9ari
hb9ari at bluewin.ch
Sat Jan 17 05:30:37 EST 2009
Hello,
I'm not a specialist, but this very interesting discussion remember some
points
during my past professional activity.
I was employed in a society involved in the design and fabrication of
RF link for broadcast TV cameras. ( My job was limited to camera remote
control
via separate UHF link(hard&firmware), PCB design and some help with RF
measurements).
The modulations used were QPSK, 16QAM and 64QAM (2K carriers, 8MHz BW)
with descending robustness and ascending bit rate.
According link conditions and/or production requirements, the optimum
"mode" was selected.
As i remember, we counted for a minimum of 10 dB crest factor and the
amplifier
plugged at the camera was designed to output over 40 dBm with 1 dB
compression,
but the max power allowed was (only) 28 dBm. We used also ultra-linear
class a PA
and this minimum 10 dB margin was always the rule. ( I remember a 50W
output COFDM PA
designed for 2.3 to 2.7GHz, a "monster"!)
We never spoke about ALC; the correct input level was adjusted with some
attenuation if necessary and the the signal quality analyzed with a
spectrum.
I had access to the "driver" (max 1W COFDM) schematic and no ALC circuit
is present.
From FPGA generating I/Q numeric, trough DA converters, modulation,
mixers, etc
all levels were defined with some tolerances for fabrication, but no ALC
We only do power measurements with bolometer, because peak power
measurement
was not an easy task. The link quality was evaluated by BER before and after
Viterbi correction. Gain linearity was specified to be under 1dB for a
8MHz spectrum.
(and other parameters like group delay, etc)
On the RX side, the specifications, to get a "freeze" free MPEG2 video,
was a minimum input RX power level of -93dBm( QPSK mode), -88dBm 16QAM
and -83 dBm (64QAM)..
For the (my) K3, i must admit that the TX linearity i measured with 2.8k
or 2.7k filter is not so good; may be one
of the reason, with RX, i've difficulties to decode ham digital pictures
with good RF levels? but no problem with DRM
using K3 IF output, LP-Pan and DREAM.(i will look for this point....i
have measured the 250Hz
and 2.8kHz filters with the software proposed by KS7D and the 2.8kHz is
far to have a good ~ flat usable width...)
I can understand that the costs are not of the same order, but the
required bandwidth
and the video/audio broadcast requirements not the same too!
As you say, just my 2 cents!
73 QRO fr Rudolf, HB9ARI
Joe Subich, W4TV wrote:
>> No, I do not misunderstand what ALC does. Perhaps I am just
>> not very good at explaining what I am trying to say. I
>> realize that ALC can only reduce the gain. That is why it is
>> necessary to set the audio drive level so that you get a
>> little bit of ALC across the whole passband range that you
>> want to use. Then, the ALC will be able to reduce the gain by
>> the necessary amount so that the RF output is constant across
>> that passband range.
>>
>
> Absolutely not! When conventional rigs are generating ALC
> their final amplifiers are ALREADY in compression and they
> are generating IMD in the PSK31 signal. PSK31 can not have
> ANY compression or clipping without causing increased IMD.
> Increasing the drive to the point that a comventional rig
> generates some level of ALC at all points virtually assures
> the generation of unacceptable IMD at most frequencies in
> the IF passband.
>
>
>> The FT-817 is not capable of 15W. But that is beside the
>> point.
>>
>
> OK, do it at 5 watts ...
>
>
>> What would this experiment prove? I know as well as you do
>> that it would not be identical. That's why I am saying that
>> some form of level control needs to be in operation across
>> the whole of the passband you wish to use.
>>
>
> That's my point exactly. The K3 does not provide level control
> across the passband it provides a constant output from the DSP
> modulator (15 KHz IF).
>
>
>> A small amount of ALC with a slow time constant will not harm
>> the IMD in PSK31 mode.
>>
>
> Wrong! ANY COMPRESSION OR CLIPPING will effect the INTENTIONAL
> amplitude shaping in the PSK31 signal designed to prevent IMD
> and control the banwidth of the transmitted signal. So called
> IMD is the result of a non-linear amplitued response in the
> audio/IF/RF chain. PSK31 drive can be thought of as a signal
> that has been "predistorted" in such a way to cancel the
> modulation sidebands. If that (amplitude) predistortion is
> lost due to compression or clipping at any point in the
> transmitter chain, the result is the appearance of the normal
> modulation sidebands (known as IMD).
>
>
>
>
>
>
>> -----Original Message-----
>> From: elecraft-bounces at mailman.qth.net
>> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Julian, G4ILO
>> Sent: Friday, January 16, 2009 4:40 PM
>> To: elecraft at mailman.qth.net
>> Subject: Re: [Elecraft] K3 pwr out on digimodes
>>
>>
>>
>>
>> Joe Subich, W4TV-4 wrote:
>>
>>> This is your fundamental misunderstanding of ALC. Traditional
>>> ALC does NOT (and can not) increase gain - it can only DECREASE
>>> gain to bring the grid/base/gate current down to the level at
>>> which the final amplifier is not being overdriven. DSP based
>>> ALC is similar, it can only adjust the modulator (DSP) output
>>> to the level determined to be appropriate at the single point
>>> in the IF filter at which the power calibration (5W/50W) was
>>> made.
>>>
>>> ALC (and power control) can not compensate for differences in
>>> gain across the passband - in other words, non-flat response.
>>> If you wanted to compensate for that non-flat response, it
>>> would be necessary to calibrate the power (5W/50W) at regular
>>> intervals across the passband and build a gain vs. frequency
>>> table. The DSP would then need to measure the frequency of
>>> any narrow band modulation and adjust the assumed gain of the
>>> power control circuit based on the gain of the IF chain at
>>> that frequency.
>>>
>>>
>>>
>> No, I do not misunderstand what ALC does. Perhaps I am just
>> not very good at explaining what I am trying to say. I
>> realize that ALC can only reduce the gain. That is why it is
>> necessary to set the audio drive level so that you get a
>> little bit of ALC across the whole passband range that you
>> want to use. Then, the ALC will be able to reduce the gain by
>> the necessary amount so that the RF output is constant across
>> that passband range.
>>
>>
>>
>>
>>> The PSK31 operator must adjust his drive (mic gain or power
>>> control) with ANY transmitter if he changes his "subcarrier"
>>> frequency. Even with conventional ALC if the drive is set
>>> to the ALC threshold when operating at the point of maximum
>>> gain, the output will be reduced when operating at other places
>>> in the passband (particularly when operating at the edges of
>>> the passand).
>>>
>>>
>> No. This misses the whole point of the convenience of "click and call"
>> operating with programs like Digipan and HRD. I never
>> suggested that the
>> drive must be set to the ALC threshold when operating at the point of
>> maximum gain. It must be set so that you get some ALC across
>> the whole of
>> the passband you wish to use (probably corresponding to no
>> more than the
>> -3dB bandwidth points, beyond that and the steepness of the
>> filter slopes
>> would cause unacceptable distortion of even a narrowband
>> signal such as
>> PSK31.) Then you will achieve constant power across that
>> range, because the
>> ALC will be able to apply varying amounts of gain reduction
>> to achieve that
>> constant power level. The K2 can do this. Why can't the K3?
>>
>>
>>
>>
>>> Try this experiment with your FT-817 ... set the power level
>>> to the maximum, turn off any compression, set your soundcard
>>> to 1700 Hz and the level for 15 Watts out of the FT-817 with
>>> NO ALC. Now, without making any other changes measure the
>>> power output with a 200 Hz tone and a 3200 Hz tone. Is it
>>> identical AT ALL THREE points?
>>>
>>>
>> The FT-817 is not capable of 15W. But that is beside the
>> point. What would
>> this experiment prove? I know as well as you do that it would not be
>> identical. That's why I am saying that some form of level
>> control needs to
>> be in operation across the whole of the passband you wish to
>> use. A small
>> amount of ALC with a slow time constant will not harm the IMD
>> in PSK31 mode.
>> The K2 demonstrates this.
>>
>> In SSB mode the K3 ALC provides the level control I am
>> describing. If I turn
>> CMP off and switch the audio source to LINE IN it works
>> exactly as I expect
>> it to work in data modes (apart from not disabling EQ, which
>> is a different
>> issue.) In DATA A mode it does not do this, so you must
>> constantly tweak the
>> drive level whenever you change frequency if you wish to
>> transmit a specific
>> power level.
>>
>> If this is how the K3 is supposed to operate then I shall put
>> it up for sale
>> right away, because it is not something that people who use
>> data modes a lot
>> want to do. (Well, perhaps there are some, because if I have
>> learned nothing
>> else from my participation in ham forums like this, it is
>> that some people
>> actually like doing things the hard way.)
>>
>> -----
>> Julian, G4ILO. K2 #392 K3 #222.
>> http://www.g4ilo.com/ G4ILO's Shack
>> http://www.ham-directory.com/ Ham
>> Directory http://www.g4ilo.com/kcomm.html KComm for
>> Elecraft K2 and K3
>>
>
>
>
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