-40db is 100 times lower than basic signal. It means if you go 100w and your 2nd harmonic is -40db, it would be about 1w. Also, your antenna could be good or bad for this harmonic frequencies so it could silence them or boost them a bit. I also agree that you cannot see harmonics on RTL dongles with perfect scale and usually they are less than they appear on dongle. If you make attenuator, you must include its attenuation and also its performance on higher frequencies.

pjeva wrote:-40db is 100 times lower than basic signal. It means if you go 100w and your 2nd harmonic is -40db, it would be about 1w.

Sorry pjeva, this is incorrect. In terms of power, 40dB is a factor of 10,000 (10dB is a factor of ~ 10x, 20dB ~ 100x, 30dB ~ 1,000x, and 40dB ~ 10,000x) so at the 100w output level, a second harmonic of -40dB below the fundamental, would be at 0.01w. I think you are thinking of Volts ? In terms of Volts, 40dB is a factor of 100, but Watts being proportional to Volts squared, this equates to a factor of 100 x 100, or 10,000 in Watts.
I think saturation, as mentioned by GUM, is likely (I think the harmonic level is probably lower than indicated) - though I don't personally have any experience of the RTL dongle.

Sorry, today is not my day. About RTL and power of lazy's tx, i think his LPF is cutting too low, at 100MHz or lower. On this dongle, you should see second and third harmonic at least.

Just to clarify "LAZY", I think you're saying that when you set the Rx gain, and the Tx output, so that you are receiving the fundamental at 0dB, the 2nd harmonic is just about visible at -45dB (so I guess this is somewhere near the minimum and / or the noise floor). You are then bringing it up, by raising the gain of the receiving unit ?
So yes, when you turn the gain up, you are amplifying the received signal(s) ie the fundamental, and all the harmonics. If the fundamental is already at saturation, then you probably won't see an increase in the fundamental, when you turn the gain up, but you will probably see an increase in the harmonics (unless it limits because it is overloaded with fundamental).
By the way, do you know what was mostly responsible for the mid-band dip you have now cured ?

To answer Builtinasheds question, just by resetting the coils to there original spacing and spreading L3 slightly. I'm going to re wind all the Output and LPF coils and see if I can get this spot on! The 2nd and 3rd and so on are not as low as before after playing around with the SDR software. Maybe the the receiver is too sensitive and not accurate enough as I can even see the 10th harmonic with the gain set at 0dB??? See the pictures below.

Here's a picture of the setup showing the Tee section SRD and power meter all hooked up.


Here's a picture of the Fundemental.


Can someone tell me what this spike at the left hand side of the screen is all about? It's there even if I turn the TX off, so must be either my PC or the SDR?


2nd harmonic


3rd Harmonic


I think i'm going to need a spec to get this right, although has been a great little project, kept me busy for hours and provided lots of nagging fuel for the other half. I now have my own workspace setup because of this for future projects!!

I'll post more when i've rewound the coils and had another go.

Lazy.

Thanks for the update. If the signal is there when the Tx is fully powered down, I guess it's picking it up out of the air. Was there a FM radio nearby ?
Otherwise maybe from the PC, possibly the hub, possibly something internal. Try moving the dongle around to see if the signal strength changes.
If it is picking the 86.45 signal out of the air, it may be doing the same with the harmonics, so it may be giving you a false reading in that way......

hi -
with my sdr - it picks up spurious signals from the power supply (charger) and also sometimes shows spikes on the edges - but if i move the freq along a bit the spike disappears.. i find running it of the battery on my laptops best.. but still see the odd spike thats not actually there..

As has been mentioned on here before, those dongles are not the way to analyse your transmitter. You will need to use the correct test equipment to get the correct results.

thetufparker wrote:if i move the freq along a bit the spike disappears

Most likely spurious mixing products because these dongles are (by necessity) wide open and with very little filtering to reject images! Get a speccy analyser!

shuffy wrote:

thetufparker wrote:if i move the freq along a bit the spike disappears

Most likely spurious mixing products because these dongles are (by necessity) wide open and with very little filtering to reject images! Get a speccy analyser!

Yes. You really want the I.F to be above the maximum frequency of operation, to push the images way out of band. That way you can deal with them with just a LPF....

I've got one of those dongles to use as a scanner and eavesdrop on people. I wouldn't trust it as a spectrum analyser. Mine picks up all sorts of crap which shouldn't be there.

builtinashed wrote:Yes. You really want the I.F to be above the maximum frequency of operation

That would work

Don't get me wrong, these dongles are absolutely brilliant for what they are, especially for 9 quid or whatever it was. Works like a treat for watching telly on a laptop or with SDR# as a scanner once you understand the limitations. Not so hot on DAB though, for some reason, I suspect that's a software problem though.

Can anyone tell me how much current this 6W board pulls at 12V?
Is it the same for the newer version, drfs06-v21?
I'm thinking of hooking one up to a battery.
Cheers

mpx wrote:Can anyone tell me how much current this 6W board pulls at 12V?
Is it the same for the newer version, drfs06-v21?
I'm thinking of hooking one up to a battery.
Cheers

It's the same for the newer version same current and same Volts as previous version.

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And what is the current? I haven't seen it stated anywhere? Thanks

mpx wrote:And what is the current? I haven't seen it stated anywhere? Thanks

I can't mind the current it someone who told me I've totally forgotten what it was but I do recall it to be same Volts thought.

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