Power levels by mode

[KG6MZS]

Hi Dan;

Forgive my ignorance:  What is "IP3?"

TIA, Eric

[M0HCN]

Third order intermodulation distortion intercept value.
Basically a measure of how much interference is generated by non linearities in the signal path in the presence of strong signals.

Typically tested by putting tow signals up with a specified spacing and measuring the level at which they result in a 'phantom' signal appearing out of the noise floor spaced one spacing away from either of the signals.

What you typically find is that where one or both signals lies outside the passband of the first IF filtering you get numbers like +30dBm or so for a good radio (Basically measuring the performance of the first mixer) , but when you re run the test with the spacing reduced to say 2Khz in an SSB bandwidth the figure drops right down, which is equivalent to saying that strong signals inside the passband are a much more serious source of interference then equally strong signals outside the IF filter passband.

This is why using as narrow an IF passband as possible is (to a first approximation) a good thing. Like all blanket statements that one has a few holes, but it will do as a general principle unless you are designing a receiver.

HTH.

Regards, Dan.

[KG6MZS]

Thanks Dan.  The abbreviations always get me.  EEs toss them around freely.

[W8JI]

The problem is more than IP3. IP3 relates to extra signals spaced signal spacing above and below the desired signals.

For example a signal at 1 kHz tone in the audio output and another with 1.2 kHz tone would have new signals at .8 and 1.4 kHz that are the result of receiver defects. This is third order intermodulation. There are also IM5 and IM7 and so on that can all fall within passband.

Odd order IM products are almost never the FIRST major problem with digimodes. The first problems are harmonic distortion and gain compression. Usually gain compression is horrible because people use a wide filter and leave the AGC active. This often makes the system go into gain compression on background noise, so every single additional signal reduces gain. This receiver defect caused by improper receiver operation (use of an IF filter wider than the desired signal) is why displays go black when strong signals appear.

It is the operator at the receiving end or receiver that is the root cause of this problem, although people are quick to blame the other fellow.

The second most common flaw is harmonic distortion. In this case something in the detector system through audio stages generates harmonics of the detected tones. This causes extra bars to appear spaced at multiples of the detected tone frequency. It is often blamed on the transmitting station, but if the extra signals are multiples of the tone frequency it is a receiver system flaw.

The third most common issue is intermodulation, which requires mixing of multiple tones or signals.

All of this stuff occurs, as well as most transmitter problems, because digital modes are not really digital modulation and detection. We pretend like we have invented some neat narrow new mode, but it often really is just an audio tone dumped into a SSB audio input on a transmitter, and detected by the SSB receiver back to tones. It is a system that really uses SSB modulation and detection to convey a digital audio signal, so naturally the weak link is the transmitter and receiver because they are analog. This is why "digital" modes have so many people dictating power and antenna levels. It is an attempt to fix dynamic range problems that are really caused by using a SSB transmitter and receiver to convert to and from audio.

If people were serious about having good digital modes that really extended range and reduced adjacent channel QRM, transmitters would be directly digital modulated and receivers would have filters to match the TX bandwidth and do digital detection. It would not go digital to audio, audio to SSB RF, SSB RF to audio, and then audio back to digital.

73 Tom

[M0HCN]

The second most common flaw is harmonic distortion. In this case something in the detector system through audio stages generates harmonics of the detected tones. This causes extra bars to appear spaced at multiples of the detected tone frequency. It is often blamed on the transmitting station, but if the extra signals are multiples of the tone frequency it is a receiver system flaw.

I have never understood these linearity issues (which is really what they are) designed into radios at audio frequency after most of the gain has been applied, The hard bit is the IF stages not building a clean audio section!
If you sold a radio with 1% harmonic distortion in the IF amp (or worse the first mixer....) at normal operating level, people would rightly be screaming that it was a steaming pile of merde, but apparently audio frequency problems are acceptable?

The third most common issue is intermodulation, which requires mixing of multiple tones or signals.

However, unlike the others this one can occur in the first mixer, so the strong signals do NOT need to be inside the passband.

so naturally the weak link is the transmitter and receiver because they are analog.

Not sure I agree with the conclusion there, for all that I agree that using an SSB radio for a mode that is not 2.4Khz wide is stupid!

transmitters would be directly digital modulated and receivers would have filters to match the TX bandwidth and do digital detection. It would not go digital to audio, audio to SSB RF, SSB RF to audio, and then audio back to digital.

Nothing wrong with using an audio frequency first TX IF or last RX IF (for that is really what it is), in fact this can have a few upsides as it removes a tricky DC nulling problem in the IQ modulator in the transmitter.
Also you really do not want to just apply raw digital switching to the phase modulator as you will get switching sidebands for days, much better to go to analogue (either at baseband I/Q or as a low first IF) and shape the transitions with a raised cosine to minimize transmitted bandwidth.

AD have an interesting combined DDS and I/Q mixer that takes its input directly in the digital domain that might be fun to play with for this stuff.

There is nothing much wrong with using analogue radios and even an audio frequency IF, providing the selectivity is there, but there is no point in trying to design a mode much narrower then the selectivity filters in the radio either, so something 500Hz wide makes sense, maybe even 250Hz but only a vanishingly small number of rigs have first IF filters narrower then that.

Regards, Dan.