low power 125khz receiver - Page 1
Forum section: electronics / projects

author: 2ndherbert
author_rank: Newbie
timestamp: March 11, 2019, 08:57:50 pm
content: Dear analog circuit designers,I am working on my thesis, the main part of witch consists of software with "minor" hardware design. I am familiar with digital design but not much into analog stuff. So far so good. My thesis includes the design of a 125khz low power, always on receiver.What i need:Get from 100µVpp 125khz (on-off keying with 9600bps) to the rectangular envelope signal with 3.3V (down to 2.5V).What i have:A very limiting budgetbattery supply with 3.6V down to 2.5V at EOLand as topping i can serve about a few µA standby current (no 125khz field)What i came up with:I thought about using a J-Fet in a common source configuration to get the voltages upto maybe some mV and than using a ultra low power comparator to further amplify the signal. The than rectangular 125khz signal i can rectify and feed through a loaded lowpass to generate the envelope.What problems i have:Finding proper j-fets for these low cutoff voltages witch are working under these low bias currents, best i can find so far is the TF412Shttps://www.onsemi.com/pub/Collateral/ENA2300-D.PDFFully understanding all the parasitic and sometimes added capacities around the common source amplifier. Are there some good design guides?I will build the circuit this week, but i am expecting a lot of trouble since getting the circuit stable was already difficult in the simulation.Am i on the road to hell or is there some hope?

author: Howardlong
author_rank: Super Contributor
timestamp: March 11, 2019, 09:15:17 pm
content: Is this RFID?

author: ogden
author_rank: Super Contributor
timestamp: March 11, 2019, 09:33:12 pm
content: You shall specify what you mean by "low power" and "always on". I would consider Tayloe detector/mixer based receiver.http://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf

author: 2ndherbert
author_rank: Newbie
timestamp: March 11, 2019, 09:45:41 pm
content: Quote from: Howardlong on March 11, 2019, 09:15:17 pmThis is not RFID. I have looked at RFID circuits but i have not found one with low power consumption.Quote from: ogden on March 11, 2019, 09:33:12 pmWith "always on" i meant that it is no option to turn the receiver off for a certain amount of time, it has to be ready all the time.Low power means max 5µA with no 125khz signal on the receiver (i don't now when there is a field that is why i referred to "standby" and always on)

author: Howardlong
author_rank: Super Contributor
timestamp: March 11, 2019, 09:54:46 pm
content: What is the 100uV pp from? Is it a tuned loop antenna?

author: radiolistener
author_rank: Super Contributor
timestamp: March 11, 2019, 11:40:02 pm
content: Quote from: 2ndherbert on March 11, 2019, 08:57:50 pmWhat is impedance of 100 uVpp 125 kHz source?

author: David Hess
author_rank: Super Contributor
timestamp: March 12, 2019, 01:08:07 am
content: I think you will get better results with a bipolar transistor in place of the FET because of greater transconductance. This also avoids the problem of trying to find a low threshold voltage FET.I would consider the two transistor cascode amplifier for each stage to make the best use of available gain.Using tuned circuits for the loads instead of resistors will also make the best use of the available voltage and current. The Q of of each tuned load will need to be compatible with the expected bandwidth to preserve the modulation.Your requirements may be unrealistic. The best single transistor 32kHz crystal oscillators operate at about that level of current.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 12, 2019, 07:57:57 am
content: Quote from: David Hess on March 12, 2019, 01:08:07 amI agree.People with little experience of Radio Receiver design often underestimate the pitfalls involved.The most likely result is a device lacking in both selectivity & sensitivity, although the OP's spec of 100uv sensitivity is not very ambitious, so that possibly may be obtainable with a plain old "Crystal Set".'Selectivity is another question, as there are the twin problems of adjacent channel rejection, & obtaining sufficient bandwidth at 125kHz to preserve the modulation.Cross modulation from high power MF AM Broadcast transmissions is another likely trap, not to mention that from SMPS.The dynamic range of a receiver operating on voltages of the order of 3.6v is unlikely to be able to handle interference from even quite distant lightning strikes, either.Receivers used professionally at such frequencies are normally complex devices because of these bugbears.Superheterodyne receivers can do this sort of job "standing on their heads", because they do not have to rely on just a few tuned stages.SDRs can achieve similar (but, in my opinion, not equal results).The trade off, of course, with such devices, is that they need much greater power to achieve their purposes.Yes, OP, you are pretty much "on the road to Hell"!

author: 2ndherbert
author_rank: Newbie
timestamp: March 12, 2019, 08:44:46 am
content: Quote from: Howardlong on March 11, 2019, 09:54:46 pmNo its not tuned loop. I am using a ferrite core coilhttps://www.tdk-electronics.tdk.com/inf/30/db/ind_2008/b82450a_a.pdfas tank circuit antenna.Quote from: David Hess on March 12, 2019, 01:08:07 amI will try to see if i can get that one going!Quote from: vk6zgo on March 12, 2019, 07:57:57 amThanks to make that clearThe device will be used minimum a few meters away from any kind of switching power supplies or similar. Interference with short disturbances is acceptable, if the recovery time is not longer than ~50ms. Using more power is no option. absolut max is 10µA.What do you think is the key to change way? Increase of available power or less sensitivity so more Vpp?

author: ogden
author_rank: Super Contributor
timestamp: March 12, 2019, 10:26:43 pm
content: Quote from: 2ndherbert on March 12, 2019, 08:44:46 amLet's talk about "always on". You mentioned that there is possibility of "no 125KHz field". For example automotive "keyless keys" are in the sleep just to briefly wake up and look for carrier when carrier found - only then they do their job. Example: by waking each 5 seconds to listen for 5 miliseconds one can increase battery life 1000 times or accordingly increase power budget for radio.

author: David Hess
author_rank: Super Contributor
timestamp: March 12, 2019, 11:22:53 pm
content: Quote from: vk6zgo on March 12, 2019, 07:57:57 amThis is also why I suggest using tuned circuits. Not only do they contribute to selectivity, but they also double the available voltage compliance.Thinking about it more, I would also use a power detection stage with a one shot to only activate the later stages including the data slicer after a potential signal is amplified.FM modulation would be easier to deal with. All of the above could be done to save power but linear operation would not be required before detection.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 13, 2019, 02:59:06 am
content: Quote from: David Hess on March 12, 2019, 11:22:53 pmIn my opinion, a radio without tuned circuits is just a detector.The OP hasn't mentioned size, but most people have a fixation on miniaturisation.That is counter-indicated in this case, as compact, high value RF inductors are lossy.This is tolerable when you haven't put strict limits upon the DC current draw of the complete device, as active gain can make up for these losses.

author: David Hess
author_rank: Super Contributor
timestamp: March 13, 2019, 08:47:40 am
content: Quote from: vk6zgo on March 13, 2019, 02:59:06 amI agree but I assume the OP was thinking of relying on the selectivity of a tuned antenna. I would use tuned circuits for their other advantages as well in this case which help with low power operation.QuoteLossy is actually an advantage if it limits Q to an acceptable value. I would have anticipated increasing the loss to lower the Q in this case anyway.