[Elecraft] When S-Meters are Useful (WAS: When do you use fast AGC?)

Wed Oct 18 05:16:38 EDT 2006

I will think about that as a potential kit. If ADC is essential, it 
would make the most sense to take an analog IF sample out of the 
receiver after the selectivity and have an outboard log amp and ADC 
converter.

My Z10000 buffer amplifier should work as an IF sample isolation 
amplifier without much change, or an even simpler circuit, such as an 
FET follower would be possible since there is no need to make it a 
wide-range universal product as with the Z10000 buffer amplifier. The 
log amp, ADC and serial output is dead easy. It should have some input 
bandpass filtering, however, to keep down the broadband noise.

If you are looking for a visual indication of level, my Z90/91 digital 
panadapter provides 60 dB or so of on-screen (and on the data port) 
dynamic range and an additional 30 dB of selectable attenuation. The 
overall accuracy is within +/- 2 dB and most do better. There is no 
provision for absolute calibration as it makes little sense to do this 
considering the variable gain in the K2 ahead of the broadband sample, 
so the readings are all relative. The Z90 Control software that I've 
written includes a provision to log the data to a text file, with values 
in dB, again arbitrary reference, so a fixed calibration would be 
required should one be interested in specific absolute dBm values.

I have some AD8307 linearity data posted on my web site. It's at Page 29 
inside the interim instruction manual that can be found at 
http://www.cliftonlaboratories.com/Documents/Quick%20Operating%20Instructions.pdf. 
The full range of 80 dB or so will not be achievable if there are strong 
nearby signals due to filter skirt issues; hence the 60 dB or so usable 
range quoted earlier.

The Z90/91's serial output protocol is open, so anyone that wants to 
write their own program is welcome to do so.

Jack
www.cliftonlaboratories.com

charliedelta at hushmail.com wrote:
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> I would be the first to order a unit Jack
>
> Theres some excellent plotting software just waiting for a S meter
> interface as you describe.
>
> http://www.g4hfq.co.uk/plphelp/plphelp.htm
>
> Theres little alternative available for those seeking a accurate S
> meter on a ham radio transceiver.  Besides i see no harm  as hams
> being professional in our approach to signal measurements. After
> all who would buy any test instrument today if it had a 200% error
> in accuracy, but thats what we have today in every radio receiver
> and transceiver.
>
> Craig
> VK3HE
>
>
> On Tue, 17 Oct 2006 02:30:47 -0700 Jack Smith
> <jack.smith at cliftonlaboratories.com> wrote:
>   
>> One possibility for an "add in" S-meter module would be an AD8307
>> log
>> amp module paralleling the IF amplifier's input. This will give
>> you an
>> honest 80 to 90 dB linear output (db in, linear output), if you
>> can
>> manage the shielding, and close to 100 dB if you don't mind a bit
>> of
>> non-linearity at the extremes.  To avoid loading down the IF amp
>> input,
>> you probably need a isolation amplifier.
>>
>> And, of course, there's always a concern when mucking about inside
>> the
>> signal chain. The buffer amplifier I offer with my Z90 digital
>> panadapter samples the K2's IF at an earlier point, just after the
>>
>> 2N5109 post-mixer amplifier, but the design could easily be used
>> to
>> sample after the filters. (The Z90 uses an AD8307 log amp
>> detector, so I
>> have some experience with it.)
>>
>> The log amp's output could then drive an external moving coil
>> meter, or
>> it could be digitized via an ADC and brought out on a RS-232
>> compatible
>> port for other purposes. Serial output would require an on-board
>> PIC and
>> would have to carefully constructed to avoid introducing unwanted
>> noise
>> into the K2, but an all-analog solution would be simple enough if
>> the
>> output device is to be a D'Arsonval meter movement.
>>
>> If there's a groundswell of demand for such a beast I will
>> consider it
>> as a future kit offering.
>>
>>
>> Jack K8ZOA
>> www.cliftonlaboratories.com
>>
>>
>>
>> charliedelta at hushmail.com wrote:
>>     
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>>> I also looked for a solution to the S meter calibration problem.
>>>       
>> I
>>     
>>> discussed the problem  with Greg Ordy w8wwv Author of the great
>>> program called S meter light. Unfortunately the K2 does not
>>>       
>> return
>>     
>>> 8 data bits via the serial port so the calibration accuracy
>>>       
>> would
>>     
>>> be close to useless.  If the K2 did return 8 data bits you could
>>> obtain 0.1 db accuracy using Gregs program. 8 data  bits
>>>       
>> returned
>>     
>>> via the data port is available on the 756PRO and several other
>>> radios. Elecraft cloned the Kenwood command set which returns 4
>>>       
>> or
>>     
>>> 5 bits or something like that.
>>>
>>> Maybe Elecraft would consider this as a future external option
>>>       
>> for
>>     
>>> the K2 . It certainly would be a first for a ham radio, that is
>>> having a inbuilt calibrated S meter. The crunched up S meter
>>> bargraph is highly annoying and close to useless on the k2.
>>>
>>> Craig
>>> VK3HE
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