receiving a carrier

On a sunny day (21 Dec 2006 02:31:12 -0800) it happened "jhon1" wrote in :

If you do not want to mix down, then you will have to build a selective amplifier for the frequency you use. What frequency is that? Are you allowed to use it? Normally one would use LC tuned circuits, or perhaps xtals at the frequency you want, the issue is stability, if the input picks up any of the signal from the next stages, then you have a problem.

"jhon1" wrote in message news: snipped-for-privacy@73g2000cwn.googlegroups.com...

You need to undertsand what is going on a bit. On a normal radio, you are not interested at all in the 'carrier', except as a 'carrier', of the modulated data (hence the name). It is harder to amplify high frequency signals, than those at lower frequencies (limitations of the amplifiers themselves), and to really accurately tune them, so what was done, was to start by amplifying the signal a little, then immediately mix to produce a lower working frequency (the I.F. - intermediate frequency), and amplify/tune this. This is the 'superhet'. This also has the big advantage that for a 'tuneable' radio (that can be used to receive different adjacent bands), the RF tuning can be quite wide band, and by changing the mixing frequency, the _same_ IF can be produced for different incoming frequencies. The IF tuning can be accurately set, and quite narrow band. to give good selectivity. Now, if you want to look at the incoming carrier itself, there is absolutely no point at all, in getting involved in a superhet. After the first stage, the signal will already be at a lower frequency, and will not give you access to the 'carrier'. Instead do a search for 'T.R.F.' (tuned radio frequency). This is a much simpler radio design, which relies on just a tuned RF stage, and amplification at the RF frequency. Normally, a envelope detector is then used to get the amplitude modulation on the carrier, but without this, such a radio, will give direct access to the incoming 'carrier'. Historically, getting good gain/selectivity with such designs, was quite hard, but a number of basic IC's exist to do this. The ZN414, and it's latter descendants, was probably the most famous, and though this is no longer available, the MK484 is an almost direct replacement.

Best Wishes

A few important questions:

What's the frequency?

How far does it have to go?

How clean does the received signal have to be?

How much power is available at the transmitter and receiver?

What country's radio laws apply?

On a sunny day (21 Dec 2006 03:46:30 -0800) it happened "jhon1" wrote in :

No way can I answer that as yo udo not give:

It would perhaps be simpler to build a separate receiver. Please answer 1,2,3 questions above.

Forget designing your own rvcr/xmt unit. The way to go these days is to just add a module to your pcb. I like RF monolithics transceivers, they comes in different frequency flavors.

1. Get yourself and old 27 MHz CB radio.

2. Strap the mic button down, but don't say anything into it.

1. Get a " Signal Strength Meter" - looks like a VU meter with an short antenna on top.

2. The needle on the meter will show you if the carrier exists and indicate its relative strength at a distance.

10- 4 li'll buddy..........

........ Phil

I wonder what for. You won't have anything to listen to at the receiver if you execute the scheme you describe. I'm no expert, but I think you may get a whistling sound during tuning or when the frequency drifts. Isn't that what happens when you're tuning around on sideband and you come across an AM carrier? Why don't you just get a shorwave receiver, build some kind of a simple tank circuit for your transmitter, and play around with them. You might learn something. From the vagueness of your first post nobody can really advise you anyway.

Google "superregen" or "super regenerative receiver" and other variants.Super-regenerative receivers are popular in applications such as garage door remote, car alarm systems and wireless door chimes. They are ideal where information rate and cost are low.

If you want to design your own, let me know; however, it may be challenging for a beginner. There are some examples on the web, as well as some commercially available units.

Super-regenerative receivers are used often at 210, 280, 312, 315, 390,

This type of receiver works like this: An RF amplifier with positive feedback (at inteded receive frequency) is allowed to go in and out of oscillation at a rate called the quench frequency. Sometimes the quench is forced externally, and sometimes a single transistor is biased and configured such that the RF transistor serves also as the quench oscillator (self quenching).

If you do it right, you can build a single-transistor receiver for a few pennies. Sensitivity is about 10uV with 75 ohm input. YMMV. In commercial applications, the challenging part of the design is meeting regulatory agency approvals, especially passing the "unintentional radiator" criteria.

Frank Raffaeli

Maybe this will give you some ideas:

Any commercial receiver can do this. In the early days of Ham radio where Morse code was used, this is exactly what was done. The transmitter sent an "unmodulated" carrier that was interrupted to generate the code. At the receiver end, the carrier was mixed with the output of a local oscillator that was tuned slightly "off" from the received carrier frequency. The result was an audio signal equal in frequency to the difference between the carrier frequency and the local oscillator frequency. This gave an audible indication of the presence or absence of the carrier. Regards, Jon

And the problem with a phase lock loop is?

Umm... if you hook up an antenna that's resonant at more or less 100MHz, the signal out of the antenna will be the carrier you transmitted -- with lots of attenuation, of course.

As a first order approximation, there's no difference between a tuned transmitting/receiving antenna system and a big attenuator. In other words, you can simulate your wireless system by just connecting up your transmitter & receiver with a cable and, say, a 60dB pad inbetween. (Throw in an extra arbitrary phase shift if you'd like -- use a random length of coax cable.)

It sounds like you don't have much experience here... could you tell us what you're trying to accomplish? Why a superheterodyne architecture won't work? When you say "it must be the same frequency" does that include accounting for things like doppler shifts? What does "the same shape" mean if you aren't modulating the carrier?

---Joel

Have fun then. There are many, very valid reasons that you don't see receivers that are not superhetrodyne.