RF stage question

It depends on what you will need for your next stage. May I suggest that you build a kit. The instructions will describe the function of each stage as you build it. I used Ramsey kits in my classes for years and they were decent as educational projects.

Regards, Tom

There used to be a company I "believe" named "Graymark" which made "educational" Electronics kits. Our school shop class used them. They were decent for "learning". "I'm" not sure about their "radio" but the "Power Supply" with alligator clip leads was cool. It was small but basically replaced batteries (then) as a sort of over sized Walwart.

It sure is nice to see some interest in this area. If you have problems finding said kit, use "any" - to keep the spark alive. Homebrew (make from scratch) if you want.

But the question can't be answered without any indication of your needs, or what the rest of the receiver is.

Most AM broadcast band radios are sensitive enough, and have no RF stage.

An RF stage often has other reasons than gain. If you want better filtering at the front of the receiver, to keep down images for instance, you might want an amplifier stage there to compensate for the losses in the tuned circuits.

Gain later in the receiver has the advantage that it's on a fixed frequency, which is the point of superheterodyne receivers; convert the incoming signal to a fixed frequency where selectivity and amplification can be had without the problems that come with making it tuneable. Put the gain at the fixed intermediate frequency, and you won't have to worry about making the stage tuneable (and maybe more important from the standpoint of construction, you won't have to get it to tune with the rest of the tuneable stages).

So you wouldn't put gain at the front of the receiver unless there was a very good reason.

Michael

You'd do well to go get an ARRL handbook (most libraries) and study that.

To maximise dynamic-range, the RF amplifier stages of high-performance HF communications receivers typically have just enough gain to compensate for the insertion loss of the first mixer.

If you want to build a superhet receiver with a tuned RF stage, you will need a variable capacitor with 3 sections to tune the antenna, oscillator and RF stage. Most AM radios use a tuning capacitor with only 2 sections to tune the oscillator and antenna without a RF stage.

But it's difficult to design your own AM radio, since you need about

90dB of gain in the IF stages, and that isn't easy to do without the whole thing oscillating at 455kHz. I've tried it, and never could get the thing to start amplyfing and stop oscillating.

Anybody know of an easy way to get 90dB gain at 455kHz without the thing oscillating?

-Bill

Did you know that you might not need it? At least for your first try.

If you dangle a piece of wire out the window ( or possibly across the floor ) as an aerial you can get more than half a volt of RF out of it i.e. measured between one end and e.g. a radiator pipe.

This is enough to power a "crystal set" directly i.e. a handful of components including a crystal ear piece (or even headphones if you can get ancient "war time" variety of e.g. 2000 Ohms impeadance - normal headphones won't work, finding out why will be a big moment for you).

Cheers Robin

Shielded transformers and neutralized IF amp stages. :-)

Cheers! Rich

Wasn't there an improvement when they brought in those newfangled pentodes arrived on the scene? Did away with the fussiness of those triodes.

(On the other hand, if pentodes had been invented before the triode, would that have delayed the "discovery" of oscillation and regeneration?)

Michael