I would like to pick up on-off keyed AM at 457kHz (avalanche rescue frequency) with great sensitivity, selectivity, and low power. Since this is just below the broadcast band, I'm hoping to find a single-chip receiver that can be tuned there. Have you come across any such animals lately?
Years I ago I wanted to receive 344kHz on the cheap, to listen to transcribed aviation weather from a facility 40 miles away. I took a cheapie AM radio, and tacked a low-value cap accross the tuning cap for the local oscillator section, so it'd oscillate down lower. Can't remember if I had to do that also upstairs in the antenna tuner section, but you get the idea. It worked just great. I don't know how much power these normally very weak "LOMs," as FAA calls them, put out when they doubled for transcribed weather. But if xmit power and distance for avalanche rescue, which is new to me, are proportionately lower, maybe this kludge will work? May not have to tack a cap to tune 475; just retweak what's there.
That's what I was thinking. The bandwidth will be quite low, so you will have to tune both the RF and the osc. If it has an RF stage, that also. You should be able to use fixed capacitors, and tune it up by adjusting the existing trimmers. If there are no trimmers, the radio isn't old enough. Note that you will need a much smaller capacitor for the oscillator than the RF. Lastly, if your IF frequency is 455 KHz, you MIGHT have to lower that by about 10 KHz if you hear a lot of squealing voices.
There used to be the old LM3820 but it is, like most other AM chips, obsolete and it was quite a power hog.
Since this is pretty much right on with the usual 455kHz IF filters in AM radios you could probably take a small AM radio, scrap the mixer and all and hook a preamp in front of the IF stage. Or get a good IF filter or filters and follow that with a high gain amp and a demodulator. If you need very small bandwidth and cost is an issue you might have to roll your own resonant circuit here or, for really narrow bandwidth order a 457kHz crystal. I'd try to design it so it can live with 3V from two alkalines and be very tolerant if these wear down to 2V. No big deal with transistors. For any decoding logic you can use the 74HC series which operates down to 2V at those slow speeds.
This should consume a whole lot less power than a mixer based design. If these need audio I guess it doesn't have to be very linear so you could possibly use a stage with minimal quiescent current.
I have used the MK484 in several designs, ranging from as low as 300khz to as high as 5 mhz. It is very versatile, if you are willing to experiment. It is easy to use, gives great results, and for those that like to tinker, a great all-around AM receiver tool. You can search for both suppliers and circuit details on Google. Well worth the try.
The key point about a superhet is that a fixed IF is easier to get gain and selectivity, since if that's tuneable it won't stay constant.
But if you've got a fixed tuned receiver, the selectivity is on the signal frequency, and you can design a good filter. Especially if it's down in the hundreds of KHz range, where the IF of many a superhet has resided.
The disadvantage in this case, where one has to have the signal on a specific frequency, is that there'll be no ceramic filters on the needed frequency, or at least not off the shelf. Oh, maybe there are, if beacon receivers use them.
But whether you put a good filter at 455KHz and have a superhet to make it tune the needed frequency, or put that good filter at the signal frequency should be irrelevant in this case.
YOu can have direct conversion SSB receivers with crystal filters, so long as you can buy or make a filter for the signal frequency. I've seen construction articles about that, someone open up the cans of HC/6 crystals and etched the crystals to get exactly where it was needed. Completely useless for any other frequency, but no spurious responses from having a mixer at the front end.
Or one could order crystals on the signal frequency.
That reminds me of some Ralph Burhan designs in the late seventies for Loran C. He'd use ICs designed for AM/FM broadcast radios, but use them on the signal frequency. He'd use the FM detector as a synchronous detector, limiting the carrier and using that to beat against the incoming signal. Rather neat, especially since he was using a very common IC.
(And as for superhet AM/FM broadcast ICs, they've got to be still available since there are still portable superhets available that don't use anything different from such receives 30 years ago, except it uses an IC instead of discrete transistors. If this is for single use, then you just pull a portable out and use it modified or extract the IC.)
Try Atmel, they make what used to be Telefunken chips. Also Philips.
You could also use a narrowband FM IF chip (Philips, On Semi.?) especially if it has the RSSI output (which is a logarithmic signal strength).
If it's a straight CW (not modulated, except for the keying) signal, a Direct Conversion receiver may be a good option. These are very common in amateur radio circles. The idea is to use a mixer to convert the signal directly down to the audio range, and have most the the receiver's gain in the audio amplifier.
Mark Zenier email@example.com Washington State resident
I went through the AM radio IC's in the NTE catalog. There are lots of IC's for RF/IF/mixer/agc, etc., but not much for low current operation.
Given it's AM and 457 kHz., I believe a TRF design would be excellent, as the current can be minimized. The NTE1055 IF looks usable, but a good OP amp may be as good if coupled with a AGC at the front-end.
For those looking for a complete "AM Tuner Subsystem" check out the NTE1214 (RF amp, osc, mixer, 3 IF amps, AGC, tune meter output).
I agree it looks very good, but I could not find a supplier except for very small quantities, AND I found several link to people wanting to buy one. I don't like to design around something that already looks obsolete.
That's not a bad idea. Surely any modulation (if there is modulation) is there to make the signal identifiable rather than to send information. A beat note can be obnoxious if you are trying to listen to an amplitude modulated signal, but it can be very useful to help determine that you have indeed come across an actual signal. Years ago when I had an SP-600, I'd reoutinely turn on the BFO when hunting signals, because it was easier to notice a carrier in the noise when there was a beatnote.
That's not a bad idea. Surely any modulation (if there is modulation) is there to make the signal identifiable rather than to send information. A beat note can be obnoxious if you are trying to listen to an amplitude modulated signal, but it can be very useful to help determine that you have indeed come across an actual signal. Years ago when I had an SP-600, I'd reoutinely turn on the BFO when hunting signals, because it was easier to notice a carrier in the noise when there was a beatnote. >>
I'm still searching for complete technical info on the avalanche
receiver/transmitter. So far I know the carrier frequency is 457 kHz. as given