14

The answer is simple: filtering. For example, let's say the desired signal is at 800 MHz, and the intermediate frequency (IF) is chosen to be 100 MHz. Mixing the 800 MHz signal with a 900 MHz local oscillator (LO) would get the signal into the desired 100 MHz IF, because 900 - 800 = 100. But also the image frequency of 1000 MHz would end up at the IF, ...


11

The short answer is that these are "standard" frequencies set aside for IF use and are reasonable free from interference. The longer answer follows if you are interested in the why. The choice of an IF frequency is one of those design tradeoffs. The lower the IF frequency used, the easier it is to achieve a narrow bandwidth to obtain good selectivity in ...


4

The reason that this is done is the difficulty of obtaining sufficient adjacent channel selectivity in the front-end tuning while still achieving high levels of image rejection across a range of frequencies as wide as the HF bands. The first intermediate frequency is higher, often in the range of 10MHz. This is used for adequate image rejection, while ...


4

One way to convert a real-valued signal into a complex-valued one is to set the complex part to zero. This has the effect of "mirroring" the spectrum: all the negative frequencies are the mirror of the positive frequencies. This can be seen by the Fourier transform property for any real $f(x)$: $$ \hat f(-\omega) = \overline{ \hat f(\omega) } \tag 1 $$ ...


4

I think I figured this out. By looking at VFO with an oscilloscope I could catch a brief glitch (~1.5ms) in the signal when the frequency is changed: It was hard to notice because you can't easily trigger on this. I had to manually trigger single captures while tuning for some time to see this. Then by googling "si5351 output glitches" I've found ...


3

Yes, attaching a general-purpose frequency counter to the local oscillator of a superheterodyne radio will result in displaying a frequency with an offset either above or below (depending on the design of the radio) the nominal frequency of the signal being received. In order to account for this, apply the reverse offset numerically, between the frequency ...


3

Try powering the LM386 temporarily off a battery. if the popping stops, then the cause is a surge being conducted into the 386 from the supply that feeds the rest of your circuit. Normally, you would bypass the power input of the 386 to ground with a hefty electrolytic capacitor (50 to 100mfd) and then shunt that with a fast-acting cap (0.01 to 0.1uF) to ...


2

While the two terms in question share the phrase "dynamic range", they are in fact, quite different metrics from one another. Therefore there is no reason to assume any type of conflict or contradiction. The basic receiver dynamic range is a measure of a receiver's ability to handle a wide range of signal strengths - from the very weakest (typically at the ...


1

Since it happens when switching frequencies, I suspect glitches of some form in the local or beat frequency oscillators' output. The mixers simply multiply the "signal" by the oscillator. Any noise or glitch in the oscillator outputs will be reflected in the mixed outputs. You may be more sensitive to BFO glitches, since the output is at DC, not a ...


1

Putting multiple ceramic filters in series is common practice. There is additional loss; twice as many filters equals twice a much loss, may have to adjust the amplifier a bit.


1

Before any decimation, a signal needs to be sufficiently bandpass or lowpass filtered such that any image frequency spectrum, that could alias with the desired spectrum after decimation, is first removed. So with a narrow enough band-pass filter inserted ahead in the signal path, a sub-sampling decimator-complex-downconverter could be done in one step. ...


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