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

[Jack Smith]

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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