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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, ...


10

I think it's more intuitive if you unlearn some things first. Oscillation is not: $$ \cos(\omega t) $$ where $\omega$ is the angular frequency in radians per second, and $t$ is time. Rather, oscillation is: $$ e^{i \omega t} $$ By Euler's formula this can be expanded to: $$ \cos(\omega t) + i \sin(\omega t) $$ If you plot this function on the complex plane, ...


8

Switching to the inverted signal is equivalent to multiplying by -1. While in an ideal mixer, the RF input can be multiplied by a sinusoidal LO, the switching mixer multiplies by a square LO. It still works as any mixer would. That is, for each frequency component in the first input $f_1$, and each frequency component in the second input $f_2$, it produces ...


7

Let's first start by developing some intuition of what a complex signal looks like. We can use GNU Radio to generate a signal that's just an unmodulated carrier, and then put that into a UI that will display the real and imaginary components over time: The result for 80 Hz is this: Notice how the real part is 90 degrees behind the imaginary part. If you ...


7

When two sinusoidal signals are multiplied through a non-linear mixer circuit, it results in the sum and difference of the mixed frequencies: $$f_s=f_1+f_2 \tag 1$$ $$f_d=f_1-f_2 \tag 2$$ Of course, negative frequencies are not possible so $f_1$ should be the higher frequency in equation 2. This multiplying or mixing process is also called heterodyning ...


6

LM387 is just a really old, ordinary op-amp as far as I can tell. It has a funny pin configuration, so if you have a PCB already made you need to make modifications. I suggest cutting the traces, scraping off the solder mask, and soldering wires in place with the right layout. Look for datasheets on the internet for the pin configuration. For old stuff ...


6

To put it simply, a balanced modulator is a mixer which has two inputs and two outputs. The outputs are (input1 + input2) and (input1 - input2). So, if the inputs are 100kHz and 1kHz, the outputs are 101kHz and 99kHz. For transmitting: If the inputs are 1MHz and (voice), the outputs are the lower sideband of the voice and the upper sideband of the voice, ...


5

This is caused by the RTL-SDR I was using to view my signal. Here's a quote from the guide at https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/: Note that this feature makes use of direct sampling and so aliasing will occur. The RTL-SDR samples at 28.8 MHz, thus you may see mirrors of strong signals from 0 - 14.4 MHz while tuning to 14.4 - 28.8 ...


5

Partial answer, because I don't know the actual electronics theory, but I hope which will help make progress towards a complete answer: A balanced modulator is a mixer with a particular feature. Basic analog mixer designs tend to include the carrier in the output, whereas a balanced modulator is one which is designed to "suppress" the carrier. Hence, if ...


4

So, I don't know the Softrock personally, but from your description, it's a switching mixer architecture. Receiver side What that means is that you mix not by multiplying the input signal with a single harmonic oscillation (a tone), but by switching it on and of. Pre-Mixer HPF Mathematically, this description is omitting one interesting detail: The ...


4

They prevent (or at least reduce) front end overload due to strong and/or messy nearby signal sources at lower frequencies. Think of the 80 meter station at Field Day blanking the 40 and 20m receivers every time it's keyed up. Chances are good that you'll be ok without them if your environment is pretty RF quiet, but it may not take all that much to cause ...


4

A balanced mixer is one that suppresses the local oscillator (LO) input in the output. A double-balanced mixer (like the one you show here) suppresses both the RF input and the LO in the output. This is often accomplished with balanced transformers, hence the name. Because T1's secondary has a grounded center tap, the voltages on its two "hot" output ...


4

The SA602AN looks to be still available from many of the usual distributors. (Check octopart.com or findchips.com to see which distributors have them in stock.) The LM387 is apparently long obsolete. A quick web search didn't reveal any pin-for-pin compatible parts still being manufactured, so if you're using the printed circuit board from the kit, then ...


4

If nobody is able to recognize what kind of signal you're dealing with by looking at your waterfall screen shots, then perhaps your best chance of identifying the signal would be to use old-fashioned RDF (radio direction-finding) techniques. If you can, consider borrowing a portable battery-powered receiver that can do 40m, and ideally also SSB, like a ...


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

All OK. Pitfall for 14 MHz RX and 9 MHz crystal filter is choice of oscillator frequency. 5 MHz is possible but a bad choice, because the harmonics of the oscillator give unwanted spurious reception. By the way: I did identical things. See pictures. In my opinion there is no preference for LSB or USB based on such passband characteristics.


4

I believe a good point of view is the concept of orthogonality. This is clear under everybody's eyes when seen in physical space, take for instance a 2-dimension space, a plane. In the example above any point on the plane can bear two indipendent informations, its x and y. The keyword here is indipendent, I can freely change one, let's say go from x1 to x2 ...


4

Based on a quick internet search, other experimenters have successfully used the scheme you propose. At this low frequency, stray capacitance is a much smaller concern than you have experienced at VHF. You may need to isolate the output and oscillator sections by separating components and/or orienting them at right angles. 630-m receive systems typically use ...


3

Using an example from Crystal Ladder Filters for All, a crystal filter response might look like this: Indeed, this filter might "prefer" LSB, because that would place the carrier near the right side of the passband which has a steeper attenuation. That would mean you'd design your IF like this: When the IF is subsequently heterodyned down to ...


3

Your answer in your question is not correct. This question is solved in the same way as the last question you asked. The formulas are the same: $$f_s=f_1+f_2 \tag 1$$ $$f_d=f_1-f_2 \tag 2$$ This time, the sum and difference is called IF (intermediate frequency) - a common term in mixer circuits. Then rearrange to find the LO (local oscillator) frequency: ...


3

No attempt was made to reject the image response of the superheterodyne. This suggests that the designers wanted nothing to phase-shift RF signals...important in an interferometer - everything between antenna feed and the 10 MHz I.F. amplifier would be quite broad-band. And gain (actually loss) of diode mixers is fairly well-controlled. Keeping those two RF ...


3

Well, there are two relatively simple methods and one slightly more complicated. In the analog world, SSB signals are created in two ways: The filter method and and phasing method. The filter method does just what you suggested - a very selective filter eliminates one sideband. Using piezoelectric crystals, filters can be made what can do that. Mostly ...


2

If I understand your question correctly, you are asking about the inputs and output of the frequency mixer in a direct-conversion transmitter. Neither of your pictures is correct, but B is closer to correct. Here is the correct picture: Here are some facts about frequency mixing that can be used to help see where you went wrong: Until a filter is added, ...


2

Yes, this will work. In fact, that is nothing but emulating an ADC with twice the sampling rate by using two synchronous ADC of the original rate, delaying one branch of the signal by half a sample clock.


2

Looking at T1, on the unbalanced side of the transformer, one leg of the input is tied to the chassis ground (the zero volt reference); the other leg is driven by a voltage source. Because the voltages on the two legs aren't equal but opposite, the input is said to be unbalanced. The current in the primary winding of the transformer creates a magnetic flux ...


2

AM synchronous detection will certainly reduce or eliminate selective fading. It generally isn't implemented simply due to the increased cost and complexity. But you will certainly be pleased with its performance if you enjoy listening to AM shortwave broadcasts. I think you would find that the easiest design that you could retrofit to the receiver would be ...


2

It looks to me like you've put a lot of effort into tracking this down. I believe you are on the right track as far as the Pi creating a lot of broadband noise. But I'm wondering if you've tried putting the SDR on a extension USB cable so that you can move it as far from the Pi as possible? I've found that getting farther from a noise source helps a lot. ...


2

An ideal frequency mixer simply multiplies its inputs. One input is the RF signal we wish to shift in frequency, and the other is the local oscillator (LO). For the moment, let's consider just an ideal frequency mixer where the LO is a pure sinusoid. Since mixers are used to shift frequencies, we can use the Fourier transform to better understand their ...


2

A balanced modulator is a kind of mixer. Specifically, it's one that works in all four "quadrants", that is combinations of each of the two input voltages being positive or negative. Along with that come certain other expectations about the device. We could draw a graph of the two input voltages $v_1$ and $v_2$, and it would look like this: Any possible ...


2

...it looks like a "quadrature mixer" is really just two separate mixers, with one fed by an LO to which a [constant] phase delay has been applied relative to the other's LO. Yes ... and. Quadrature versions of the modulating signal may comprise the "baseband" inputs to the quadrature mixers: Here's where "the phase shift is 90 degrees for all frequencies" ...


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