[Elecraft] IMD and supply voltage

[Wayne Burdick]

Frank,

All radios that run from an external DC supply (to make them usable with emergency power sources, such as a 12-V car battery or solar panel) are subject to increasing TX IMD as supply voltage goes down. But such radios are virtually all specified (by the manufacturer) and tested (in QST reviews, for example) at a standard bench supply voltage of about 14 V. 

If you're running from lower voltages, such as from a depleted 12 V car battery, the radio is not likely to put out full power on all bands. This effect will vary with band, load impedance, and the actual voltage. You could simply crank the power all the way up and accept whatever TX IMD results from this. Instead, we recommend reducing power manually if you know the voltage is low.

The K3 and K3S, like our other radios, are very popular for Field Day and other applications where a battery might be used. So we provide two ways for you to monitor the supply voltage:

1. The CONFIG:BAT MIN menu entry. This sets the voltage at which you will periodically see a BAT LOW warning on the display. 

2. Voltage display: Tap DISP and rotate VFO B until you see XX.X VDC. You can also display current drain and PA temperature this way.

73,
Wayne
N6KR


On Oct 29, 2015, at 8:09 AM, frank <frank at k5dkz.com> wrote:

> This subject keeps cropping up.  Is this a serious issue ?
> 
> I have heard that IMD improves by 10db with a one volt increase in supply voltage.  Is this really the case or just a measurement error?  10db is a large difference.  One volt is a small variation.
> 
> What is the minimum acceptable IMD?  How linear does the amp need to be?  If 36db is better than 26db is 100db even better or just overkill?
> 
> Are other solid state amplifier products similarly effected?
> 
> What is the IMD performance of the K3S at 10 watts over a supply voltage variation from 11 volts to 14.7 volts?  I ask because I believe those ten watts are generated by Dmos devices as well.  Do these devices only misbehave with varying supply voltage at higher (100w) power levels and lower supply voltages?
> 
> Should I wait to purchase a K3S until this issue is resolved?  (Assuming this is a valid issue)
> 
> Back in the good old days we used to solve problems.  Today we don't have problems, just issues.  
> 
> -- 
> Frank - K5DKZ
> KX3 - 7550
> PX3 - 1143
> KXPA100 - 1566
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