27

It's a feature not a bug. When turned on, the weather alert function checks the local NOAA station for its alert tone every 5 seconds for half a second... doing so takes over the receiver. I'd turned it on, and forgotten I'd done so; and how that feature is implemented never registered in my mind. Turning it off solves my "problem." Thanks to ...


9

10⁻⁴ which I think is too low power. Why? Gut feeling says that is actually pretty good a transmission for such a distance. Notice how your free space path loss is $$\left(\frac{\lambda}{R}\right)^2\cdot\frac1{16\pi^2}\approx \left(\frac{\lambda}{R}\right)^2 \cdot \frac1{160}$$ and $\frac\lambda R=3\cdot10^{-3}, \left(\frac\lambda R\right)^2=9\cdot10^...


7

Yes, this most certainly was a thing, and people actually did it. This type of communication has been done at ELF frequencies in the 3-30 kHz range. Their linear amplifiers were ordinary Hi-Fi audio amps. I don't know if LF frequencies higher than that are useful for that, but the hams on the 2200m band (137 KHz) use above-ground antennas. A lot of ham ...


6

So, the job of a BFO is essentially "faking" the carrier oscillation of the AM transmission that was suppressed, together with the other sideband, so that the rest of the receiver can demodulate the SSB signal. As such, it's the input to a specific nonlinear device: the mixer (sometimes combined with a power detector, then called "product ...


5

That looks like you're strongly overdriving your receiver. Drastically reduce gain. This signal might not come from 137.5 MHz at all but be mixed there by the nonlinear effect of overdriving your amplifiers. The fact you can receive FM (a different signal at a different frequency) doesn't mean the gain is appropriate for this signal. Also, FM, being FM, is ...


4

It might be harmonics of the oscillator, as you suppose, but it also might be that any mixer doesn't only produce the difference frequency of its two inputs ($|f_\text{RF}-f_\text{local oscillator}|$, here), but also higher-order intermodulation products (like $|2f_\text{RF}-f_\text{LO}|$ and $|f_\text{RF}-2f_\text{LO}|$). I think I've posted multiple ...


4

The BFO is one input to the mixer, the other being the RF signal you're wanting to receive. An ideal mixer simply multiplies its inputs: if at one instant one input is 2V, and the other input is 3V, the output will be 6V. This is useful because multiplication like this can accomplish a frequency shift. The objective in receiving USB is simply to take all the ...


4

First and foremost, varicaps aren't mechanical. If you want a rapidly tunable thing, that's a necessity; even if you just want something tunable by electronics, it's very desirable (otherwise you'll need to motorize your air gap cap...). Air gap capacitors can be made to withstand very high voltages, which makes them desirable for anything that has to handle ...


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

There's multiple way that multiple signals can be sent at the same time on the same frequency. One is to have multiple antennas, where even though the antennas themselves can be omnidirectional, they can be used to "beam form" by carefully offsetting the signal. And likewise the receiver can do this trick. Also see this article on MIMO. Wifi can ...


3

The dynamic range of (shortwave) received signal strength varies from noise equivalent field strength (in the order of 250 nV/m in 3 kHz SSB bandwidth) to 1 mV/m (for radio ham transmissions via the ionosphere). Local signal can go over 1 mV/m. In short: dynamic range is in the order of 72 dB. AGC doesn't have to cover that range, but 50 dB is "nice to ...


3

Transmission between antennas is "two port in to two port out". But: in between there is a transmission path with reflections. Some frequencies are attenuated and some other frequencies are even stronger than average. That is called Rayleigh path (sorry, not complete for the moment). The transfer is frequency-dependent AND antenna position ...


3

Your calculation looks all correct. A quick conceptual way to check it is this: picture a 200 watt light bulb 20 metres away, work out how much light falls on an area of about ${\lambda\over{2}}$ by $ {\lambda\over{2}} $ This is the power received by the second antenna. The area of a 20 metre sphere is 5000 $m^2$ and the capture area is ${1\over1600} m^2$ ...


3

https://www.gpscentral.ca/manuals/Icom_IC-R8600_ENG_Manual.pdf Page 21 explains the difference between regular squelch and s-meter squelch on any Icom radio. Noise squelch The Noise Squelch allows the audio to be heard only while receiving a signal that includes less noise than a set level. Activates when the squelch level is set to between 30% and 50% in ...


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

A direct-sampling SDR doesn't require a mixer, which can simplify the design. Also, provided the receiver has sufficient dynamic range and processing power, a direct-sampling receiver can monitor all the bands at once. On the other hand, a direct-sampling SDR must have an ADC and DAC at a sample rate above twice the maximum frequency. So for an SDR that ...


2

I am guessing that you programmed a receive tone requirement. Try programming only the transmit tone and leaving the receive tone not required. I don't have Chirp available right now so I can't give you the names of these fields. Best, Ed Greenberg KM6CG


2

Everything works very well, but on some frequencies I hear loud AM broadcast stations. I think this is caused by harmonics from the VFO, but I am not sure. What you're possibly hearing is intermodulation distortions created by your receiver from the mixer and amplifier. A local AM broadcasting station has high radiated power, making the problem even worse. ...


2

the I and Q components of a sample correspond to the complex representation of a portion of a sine wave described by $I \cdot \cos(2 \pi f t) + Q \cdot \sin(2 \pi f t)$ w.r.t. $t$, where $f$ denotes the frequency of interest This is correct (if we suppose the incoming signal is a sine wave, i.e. an unmodulated carrier). I don't think that the receiver ...


2

Suppose that a sample is taken at a time $t$, I don't think that the receiver could just multiply the instantaneous strength $V$ (voltage?) of the incoming signal by $\cos(2\pi ft)$ and by $\sin(2 \pi f t)$ to recover $I$ and $Q$ (as the diagram in section "Receiver Side" of the linked article appears to suggests) since this would carry no more ...


2

This is a basic question and needs an answer. Regenerative receiver or audion. Single LC-filter with undamping: Q-multiplier. The frequency transfer of such a filter can be obtained from a simulation. See figure. As you can see: the AM demodulated audio response is affected when selectivity is too high. The example shows 1 MHz mediumwave. Audio bandwidth of ...


1

In most typical SDR receivers, I and Q are not determined from the instantaneous RF voltage input, but from a reduced bandwidth slice of the RF spectrum. The slice is taken by quadrature heterodyning/mixing (with a quadrature local oscillator (LO) near the frequency slice of interest), thus producing two signals. This pair of mixer results is typically low ...


1

Yes, multiple signals can exist on the same frequency at once. In a net, this could happen if two people tried to talk at the same time. This is called a "double". When this happens, the signals mix, or add together. If the two signals are AM, the amplitudes of the radio waves add, which corresponds to the amplitudes of the audio, so you get ...


1

There's a huge variance in signal levels — the lowest signal that's readable above the noise floor and the strongest signal that won't overload the receiver are probably at least 60dB apart. To get that into a comfortable listening range without AGC, the operator has to adjust the receive level. Maybe that doesn't sound like a big deal (can't you just set it ...


1

IMD will respond more to attenuation as the receiver will be operating in a more linear region. This is your best bet for detecting IMD: If you add 3 dB of attenuation (or reduce gain by 3 dB) and you see some signals or the noise floor go down by more than 3 dB, you've significantly reduced IMD. There is another way to determine if attenuation should be ...


1

On some (most?) receivers, if the RF gain is turned up too high, intermodulation distortion increases due to increasing non-linearities (maybe even clipping, etc.) I'd argue that's all receivers, even, since that's exactly what "turned up too high" would imply. You get a non-linear effect, and that always leads to intermodulation. If that doesn't ...


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

if whistler can upgrade these other legacy scanners from RS and GRE to do DMR/NXDN and Phase II they can do it with this as well. Says someone with technical insight into the engineering of these radios? There has to be a way. That's unfounded wishful thinking. Generally, exchanging the CPU and DSP in any board pretty much means a redesign of at least ...


1

https://www.alibaba.com/countrysearch/CN/ferrite-coil.html?spm=a2700.7724857.22.2.39a54807W9PFes Many Chinese suppliers offer these on Alibaba


1

Direct conversion with lower distortion vs mixers with inexpensive ADC is only the first differentiation between the two methods. The original advantage of using a mixer is that you could vary the intermediate frequency used by the mixer to change the frequency of interest and have a fixed frequency output from the mixer to feed to second stage filters, ...


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