15

The earliest Soviet satellites, like Sputnik 1, transmitted what amounted to a CW stream -- just pulsed RF at pretty low power (later Sputniks transmitted data by pulse width modulation, still essentially CW). In order to hear it, you needed a Beat Frequency Oscillator -- a BFO -- and with a common radio receiver, the signal generator was standing in for ...


9

I'm going to discuss filters and not oscillators, because the reasons are pretty much the same. An oscillator is just a filter with enough gain to put it on the edge of stability. It certainly is possible to use RC filters in RF design, and sometimes you do see them in non-critical filters, especially those that don't require a steep filter or high power ...


7

There is no hard and fast rule. Consider that in some LC circuit applications, a lower Q may be desirable in order to achieve a wider bandwidth. In other cases, a very high Q may be desirable for narrow selectivity, for example. Both the inductor and the capacitor in a resonant circuit may affect the Q of the circuit. The Q of the inductor is determined by ...


5

The problem is most likely the type 43 core material. It is quite lossy at 21 MHz and is a poor choice for the auto-transformer. Unlike a transmission line transformer, the auto-transformer relies on the coupling of the two coils to partially occur by passing flux through the core. The complex permeability of type 43 material indicates high losses for this ...


4

To understand better, I'd suggest you follow this: 1) If you have an AM transmitter at 10.000 MHz (let's use simple numbers). If someone whistles in the microphone, at, say, 1kHz, two sidebands will appear, at 9.999MHz and 10.001 MHz (and a carrier at 10.000MHz). 2) If you convert the AM tx into USB, and transmit at 10.000 MHz, the whistler will appear at ...


4

Filtering square wave aims to produce as pure as possible sine wave as the local oscillator frequency. If you would feed all the harmonics to the non-linear modulator, the output would contain all the strange sum frequencies that can be very close to the wanted signal. For example in SSB transmitter the unwanted sideband could mix with a harmonic of the ...


4

The oscillator may really be running at half of the received frequency. Search for Poliakov mixer on the web for an example of this.


4

Q1 forms a Colpitts crystal oscillator. It uses the reverse biased D2 as a varicap (variable capacitor) to pull the crystal frequency by 700 to 800 Hz, This frequency beats with the received frequency to produce the audible CW tone. The adjustability of the pull through W1 also allows a simple, limited range RIT function during receive.


4

didn't do it, but back in the day stumbled across this, and thought it might be on-topic. EDIT: inserted original text below. To: Multiple recipients of list From: carreiro@netcom.com (Paul Carreiro) Subject: Help in FT-243 Grinding Greetings fellow boatanchorites! I need help from all the BA gurus out there practiced in the mysterious ...


4

It is the thickness of the crystal that primarily determines its operating frequency. When crystals are manufactured, they are gradually ground down in a special fixture. The operator monitors the process by keeping a shortwave radio nearby, which picks up the electrical noise created by the stress of the grinding action on the crystals, which is relatively ...


3

You can use a radio receiver that has appropriate coverage, but the catch is that you have to sweep across the entire frequency range the oscillator might be oscillating at — you're limited to the bandwidth of the radio's filters, so you can't get an immediate answer like a frequency counter would give you. First, you need to make sure the signal is getting ...


3

This falls under the heading of "Depends on the Commercial Rig"... Some commercial Ham gear is equipped with a connection for external VFO, so for those it's a "plug and play" solution. For other gear it would depend on your ability to find the correct spot in the signal chain to inject the signal from your VFO. I know of hams who have built a DDS VFO and ...


2

I would suggest looking at a couple of sites: http://www.radio-electronics.com/info/rf-technology-design/superheterodyne-radio-receiver/basics-tutorial.php http://users.tpg.com.au/users/ldbutler/Superhet.htm Looking at Figure 4 on this second site shows that the RF front end and the local oscillator have a ganged tuning capacitor. The difference in ...


2

A small extra bit of information... Even though many VFO circuit descriptions, as you say, make it a point mentioning the 'cleanliness', by far most mixer circuits then proceed to convert the sine wave into something horrible :) Mixers, in order to function as such, must be non-linear. Ideally, they should multiply both input signals, and produce neat sum ...


2

Does the VFO signal necessarily have to be a sinusoid? Absolutely not. For each frequency component on a mixer's inputs, it produces at the output the sum and difference of those frequency components. Of course, if one of the inputs is sinusoidal, then it has just one frequency component. However, if there are more frequency components it still works. ...


2

Not really a problem – just a technological challenge. The challenge, in almost any case, lies in the fact that per definition, the input is a harmonic of the output of the frequency divider, and since you don't want to have both the "high" input frequency and the "low" output frequency in your LO signal, you'll have to strongly suppress the frequency of ...


2

This appears to be a design that uses a harmonic mixer (or a subharmonic mixer, both names are used on the Wikipedia page). While these are more commonly used at microwave or millimeter wave frequencies in contemporary equipment, they were probably reasonable, low-cost solutions for these single-conversion receivers. As described in the text, VD1 and VD2 ...


2

Mixing products from the oscillator. Best way is to use the oscillator on a higher frequency. Example: receiving 10 MHz, mf is say 9 MHz, so the oscillator is on 19 MHz. Demodulation does it´s work on 9 MHz so no [you hope] no mixing problems. You can also use a oscillator in 1 MHz, mf is 9 MHz bus you see the problem harmonics from the 1 MHz oscillator. 1,...


2

When designing circuits for the VHF region, careful design and construction practices must be utilized in order to obtain stable and repeatable gain. Here are a few thoughts that may improve the reliability of your circuit: 1.) Increase the drain resistance. This will reduce the tendency for UHF oscillations (and reduce the gain). 2.) Use toroids for your ...


2

The transient response of an RC circuit driven to oscillation by negative feedback generates a much higher proportion of harmonics than a similar LC circuits response. These harmonics have to be either filtered out with additional circuitry, or the oscillation needs a significant amount of post-processing (such as using it to clock an SDR or a digital ...


2

See http://www.leapsecond.com/museum/fei5650a/FE5650A-pin-out.gif for the pin out of option 58. http://www.leapsecond.com/museum/fei5650a/ supplies more detail.


1

I thought I'd come back to this having discovered the real reason why I wasn't developing the expected voltage across the 50 ohm load. Glenn's answer was good and is still very relevant, but I made a silly and simple mistake and I'd like to post it up as an answer in case someone else has the same issue. Basically, I had reversed the windings on the toroid....


1

There is definitely a best L to C ratio. You write: For 7030 kHz for example L=2.33 μH and C=220 pF, or L=23.3 μH and C=22 pF, or L=513 nH and C=1 nF, etc. If we increase the inductance, we can use a thinner wire for the same unloaded quality factor in all those cases. Assume the unloaded Q=300 in all the cases. If you make a parallel resonator in a 50 ohm ...


1

I see two coils in close proximity in the "as built" photo. One coil is connected to the detector; the other to the antenna -- and their axes are colinear. Whether this was the intent or not, your receiver is acting as a regenerative type -- detected signal in one coil is coupling to the other. Without a control to limit current in the coil that's acting ...


1

I can second the comments about bon-ami. Way back 2/3 of the way through the last century, we also used some of the super fine abrasives used for the later stages of grinding amateur telescope mirrors. Try Googling "Amateur Telescope Making Supplies" and "Telescope Mirror Blanks" for suppliers. - W6HPA


1

I only scanned that whole quote. I have my father's W9EHS SK, FT crystal grinding equipment. It has two special grinding compounds and the two plates. One of the plates has a rosette-like shaped indent that helps guide the blank in the figure 8 pattern. There is a special FT holder that allowed easy insertion and removal. At times, the edges had to be ...


1

RC oscillators have high levels of sideband noise. I do not know whether one could mitigate that by connecting RC filters in series, but it seems unlikely to me. A good LC oscillator uses the fact that the impedance depends on the frequency. With a parallel crystal resonance, the amplifier would see a low impedance far from the resonance so the wideband ...


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