Check the FCC requirements for harmonic radiation imposed on broadcast stations. Once done you will have a suitable "Aha" moment, one hopes, when you figure out that X dB down from Y dB over S9 (er 50 uV at 50 ohms unless you are Collins in which case it is 100 uV at 50 ohms) and just how big a signal S9 really is. Of course you will see harmonics. And it's nothing to do with your receiver. Now you use the word "images". If the signals are not on frequency they are leakage through the front end filtering and that implies they would properly be called aliases. Images are a superheterodyne receiver phenomenon and occur on the opposite side of a conversion process's IF frequency from the tuned frequency. It will be a different frequency. And while the AirSpy HF+ SDR with your computer is "sort of" a superhet receiver the IF in question is possibly 0 Hz. So images are "interesting" to contemplate. (I've not tried to figure this out.) A good set of numbers taken with variable center frequency tuning can give enough numbers to determine all the variables that might be involved. That will help you define your local "Don't DO that!" conditions.
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(And if the tech on the phone asks you to make sure your computer is turned on, fire back at him, "I didn't know computers had a sex life!")
On 20180313 06:42, Chris Smolinski wrote:
I notice the same levels of intermod on LW/LW with firmware version 1.6.2 vs previous versions, it does not seem any worse. This is using SdrConsole which AFAIK keeps the AirSpyHF+ AGC turned on.
While playing around with SDR# I was of course able to generate lots more intermod/etc by switching AGC off and turning the gains up too high and overloading things, as you'd expect.
I don't have any nearby strong MW stations, the nearest is about 10 miles away, and I think 5 kW. With AGC on I can occasionally see images from it on LW and MW, although careful tuning can eliminate them. (Setting the center frequency such that the station is outside the 768 kHz passband)
Any guesses when there will be a library update to further improve the AirSpyHF+ filtering?
Black Cat Systems
Westminster, MD USA
On Mar 13, 2018, at 8:33 AM, Giovanni Carboni <@IZ5PQT> wrote:
I had to disinstall version 1.6.2 of the firmware (20180309) on my AirspyHF+ since it caused horrible
intermodulation below 1 MHz.
Less than 15 km from my QTH there is a 50 kW MW TX on 657 kHz (Rai Coltrano). The signal at the
radio antenna input is -21 dBm measured with an ELAD S2 and checked with a scope. The two measurements
agree within errors (20 mV peak on the scope). It is a quite large signal so I am clearly in an extreme
With all firmware versions until 1.6.1 the receiver did not show intermodulation problems (except of course
for a narrow band around 657 kHz). With 1.6.2 a terrible
intermodulation occurred making reception impossible on all the MW spectrum up to 1000 kHz. I did not check
carefully but it looks like reception below 500 kHz was not affected.
I had to go back to 1.6.1 and things became normal.
I do not have instrumentation to perform more accurate measurements but the test is
easily reproducible in a lab.
73 Giovanni IZ5PQT