Re: [softrock40] How to measure IMD?
I looked at your images this morning - you seem to have no unwanted sideband suppression - the upper and lower sidebands are identical in level - like you are operating in DSB-SC (double sideband, suppressed carrier) mode. I don't know if this is your intention or not, but that's what it looks like.
Your oscilloscope envelope image also does not look like a standard SSB signal, but some of that is due to the sweep speed - the envelope display should be with a sweep speed much slower than the value needed to resolve the individual RF cycles. You are looking for the modulated envelope display, so something on the order of 500us/div will be in the neighborhood of the desired range, not 20ns.
It looks as if you have about 23 to 25 dB 3rd order intermodulation suppression, measured from the test tone. That's not all that great.
Also, the higher order IMD products are much higher than one would normally like, particularly if you will be operating with an amplifier.
Before going further, I would try to find the reason the signal is in DSB mode, not SSB.
One normally measures with a span around 10 KHz, and RBW of 100 or 30 Hz - looks like you have an HP8568 or 8566 spectrum analyzer (I use an 8568B here) - in order to resolve the individual products better than with the 300 Hz RBW used in most of your images. But that's something to be looked at after you get it running in SSB mode.
On 6/14/2013 7:36 AM, Chris Wilson wrote:
> Not comprehensive guides but should give some insights.
> Two-tone test and look at the Spectrum Analyser for additional products.
> 5C-043.pdf shows what you should see and the difference between the
> carrier peak and the 3rd order product peak as seen on the Spectrum
> Analyser gives you the figure.
> 73 ... Sid.
Thanks Sid and Alan.
I made an Audacity dual tone file with combined frequencies of 1330
Hz and 550 Hz, and checked the resulting file looked clean from the
audio card, on a `scope. I then ran it into the Softrock Ensemble II
RXTX audio input. I measured the TX output at the aerial socket via a
30dB attenuator into the SA. I played with RBW and reference level
settings as I wasn't sure of the best way to show the results. The
image *55.jpg is with the Softrock at full 1 Watt power with sensible
modulation levels. The earlier SA scans are with about 750 mW output.
The `scope scan is with the Softrock at full power. I can't see any
clipping. I sniffed the output into a dummy load. I am sure I am
doing some things wrong, criticism welcome. Thanks! Once I am more
confident of technique I want to look at the output after power
Chris Wilson. 2E0ILY
- '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