Designing 457kHz receiver for use in near field - Page 1
Forum section: electronics / rf-microwave

author: arildj78
author_rank: Regular Contributor
timestamp: March 20, 2019, 11:54:30 am
content: I am designing my first RF circuit. It will detect the signal strength of an 457khz avalanche beacon. The receiving antenna will be within 150m of the transmitter.The plan is a magnetic field antenna (coil on ferrite rod) connected to a receiver. Finally the signal strength will be sampled by an ADC and processed by a micro.The questions I have is as follows:•When designing the antenna, is it best to add as many turns as possible or should I aim for 50ohm at 457kHz?•What are the considerations when designing the input amplifier? Is it best to match the input impedance to that of the antenna for maximum power transfer or would it be better to have very high impedance to optimize for voltage amplification?•When only detecting signal strength, what blocks would the receiver have and are there any benefits from mixing down to a lower intermediate frequency? For best sensitivity, which sequence should the receiverblocks be connected? Should I start with a filter and then amp, or the other way around?Apart from the specifics mentioned above, what would be a good reference for standard topologies?Cheers from north of the arctic circle, Arild.

author: babysitter
author_rank: Frequent Contributor
timestamp: March 20, 2019, 08:38:00 pm
content: Not ready to use for your reason, but have a look at amateur radio direction finding receivers für 3.5 MHz,e.g.http://www.df1fo.de/index.html#80mRXfor antenna and stuff.I think to find such nearby beacons you don't need to use high gains.The number of 457 kHz gets you into the passband of old-style 455 kHz IF components which you might be able to source from scrap radios.Vy 73Hendrik

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2019, 11:26:59 pm
content: Quote from: arildj78 on March 20, 2019, 11:54:30 amDo a search for amateur radio articles aboutloop antennas with a sensing element. One of their features is unambiguous direction finding instead of having a more common bidirectional antenna pattern which can lead you away from the source. (2)QuoteI would use a tuned input circuit to at least somewhat reject interfering signals making this question irrelevant as any amplifier input impedance can be used. The antenna might be part of this tuned circuit as with AM radio loop-stick antennas and the loop antenna mentioned above. (1)QuoteThere are two benefits to using one or more intermediate frequencies:1. This allows for a narrow single frequency filter for greater selectivity.2. This allows for greater gain because if too much gain occurs at one frequency, feedback from the output to the input will cause problems. Splitting the gain up among different IF frequencies prevents this.There is no requirement to mix *down* to intermediate frequencies. This is simply the most common configuration because more gain will be available but when starting with a low RF frequency, mixing to a higher IF has advantages.Filtering is better used before amplification unless the filter introduces excessive loss which would degrade the noise figure.(1) Low frequency amateur radio fox-hunters are a select group compared to even the VHF and UHF fox-hunters. My experience is mostly with VHF and higher.(2) Also check out page 8 ofthis PDF. It is the antenna I have seen used for low frequency direction finding before.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: March 21, 2019, 11:41:38 pm
content: What if you make a resonant tank large coil and capacitor with infinite impedance for carrier frequency, then add a tap from coil or capacitor divider to get 50 ohm?So the tank is 184uH coil antenna as parallel tank with 618pF made by putting 680pF in series with 6800pF. The 6800pF cap is a 50 ohm output to amplifier.

author: arildj78
author_rank: Regular Contributor
timestamp: March 22, 2019, 05:27:20 pm
content: I'm trying to wrap my head around what you mean. I will try to figure this out.A

author: arildj78
author_rank: Regular Contributor
timestamp: March 22, 2019, 05:30:48 pm
content: Uups.. I got lost in that schematic. If we take it from the top.It looked to me like that circuit used JFETS as input amplifier. What is the benefit from using a JFET and not a MOSFET at the input?A

author: floobydust
author_rank: Super Contributor
timestamp: March 22, 2019, 07:02:11 pm
content: I find AM radio gets in the way. A station at around 580kHz will get picked up by a tuned antenna at 500kHz. The Q of the antenna tank circuit has to be high.500kHz is popular for lightning detection. I abandoned an AS3935/MA5532(tiny Q=30) antenna design because the Q was too low, it picks up enough adjacent AM radio stations and gives false lightning detection counts if an announcer is yelling, lol.Stormwizehas some antennas, all large ferrite rods. You can buy large ferrite rods 4" to 8" long x 10mm dia on eBay for crystal radios at low cost.To get 50 ohm output, you just wind 1-2 turns on the rod to get a transformer output. Or place an AM radio nearby so its loopstick antenna couples to the larger ferrite rod antenna. You'll need many turns on the main antenna, with a tuning capacitor to get 457kHz.You could start with the old ZN414/MK484 now replaced byTA7642and just try it out. The IC costs pennies. There are lots of example circuits using it.I think you need to have an IF for better selectivity. But not sure how much attenuation snow gives and 150m seems close to the beacon- unless it's very low power.Better radios useTEA5551Tsingle-chip AM radio with a ?kHz IF but the Toko magnetics would be hard to find.I would think drift will be a huge problem with arctic temperatures and cold batteries, for the beacon or the receiver.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2019, 03:36:00 am
content: Quote from: arildj78 on March 22, 2019, 05:30:48 pmA JFET is a little tougher and operates in depletion mode which may be an advantage.

author: Marco
author_rank: Super Contributor
timestamp: March 23, 2019, 04:14:12 am
content: Quote from: floobydust on March 22, 2019, 07:02:11 pmAdd a quadrature lock in detector?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2019, 10:43:20 am
content: With a high impedance FET amplifier close to the antenna there is no need to have the antenna impedance matched to 50 Ohms. A resonant loop or ferrite rod antenna tends to be relatively high in impedance anyway.In the 500 kHz range there is usually plenty of background noise from AM radio, SMPS, power line comunications and so on. So no need to push the amplifier and receiver noise level to the extremes. One may have to live with rather high signal levels from AM radio at frequencies relatively close. It may help to have extra analog filtering besides the antenna, e.g. an 455 kHz ZF filter.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2019, 04:34:42 pm
content: Those little tuned h-field antennas like I linked have a high Q so they operate as the preselector although this also makes them finicky. I had problems with humidity detuning my 2 meter shrunken quad but it should not be as bad with a 457kHz antenna.