Re: [softrock40] Re: How to measure IMD?
My comments inserted below--
On 6/14/2013 5:44 PM, warrenallgyer wrote:
> I think we are confusing two issues. PEP, and the peaks of a complex signal as displayed on an S/A, are NOT the same.
Yes, the peaks displayed on your S/A are apparently shown relative to
the total power contained in all of the signals on the display. As long
as the "secondary" signals are relatively weak, each of the "primary"
signals will be 6 db down from the peak of the composite signal (for
which there is no spectral component to be displayed).
> PEP, which is the reference for the -30 dB IMD3 specification, is the peak power capability of a transmitter. It can can only be measured on an S/A when the modulating tone is a single frequency.
The S/A should not be used to measure PEP. It is only for the display
of spectral components relative to the peak of the composite waveform.
> The peaks of a complex signal on an S/A represent the peak power on that particular portion of the bandpass at that particular moment. By definition these peaks are 6 dB below the transmitter PEP capability in the modulating signal is a two tone test.
Only if the spurious components are quite small.
> For a given P-P audio modulation level, if two tones are used, the power is dispersed between the two resulting RF peaks and the associated products.
> If you remove one of the tones while maintaining the P-P input level of the remaining tone, the RF level of the remaining peak on the S/A will increase by 6 dB and will be the only case where the displayed power level and the PEP match for that particular modulation level.
This is where you will confuse many observers. The RF level does not
increase. Only the display changes since all of the power is
concentrated in the single signal. Remember, the display is normalized
to show total power at 0 dB, regardless of the waveform. If there were
significant clipping of the signal and the S/A bandwidth was wide enough
to include harmonics, then the level of the single signal would not rise
6db relative to the two tone signal. If the S/A bandwidth is only wide
enough to show the audio passband, then, yes, the single signal would
show at 0dB--6dB above the two peaks of the two tone signal.
> Warren Allgyer - 9V1TD
- 'cause IMD, as measured using the ARRL protocol, is specified with reference to PEP, which is accepted to be 6 dB higher than the two tone level.
I would be interested to see the mil protocol and specification if you have it.
(AKA Warren Allgyer - 9V1TD)
--- In firstname.lastname@example.org, FRANCIS CARCIA <carcia@...> wrote:
> The ARRL spec is the 6dB gift method not the mil method. If you add 6dB to the two tone level why don't you add 6dB to the IP3 also since it is the result of the two tones. Splain that Lucy.
> I'm attending the school of hard knocks trying to modify a 1200 watt MRI board to HF. Fairly easy to make power but a different issue to get good IMD on all bands.
> Frank WA1GFZ